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== General ==
== General ==
In this section a more in depth literature review on the specific subject of reforestation after forest fires is done to assess whether or not a robot is a desirable artifact to be created for such a purpose. Several aspects are investigated including but not limited to biodiversity, need for controlled seeding, effectiveness of current and the costs of current methods. The general literature review concerning itself with the possibilities of robotics technology and the contemporary issues involving reforestation can be found in [[General Literature Review]]. General information about the project can be found at [[PRE2017 4 Groep6]].
In this section a more in-depth literature review on the specific subject of reforestation after forest fires is done to assess whether or not a robot is a desirable artifact to be created for such a purpose. Several aspects are investigated including but not limited to biodiversity, need for controlled seeding, the effectiveness of current methods and the costs of current methods. The general literature review concerning itself with the possibilities of robotics technology and the contemporary issues involving reforestation can be found in [[General Literature Review]]. General information about the project can be found at [[PRE2017 4 Groep6]].
 
==Biodiversity & Need for Control in National Parks==
National Parks are located in most countries spread all over the world. In the United States only, the 59 national parks which were acknowledged at the end of 2017 span well over 400.000 km<sup>2</sup> (Sawe, B.E. 2017)
<ref>
Sawe Benjamin Elisha (2017) How many national parks are there in the United States, World Atlas. Retrieved from: https://www.worldatlas.com/articles/how-many-national-parks-are-there-in-the-united-states.html. Accessed at 14-05-2018.
</ref>.
Even though National Parks are most commonly known as a touristic attraction, the reason they have originally been established is ''‘’to conserve the scenery, natural and cultural resources, and other values of the park in a way that will leave them unimpaired for the enjoyment of future generations.’’'' (The NPS Organic Act, 1916)
<ref>
National Park Service (1916) the NPS Organic Act. Retrieved from: https://www.nps.gov/subjects/air/npsresponsibilities.htm. Accessed at 14-05-2018.
</ref>.
Meaning that, only taking the natural factors into account, the National Parks have to keep the wildlife as intact as possible. Thereby protecting it not only from human influences, but also from certain natural ones. The latter include, but are not limited to, typhoons, droughts, floods, earthquakes, and fires. Even though these phenomena are considered events which happen every once in a while and which are part of nature, their effect on a National Park is usually catastrophic. Usually, after one of these events, a substantial portion of the park is completely devastated, meaning that the wildlife needs to recover. <br>
 
This paper will be limited to the phenomena of a forest fire originated from natural causes and the recovery of the National Park afterward.
In order to recover a National Parks ‘natural scenery’, a clear definition of this must first be established. The natural scenery can be divided into three categories, being animals, plants, and non-living elements. This latter category spans the general topography of the Park, e.g. rivers, lakes, and mountains.
Together these three categories form what is commonly known as an ecosystem, which is a term describing the relation between organisms and the physical environment they live in. Therefore, the goal of conserving the natural scenery can also be reformulated into conserving the current ecosystem. As the non-living elements are usually not as heavily influenced as the other two categories, they are from now on ignored in this analysis of the recovery of the National Park.
Another concept linked to the conservation of nature is biodiversity. Biodiversity is the variability among living organisms from all sources, within and between species. The degree of biodiversity shapes the ecosystem, if there are many different species living in the park, they interact in another way with each other and their physical environment when compared with a situation wherein there are only a handful of species living in a similar environment. This shows that the biodiversity is of vital importance for the ecosystem and that a change in the park's biodiversity will result in a transformation of its ecosystem. (Greenfacts, 2018)
<ref>
Greenfacts (2018) Biodiversity and Human Well-being. Retrieved from: https://www.greenfacts.org/en/biodiversity/l-3/1-define-biodiversity.htm. Accessed at 16-05-2018.
</ref>
The substantial dependence of natural scenery on the ecosystem and of the ecosystem on the biodiversity states that for a National Park to conserve its natural scenery, at least the biodiversity cannot change by any significant factor. It is, however, not claimed that this is enough for a park to conserve the natural scenery. It could be argued that other factors, like noise or horizon pollution, mean that the natural scenery is not conserved, but this is not discussed in this paper. Only the contribution of the biodiversity is taken into account. <br>
 
Coming back to the recovery after a natural catastrophe, to say that a forest fire has a significant effect on the park's biodiversity is an understatement. Depending on the fire’s size, temperature, and the propagation speed it will wreak havoc over a large area of the national park which has disastrous consequences for all wildlife within the area. In order to get the park back to its original level of biodiversity, a certain degree of control is needed, as the original ratios of species should be established once again. Regaining this biodiversity is divided in both the animal and plant life, and can be done in multiple ways. This paper will be limited to the methods of regrowing the plant life, to be referred to as reforestation from now on. In order for reforestation to be effective, it needs to give all species a fair chance to return to their original population. This also requires to take into account the species-specific growth preferences, meaning factors such as temperature, nutrition, degrees of freedom, and exposure to sunlight. All these factors are heavily dependent on the plant's location, and thus on the location where the initial seed starts to sprout after the fire. Hence, the location of the seeds is of vital importance for reforestation. The three currently most used methods for reforestation are aerial, manual and natural reforestation, which will be further elaborated on below.


== Current methods of reforestation ==
== Current methods of reforestation ==


=== Natural reforestation vs Artificial reforestation ===
===Natural Reforestation===
A forest can be recreated with natural reforestation. Natural reforestation relies on nature to return an area to forestland after the area is deforested, this can happen through seeds that are carried by the wind, transported or buried by animals or that are dropped by mature trees<ref>North Carolina Forestry Association. (2017, February). Forest Management Basics. Opgehaald van North Carolina Forestry: https://www.ncforestry.org/teachers/forest-management-basics/</ref>. In contrast to artificial deforestation, natural reforestation happens without the help of humans or machines. Artificial reforestation has certain important benefits why it is often preferred over natural reforestation. It provides better control over tree spacing, more control over the species present in the new forest, the opportunity to plant genetically improved seeds or seedlings, and a higher rate of tree survival<ref>North Carolina Forestry Association. (2017, February). Forest Management Basics. Opgehaald van North Carolina Forestry: https://www.ncforestry.org/teachers/forest-management-basics/</ref>. It can be summarized that when using artificial reforestation, the reforestation can be better managed than with natural reforestation. Why is it preferred to have more control over reforestation? Reforestation guidelines help minimize exposure to mineral soil, and thus decrease the impact on the nutrient balance of the site and provide the flexibility to successfully regenerate certain desired species. Reforestation guidelines encourage approaches to regeneration of deforested areas that result in tree species diversity, appropriate species selection for a particular site and maintenance of habitat structure. Artificial reforestation thus has benefits for wildlife habitat and forest soils<ref>nrs fs fed. (2014). Reforestation</ref>. Another source states that only 7.9% of reforestation is done with natural reforestation. This number is so low because Where natural methods of natural regeneration fail or are unrealistic, artificial planting ensures the attainment of the main goal - sustainability of forest ecosystems<ref>Jan Lukaszewicz, W. K. (2002). THE ROLE OF ARTIFICIAL AND NATURAL REGENERATION IN INCREASING THE SUSTAINABILITY OF FOREST ECOSYSTEMS IN POLAND. </ref>. Therefore it can be concluded that artificial reforestation is preferred over natural reforestation. There are different methods of artificial reforestation. The two most common ones will be further explained below.  
One method of reforestation is natural reforestation. Natural reforestation relies on nature to return an area to forestland after the area is deforested. This returning of vegetation can happen through seeds that are carried by the wind, transported or buried by animals or that are dropped by mature trees (North Carolina Forestry Association, 2017) <ref name="NCFA">North Carolina Forestry Association. (2017, February). Forest Management Basics. Retrieved: https://www.ncforestry.org/teachers/forest-management-basics/. Accessed at 18-05-2018.</ref>. This already leads to the first constraint of natural reforestation; there must be adequate living trees and animals around to enable natural reforestation, i.e. a nearby ecosystem has to be present. If there are no trees in the entire environment, there is only a negligible probability that seeds will be transported to the area. However, this study is concerned about returning a forest after a wildfire in a National park, in most cases, the fire is eliminated after a while due to human interference and this results in sufficient living trees left in the neighborhood to provide seeds.
 
In contrast to artificial deforestation, natural reforestation happens without the help of humans or machines. Because natural reforestation happens without the interference of humans, there is absolutely no control over natural reforestation. This means that the most dominant species, or the species that have not been destroyed by the fire, will regrow on the devastated area whilst other species which were originally located in this area are either all destroyed by the fire or take much longer to regrow, endangering them to vanishing from the National park since they cannot spread their seeds anymore. As is stated in the introduction from this wiki page, in order to rehabilitate the Natural park the biodiversity must stay in its original state to the best possible extent. This may happen with natural reforestation when the National park only consisted of one or a few species. However, when the National park consisted of many different species, which is often the case to create a nice scenery, natural reforestation provides no control and thus no guarantee that the old ratio of species will regrow. It can be said that the natural reforestation will always be good since nature can do what she wants, this is however not the case for a National park. Some species will always be dominant over other species, e.g. weeds in your own backyard. These dominant species are however not the (only) species you want to have in a National park. In order to recreate the original ratios of species in the National park, some level of control is needed which cannot be created with natural reforestation.
 
The benefit of natural reforestation is that the costs are extremely low. Because natural reforestation happens without the interference of humans, technology or other materials, there is also no money invested in the reforestation. <br>
 
In the introduction, it is also stated that in order to achieve the preferred ratios of species, the different seeds need to be planted at different depths in the soil. It is obvious that this situation cannot be achieved by means of natural reforestation. Because most natural reforestation happens with seeds that are dropped by mature trees, these seeds will all end up on the ground and none will be buried, unless they are taken away by local wildlife. The level of control that is thus needed to recreate a National park cannot be obtained with natural reforestation.  


===Viability of direct seeding===
Another source confirms this conclusion and states that only 7.9% of reforestation is done with natural reforestation (Lukaszewicz, 2002)<ref>Jan Lukaszewicz, W. K. (2002). THE ROLE OF ARTIFICIAL AND NATURAL REGENERATION IN INCREASING THE SUSTAINABILITY OF FOREST ECOSYSTEMS IN POLAND. Retrieved from: http://www.fao.org/docrep/ARTICLE/WFC/XII/0323-B1.HTM. Accessed at 16-05-2018. </ref>. This number is so low because where natural methods of natural regeneration fail or are unrealistic, artificial planting ensures the attainment of the main goal - sustainability of forest ecosystems. <br>
While direct seeding has been a valid option for reforestation for centuries, over the last 5 decades the quality of seedlings has improved rapidly. This caused seedlings to be chosen more often over direct seeding since seedlings have a higher establish rate.
Worldwide forest restoration programs, of which a few have started recently, will favor direct seeding again since direct seeding uses less labor hours and the seeds are cheaper and easier to produce then seedlings. To increase the establish rate of direct seeding one has to consider that seeding is more than delivering seeds to the site:
The time of seeding for different seeds impacts the establish rates, the quality of the seeds and the soil also should be inspected. lastly managing competitive vegetation should also improve establish rates. <ref>Grossnickle SC, Ivetić V (2017) Direct Seeding in Reforestation – A Field Performance Review. Reforesta 4: 94-142. doi: https://dx.doi.org/10.21750/REFOR.4.07.46</ref>


Artificial reforestation has certain important benefits as to why it is often preferred over natural reforestation. It provides better control over tree spacing, more control over the species present in the new forest, the opportunity to plant genetically improved seeds or seedlings, and a higher rate of tree survival (North Carolina Forestry Association, 2017) <ref name="NCFA"/>. It can be summarized that when using artificial reforestation, the reforestation can be better managed than with natural reforestation, which is exactly what is necessary to recreate the National park as is stated in the introduction (United States Department of Agriculture, 2014) <ref> United States Department of Agriculture, Forest Service, Northern Research Station,2014. Retrieved from: https://www.nrs.fs.fed.us/fmg/nfmg/docs/mn/Reforestation.pdf. Accessed at 18-05-2018.</ref>. Two main methods of artificial reforestation are manual reforestation and aerial reforestation. These two methods will now be discussed.


Broadcasting the seeds by hand is a valid way of reforestation, but has some drawbacks. The seed establish rates are very low, mostly around 20%. This can be improved however by different methods, such as manually cultivating the ground or using straw mulching. The effect of such methods differ heavily between kinds of vegetation. The aforementioned methods have been tested on 3 species of plants in Greece.<ref>Brofas, G., & Karetsos, G. (2002). Revegetation of mining spoils by seeding of woody species on ghiona mountain, central greece. Land Degradation and Development, 13(6), 461-467. doi:10.1002/ldr.529</ref>


Since the research shows that the effects are not consistent this means research will have to be done on all plants in the region of reforestation in order to use the broadcasting of seeds to achieve an acceptable result.
===Manual reforestation===
Manual reforestation is the oldest and most well-known method of replanting areas devastated by wildfires and the overall replanting of trees in general. However, manual reforestation is also a very slow method. It requires a large workforce and costs can quickly rise beyond proportions. It is estimated that manual reforestation can cost up to 62.000 USD for the initial replanting of a mere 1 by 1 km lot (Engel and Parrotta, 2001) <ref name = "manual"> Engel, V. L., & Parrotta, J. A. (2001). An evaluation of direct seeding for reforestation of degraded lands in central Sao Paulo state, Brazil. Forest Ecology and Management, 152(1-3), 169-181., https://doi.org/10.1016/S0378-1127(00)00600-9 </ref>. Maintenance is also required in the following years to make sure the forest grows as it is expected to be. This can cost up to 120.000 USD in the first 2 years only (Engel and Parrotta, 2001)<ref name = "manual"/>. This is a significant investment for national parks or other involved parties and other methods might be more cost efficient. More than 182.000 USD has to be spent on workforce and material. This is by far the most expensive and most labor intensive method and therefore not the most wanted solution. Manual reforestation is, however, preferable over methods for small areas.  


This method of reforestation also poses significant health risks to the workforce (Finlay et al., 2012) <ref name = "health"> Finlay, S. E., Moffat, A., Gazzard, R., Baker, D., & Murray, V. (2012). Health impacts of wildfires. PLoS currents, 4. https://dx.doi.org/10.1371%2F4f959951cce2c </ref>. Since our focus lies on wildfires in national parks, this implies high pollution rates for soil, air, and water in the target areas. Ashes and toxic residues are often found in and near the devastated areas. There is an increase in mortality rate and symptomes related to the lungs occur at a significantly higher rate (Finlay et al., 2012) <ref name = "health"/> . This is perhaps the most important factor when deciding which method is to be used to replant an area after a wildfire.


==== Aerial seeding ====
Manual reforestation also has some benefits over the other primary replanting methods. This method is more precise compared to the other methods. Seeds can be planted at a more optimal depth and invasive or other unwanted species can be easily removed by the workforce on-site (Waldrop, 1998) <ref name = "conference"> Waldrop, T. A. (1998). Proceedings of the Ninth Biennial Southern Silvicultural Research Conference. Gen. Tech. Rep. SRS-20. Asheville, NC: US Department of Agriculture, Forest Service, Southern Research Station. 628 p., 20. https://doi.org/10.2737/SRS-GTR-20</ref>. This means that the destroyed forest can be changed to a more healthy ecosystem by having the workforce suppress and promote certain species. The germination rate is therefore also relatively high compared to the other primary replanting methods (Waldrop, 1998) <ref name = "conference"/>. However, this rate also depends on other factors like seed quality and differs per species. Seeds are also less prone to animals that use them as food as they are planted directly into the ground and are not easily accessible by animals. Seedlings and young trees can also be planted instead of seeds themselves to kickstart the growth of the forest. This method bypasses the inherent danger of the seed just lying on the ground. However, seedlings and saplings are more expensive and are fairly more difficult to move on-site when compared with the relatively small seeds. Recent advancements in seed quality have also made seed-only methods more beneficial, both in terms of costs and survival rate.
Aerial seeding is perhaps the most novel method for reforestation among the other options, which have generally existed for many centuries. Its main premise is a reduction in labour, as seeds can be sown at a much higher rate than manual seeding could ever produce and time-effectiveness, as an airplane can easily cover an area of several hectares at a much quicker rate than manual seeding using volunteers. However the question remains if this method is truly beneficial in case of actual saplings it produces and the costs it inherently carries, considering an aircraft of several metric tons needs to be lifted in the air burning kerosine and enormous amounts of seeds are spread.
Contrary to intuitive belief aerial seeding is in most cases not a standalone method, in order to be effective more often than not some preliminary ground work is required to prepare the area to be seeded (in terms of boosting the receptiveness of the ground to the dropped seeds) (Régnière, 1982) <ref name="probability model"> Régnière, J. (1982). A probabilistic model relating stocking to degree of scarification and aerial seeding rate. Canadian Journal of Forest Research, 12(2), 362-367. </ref>. A very crude probabilistic model, taking into account two classes of possible areas (highly receptive due to site preparation or natural levels) and a constant occupation of highly receptive area per unit of area exists <ref name="probability model" />, which reveals that higher seeding rates do in general lead to more saplings, however the relation is only linear in the case of pure natural occupation. If the site is prepared and occupation rates become higher, the relation between the number of saplings per unit approaches more or less rooted relationships. This model further reveals that the variance of the pattern in saplings per unit area severely depend on the width between airplane runs, with lowest variance only occurring at 1 meter distances. However, for each and every combination of width between airplane runs and seeding density a minimum in variance exists to create an optimal balance between the two. Using a purely random (obtained by uniform seeding density created by narrow spacing) seed distribution as a measure for maximum obtainable sapling rate, it was found that the efficiency of a real process with a limited spacing decreases rapidly as the spacing becomes larger, although there is a compensating effect for higher seeding rates, albeit the amplitude of this compensation is much smaller than the amplitude of the decrease in efficiency at wider spacing. Very counterintuitive, for equal spacing between the airplane runs, the efficiency of the process first drops to a minimum of 88% after which it slowly increases for larger seeding drop densities, meaning that more does not necessarily mean better, unless ridiculous amounts of seeds are used.  


Furthermore, a second study in China comparing growth rates and carbon stock levels (equivalent for biomass) of both aerial seeded forests and naturally regenerated forests showed differences effectiveness between the two methods, with aerially seeded forests needing longer time to completely develop and hence always following behind naturally regenerated forests (Xiao et al., 2015) <ref name="Chinabois"> Xiao, X., Wei, X., Liu, Y., Ouyang, X., Li, Q., & Ning, J. (2015). Aerial seeding: an effective forest restoration method in highly degraded forest landscapes of sub-tropic regions. Forests, 6(6), 1748-1762. </ref>. This study was performed using the same tree species in previously highly naturally degraded areas.  In the early years (10-20 years) of the new forests the naturally regenerated forests seem to do better in terms of overall carbon stock, with an eventual conversion of the carbon stock values for the aerially seeded forest to carbon stock values of the naturally regrown forests for elder forests (50+ years). This faster growth rate of naturally regenerated forests is most likely caused by the self-capacity of natural forests to sustain themselves, which can only kick in at a later stage for an artificially planted forest by means of aerial seeding and by a higher occupation of carbon stock of the forest floor litter layer in the natural forests which provide many nutrients for trees. Thus overall, aerially seeded forests will eventually over time reach a state where the difference in their carbon stock is no longer statistically significant, however this approach in carbon stock is always from below.
Manual reforestation is also already augmented by utilizing machines, by preparing the ground beforehand by means of subsoiling, machines can increase the growth rate and survival rate of the seeds (Waldrop, 1998) <ref name = "conference"/>. Subsoiling also provides the option for placing the seeds even deeper into the ground, which can be beneficial for certain species. However, planting is still a labor-intensive practice and these large machines for subsoiling, often used in agriculture, cannot be used in all areas in national parks. Without machines, manual reforestation can still reach hill and mountain like areas that are normally not easily accessible for large machines.


Hence taking into account the findings of both Régnière <ref name="probability model" />and Xiao et al. <ref name="Chinabois" /> it is safe to conclude that aerial seeding can possibly be an effective method for reforestation, however it is largely context dependent on the problem. Considering one needs a pre-prepared area to increase survivability of seeds and the airplane needs to make very tightly spaced runs to create a somewhat universally spread forest, the process becomes very time intensive in excecution, whereas the main appeal of aerial seeding would be the simplicity in saving time as compared to manual seeding. In situation where natural natural deposits for seeds are scarce aerial seeding would be an optimal solution, since it simply would take nature too long to naturally recover the forest, even though the growth rate of natural forests are higher. However, considering we are dealing with forests fires this last situation is not very likely, as most often due to human intervention forests do not completely vanish due to wildfires, rendering plenty of natural seed deposit left. In terms of control aerial seeding gives little opportunities unless one chooses to specifically seed only one species of plant, as for any other desired degree of control a mixture of seeds has to be spread, for which different survival rates exists. Albeit, if one desires empty patches of land in the forest this could be done by severely increasing the spacing of airplane runs. In terms of biodiversity a similar conclusion can be drawn. Some level of biodiversity can be reached by mixing seeds from differents trees and plants together for the airplane to drop, however this increases the difficulty of pre-preparing the site for different types of trees/plants to coexist together, whereas such an situation would also eventually be reached by nature.  
A small robot could significantly decrease overall costs while still being able to a reach difficult areas. There are also no extra health risks for the human workforce since they can perform their function off-site. If high precision can be achieved without a significant loss of speed and a high germination rate, then small robots might provide an elegant, cheap and fast solution to the given problem.




==== Manual reforestation (Volunteering) ====
===Aerial reforestation===
Manual reforestation is an ineffective method. The cost of replanting a 1 km by 1 km field can cost up to 62,000 usd. This is not a small investment, and maintenance can further increase these costs. Manual reforestation also require a significant amount of workers, it might not be feasible to get sufficient manpower. Robotics might provide a cheaper and more efficient alternative, maintenance costs can be lower than the labour costs of human workers, this has been shown in various other sectors within our industry.<ref> Vera Lex Engel, John A. Parrotta, An evaluation of direct seeding for reforestation of degraded lands in central São Paulo State, Brazil, 2001, https://www.fs.fed.us/research/publications/misc/78142-2001-Foreco-Engel-Parrotta.pdf </ref>
Aerial seeding is perhaps the most novel method of reforestation among the other options, which have generally existed for many centuries. Its main premise is a reduction in labor, as seeds can be sown at a much higher rate than manual seeding could ever produce and time-effectiveness, as an airplane can easily cover an area of several hectares at a much quicker rate than manual seeding using volunteers. However, the question remains if this method is truly beneficial in terms of actual saplings it produces and the costs it inherently carries, considering an aircraft of several metric tons needs to be lifted in the air burning kerosine and enormous amounts of seeds are spread. <br>
Contrary to intuitive belief aerial seeding is in most cases not a standalone method, in order to be effective more often than not some preliminary groundwork is required to prepare the area to be seeded (in terms of boosting the receptiveness of the ground to the dropped seeds) (Régnière, 1982) <ref name="probability model"> Régnière, J. (1982). A probabilistic model relating stocking to a degree of scarification and aerial seeding rate. Canadian Journal of Forest Research, 12(2), 362-367. https://doi.org/10.1139/x82-052 </ref>. A very crude probabilistic model, taking into account two classes of possible areas (highly receptive due to site preparation or natural levels) and a constant occupation of highly receptive area per unit of area exists (Régnière, 1982) <ref name="probability model" />, which reveals that higher seeding rates do in general lead to more saplings, however the relationship is only linear in the case of pure natural occupation. If the site is prepared and occupation rates become higher, the relation between the number of saplings per unit area approaches a more or less square root relationship. This model further reveals that the variance of the pattern in saplings per unit area severely depends on the width between airplane runs, with lowest variance (corresponding to a uniformly seeded area, or in the case of multiple sown species a random pattern) only occurring at 1 meter distances. However, for each and every combination of width between airplane runs and seeding density a minimum invariance exists to create an optimal balance between the two. Using a purely random seed distribution (obtained by a uniform seeding density created by narrow spacing between airplane runs) as a measure for maximum obtainable sapling rate, it was found that the efficiency of a real process with a limited spacing decreases rapidly as the spacing becomes larger, although there is a compensating effect for higher seeding rates, albeit the amplitude of this compensation is much smaller than the amplitude of the decrease in efficiency at wider spacing. Very counterintuitive, for equal spacing between the airplane runs, the efficiency of the process first drops to a minimum of 88% after which it slowly increases for larger seeding drop densities, meaning that higher seeding rates do not necessarily mean better unless ridiculous amounts of seeds are used.


== Biodiversity ==
Furthermore, a second study in China comparing growth rates and carbon stock levels (equivalent for biomass) of both aerial seeded forests and naturally regenerated forests showed differences effectiveness between the two methods, with aerially seeded forests needing longer time to completely develop and hence always following behind naturally regenerated forests (Xiao et al., 2015) <ref name="Chinabois"> Xiao, X., Wei, X., Liu, Y., Ouyang, X., Li, Q., & Ning, J. (2015). Aerial seeding: an effective forest restoration method in highly degraded forest landscapes of sub-tropic regions. Forests, 6(6), 1748-1762. http://dx.doi.org/10.3390/f6061748 </ref>. This study was performed using the same tree species in previously highly naturally degraded areas.  In the early years (10-20 years) of the new forests, the naturally regenerated forests seem to do better in terms of overall carbon stock, with an eventual conversion of the carbon stock values for the aerially seeded forest to carbon stock values of the naturally regrown forests for elder forests (50+ years). This faster growth rate of naturally regenerated forests is most likely caused by the self-capacity of natural forests to sustain themselves, which can only kick in at a later stage for an artificially planted forest by means of aerial seeding and by a higher occupation of carbon stock of the forest floor litter layer in the natural forests which provide many nutrients for trees. Thus overall, aerially seeded forests will eventually over time reach a state where the difference in their carbon stock is no longer statistically significant, however this approach in carbon stock is always from below and will never exactly converge. 
Biodiversity is the measure of variability of living organisms. For a national park, this is to be interpreted as the amount of different trees, plants, animals and all other living organisms that can be found there. A parks biodiversity forms the foundation of a parks ecosystem, (Greenfacts, 2018)
 
<ref>
Hence taking into account the above findings (Régnière, 1982) <ref name="probability model" />  (Xiao et al., 2015) <ref name="Chinabois" /> it is safe to conclude that aerial seeding can possibly be an effective method for reforestation, however it is largely context dependent on the problem. Considering one needs a pre-prepared area to increase the survivability of seeds and the airplane needs to make very tightly spaced runs to create a somewhat universally spread forest, the process becomes very time intensive in execution, whereas the main appeal of aerial seeding would be the simplicity in saving time as compared to manual seeding. In a situation where natural deposits of seeds are scarce, aerial seeding would be an optimal solution, since it simply would take nature too long to naturally recover the forest, even though the growth rate of natural forests are higher. However, considering we are dealing with forests fires in national parks this last situation is not very likely, as most often due to human intervention national parks do not completely vanish due to wildfires, rendering plenty of natural seed deposit left. In terms of control, aerial seeding gives little opportunities unless one chooses to specifically seed only one species of plant, as for any other desired degree of control a mixture of seeds has to be spread, for which different survival rates exist. Albeit, if one desires empty patches of land in the forest this could be done by severely increasing the spacing of airplane runs. <br>
Greenfacts (2018) Biodiversity and Human Well-being retrieved from: https://www.greenfacts.org/en/biodiversity/l-3/1-define-biodiversity.htm
In terms of biodiversity, a similar conclusion can be drawn. Some level of biodiversity can be reached by mixing seeds from different trees and plants together for the airplane to drop, however this increases the difficulty of pre-preparing the site for different types of trees/plants to coexist together as each seed will have a different optimal depth and nutritional needs, whereas such a situation would also eventually be reached by nature. In terms of costs however, a study of the Society of American Foresters conducted in 1948 revealed that the costs of aerial seeding would be $7.25 per hectare (Westveld, 1949) <ref> Westveld, M. (1949). Airplane seeding: A new venture in reforestation. Unasylva, 3(3), 95-99. Retrieved from: http://www.fao.org/docrep/x5350e/x5350e03.htm. Accessed at 20-05-2018. </ref>, so taking into account inflation this would yield a cost of $75.39 (Bureau of Labor Statistics, 2018) <ref>Bureau of Labor Statistics, United States Department of Labor. Retrieved from: https://www.bls.gov/data/inflation_calculator.htm. Accessed at 16-05-2018. </ref>. More recent sources report a cost of $60 per hectare for aerial seeding process only, with a whopping $630 per hectare if site preparations are taken into account as well (Canadian Silviculture, 2005) <ref> Canadian Silviculture, spring 2005. pp 9-13. Retrieved from: https://http://www.silviculturemagazine.com/issues/spring-2005. Accessed at 16-05-2018. </ref>. The discrepancy between the values obtained by means of inflation and the more recent value is most likely caused by technological improvements making the process cheaper and more efficient, thus countering the inflation. <br>
</ref>
A popular novel alternative method for aerial seeding is by the employment of drones. Although this technique has a lot of potential, in its current state it is rather limited; communication technologies only allow a maximum distance of a couple of hundred meters for commands to be received by the drone (Fortes, 2017) <ref> Fortes, E. P. (2017). Seed Plant Drone for Reforestation. The Graduate Review, 2(1), 13-26. Retrieved from: http://vc.bridgew.edu/grad_rev/vol2/iss1/7/. Accessed at 18-05-2018. </ref>, and even if the drones are build to be autonomous to overcome this distance limitation due to communication problems, they are still severely limited by their battery capacity which usually allows operation for somewhere between 30-90 minutes (Anderson, 2016) <ref> John Anderson, New Atlas, (2016, September).Retrieved from: https://newatlas.com/tree-planting-drones-droneseed/45259/ .Accessed at 17-05-2018. </ref> (Bustamante, 2015) <ref>Bustamante, L.A.E. ,KÖLN, T. Forest Monitoring with Drones: Application Strategies for Protected Riverine Forest Ecosystems in the Atlantic Forest of Rio de Janeiro, Brazil (Doctoral dissertation, UNIVERSITY OF APPLIED SCIENCES). Retrieved from: http://intecral-project.web.th-koeln.de/wordpress/wp-content/uploads/2014/05/Thesis_Luis_Esquivel_021215_2.pdf. Accessed at 20-05-2018.</ref>.
meaning that a big chance in biodiversity would result in a big impact in the parks ecosystem. The goal of a national park is to conserve the scenery and the natural and historic objects and wildlife therein, which cannot be done if the ecosystem changes drastically. This leads to the conclusion that for a national park to fulfill its purpose, a drastic change in the ecosystem, and thus in the biodiversity, has to be avoided.
 
To this extend, natural reforestation is not sufficient regrowth method. When a forest fire occurs, the ground is covered in ashes and everything has been heated to arbitrary high temperatures. Even though some forest fires are beneficial for the fertility of the area, those which are too hot are not. Different kinds of plants are more suited to deal with those problems than others, meaning that those plants have a clear advantage during natural reforestation.  
All in all it can thus be concluded that in the case of a forest fire a robot can certainly be a proper solution as it gives a decent level of control if the resolution of the actuators is big enough and will certainly be an incentive for National Parks to switch to this novel technology if it can operate at a cost of less than $630 per hectare.
Beyond this, the fire’s size has impact on the way reforestation occurs. As some plants,  take lodgepole pines for example, are more effective at spreading their seeds over farther distances, these will start to recolonize the middle of the burned area fairly soon, while other plants, mostly smaller ones, take longer to get to the middle of the burned area, perhaps even multiple generations.(Turner, M.G. et al. 1997) <ref>
 
Turner, M.G. et al. (1997). Effects of fire size and pattern on early succession in Yellowstone National Park, Ecological Monographs 67(4) pp. 411-433 Retrieved from: https://doi.org/10.1890/0012-9615(1997)067[0411:EOFSAP]2.0.CO;2
==Conclusion==
</ref>This phenomenon is the reason that the bigger the fire is, the more tree seedlings sprout, and the less vascular species get the change to grow, causing a decrease in the general species variety in the regrown part of the forest.
In the conclusion, a decision will be made on which current method of reforestation is most effective. This decision will be based on several factors which the research group considers important. The factors are:
 
* Effectivity with respect to time
* Costs
* Labour intensity 
* Ability to restore biodiversity
* Effectivity with respect to resource
* Level of control
* Imposed health risk
 
The three methods will each get one of the four rankings per factor (--, -, +, ++). The ranking they get will be based on the literature review that is done per reforestation method. The reforestation method with the best overall score is considered to be the best reforestation method in the case of a forest fire in a National park. However, as the primary factors determining the effectiveness of reforestation are the degree of biodiversity and level of control, these criteria are assigned a weight of 2, whereas all other criteria have a regular weight of 1. 
 
Natural reforestation and Aerial reforestation both receive one + with respect to biodiversity, this is because they have the ability to regrow several different species but the more dominant species will take over and the natural scenery will not recover. Manual reforestation receives two + because, with manual reforestation, complete recovery of the natural scenery is possible.
Manual reforestation receives one + with respect to the time effectivity, this is because of a forest recovers faster when seeds are planted. Aerial and natural reforestation get a - and a -- respectively because aerial reforestation is only beneficial with respect to time in a non-fertile area and natural reforestation is very slow.
Natural reforestation receives two + with respect to effectivity with resources because natural reforestation uses no additional resources and thus also has no waste. Manual reforestation receives one + because different seeds can be planted in the correct environment and therefore only the minimal number of seeds are used. Aerial reforestation receives two - because the seed-tree ratio is extremely high.
Because of this high ratio and much fuel costs aerial reforestation receives two - with respect to costs. Manual reforestation also gets one - for costs because manual labor is always very expensive. Natural reforestation is free and therefore gets two +.
Because manual labor is so intensive, manual reforestation receives two - when looking at the factor of labor intensity. With natural reforestation no labor is needed, thus this method receives two +. Aerial reforestation involves labor, this is however less intensive than with manual reforestation, the area that can be covered in a small amount of time is very big and this method therefore receives one +.
Nobody manages the reforestation when it is done with natural reforestation, this method scores two - when looking at the level of control that can be fulfilled. Because with manual reforestation the location of each species can be determined you can exert a good level of control and this method thus receives two +. Aerial reforestation receives one + because you can determine the area at which you want to plant trees but you cannot control it per centimeter.
Manual reforestation receives two - with respect to imposed health risk since it involves sending people in polluted and hence mildly toxic areas. Natural reforestation is ranked with two + on this aspect since the only involved health risk is minor injuries which might occur during the upkeep of the forest in the years to come, but this is something all methods of reforestation have. Aerial seeding receives one + for imposed health risk. This is because pilots will be exposed to low level of kerosine vapor when refueling their planes, which is a toxic substance. However, these vapor concentrations are low as they are quickly diffused into the surrounding area. Furthermore, flying at an alleviation exposes the pilots to higher levels of radiation, however seeding heights are rarely above the cloud level and are still in the lower level of the atmosphere where most of the radiation has already been filtered out, thus not posing a significant extra dose when compared to groundwork.  


{| class="wikitable" style="text-align: left; color: black; border:1px solid black; border-collapse: collapse;" border="1"
|+ '''Table 3: Evaluation of the reforestation methods'''
!
! Natural reforestation
! Manual reforestation
! Aerial reforestation
|-
| Ability to restore biodiversity (x2)
| +
| ++
| +
|-
| Effectivity with respect to time
| --
| +
| -
|-
| Effectivity with respect to resources
| ++
| +
| --
|-
| Costs
| ++
| -
| --
|-
| Labour intensity
| ++
| --
| +
|-
| Level of control (x2)
| --
| ++
| +
|-
| Imposed health risk
| ++
| --
| +
|-
| Result
| +4
| +5
| +1
|}




== Need for control ==
The result shows that natural reforestation and manual reforestation have a tight score of +4 and +5 points respectively. If the grating scale had a higher resolution (more than 4 options and/or less discrete weights) better distinction between the two could have been made. For now, they can thus be considered about equally good for reforestation after a forest fire with respect to the factors the group considers important, with the preference for manual reforestation arising from the evaluation matrix from table 3. Aerial reforestation, despite being commonly a used solution to a reforestation problem in the scope of a national park, is actually least favored. This is because in both important factors (biodiversity and level of control) it did not emerge victoriously.  
Seeds of different species have different optimal depths for sowing, with some growing best if they are buried a few inches deep in the soil, while others, including many grasses and herbs, need exposure to light to germinate and so need to be on the surface<ref>Goosem, S., & Tucker, N. (2013). Repairing the Rainforest . Cairns: Wet Tropics Management Authority and Biotropica Australia Pty</ref>.. This means that if the seeds are all buried at the same depth or not buried at all, one species will be at the perfect level to grow and will dominate the other species. In order to have the biodiversity that is preferred for the area, the seeds of different species need to be planted at different levels to create a good growing environment for every species. This can only be done with a level of control that cannot be obtained with aerial seeding. A rule of thumb when growing vegetables and grains is to sow the seed at a depth of one to two times the width of the seed.  


As discussed previously, a National Parks goal is to conserve the scenery of the area, meaning that if a fire occurs, the National Parks aims to get the park back to its original state. This can not be done by means of natural reforestation, as this does not give all species which used to live in the burned down area a big enough chance to ensure the conservation of the scenery. This means that the method of natural reforestation has insufficient means of control to be a useful solution to the problem at hand.
This conclusion is interesting when a prototype of a robot which combats deforestation as a result of forest fires in National parks, is designed. This robot must follow the main design of manual reforestation since this method is considered most efficient. However, as can be seen in the table, there are also some improvement points for this method. Manual reforestation is very labor intensive and it is expensive as well. When a prototype for a reforestation robot is made, the robot must improve the way manual reforestation is done now with respect to labor intensiveness and costs, but the robot must be able to restore biodiversity and have a level of control at least to the same extent as manual reforestation methods have nowadays. A robot would, therefore, be a desirable way to improve manual reforestation since a robot is a good alternative to decrease the labor intensiveness of a job.


== Bibliography ==
== Bibliography ==
<references />
<references />

Latest revision as of 09:47, 21 June 2018

General

In this section a more in-depth literature review on the specific subject of reforestation after forest fires is done to assess whether or not a robot is a desirable artifact to be created for such a purpose. Several aspects are investigated including but not limited to biodiversity, need for controlled seeding, the effectiveness of current methods and the costs of current methods. The general literature review concerning itself with the possibilities of robotics technology and the contemporary issues involving reforestation can be found in General Literature Review. General information about the project can be found at PRE2017 4 Groep6.

Biodiversity & Need for Control in National Parks

National Parks are located in most countries spread all over the world. In the United States only, the 59 national parks which were acknowledged at the end of 2017 span well over 400.000 km2 (Sawe, B.E. 2017) [1]. Even though National Parks are most commonly known as a touristic attraction, the reason they have originally been established is ‘’to conserve the scenery, natural and cultural resources, and other values of the park in a way that will leave them unimpaired for the enjoyment of future generations.’’ (The NPS Organic Act, 1916) [2]. Meaning that, only taking the natural factors into account, the National Parks have to keep the wildlife as intact as possible. Thereby protecting it not only from human influences, but also from certain natural ones. The latter include, but are not limited to, typhoons, droughts, floods, earthquakes, and fires. Even though these phenomena are considered events which happen every once in a while and which are part of nature, their effect on a National Park is usually catastrophic. Usually, after one of these events, a substantial portion of the park is completely devastated, meaning that the wildlife needs to recover.

This paper will be limited to the phenomena of a forest fire originated from natural causes and the recovery of the National Park afterward. In order to recover a National Parks ‘natural scenery’, a clear definition of this must first be established. The natural scenery can be divided into three categories, being animals, plants, and non-living elements. This latter category spans the general topography of the Park, e.g. rivers, lakes, and mountains. Together these three categories form what is commonly known as an ecosystem, which is a term describing the relation between organisms and the physical environment they live in. Therefore, the goal of conserving the natural scenery can also be reformulated into conserving the current ecosystem. As the non-living elements are usually not as heavily influenced as the other two categories, they are from now on ignored in this analysis of the recovery of the National Park. Another concept linked to the conservation of nature is biodiversity. Biodiversity is the variability among living organisms from all sources, within and between species. The degree of biodiversity shapes the ecosystem, if there are many different species living in the park, they interact in another way with each other and their physical environment when compared with a situation wherein there are only a handful of species living in a similar environment. This shows that the biodiversity is of vital importance for the ecosystem and that a change in the park's biodiversity will result in a transformation of its ecosystem. (Greenfacts, 2018) [3] The substantial dependence of natural scenery on the ecosystem and of the ecosystem on the biodiversity states that for a National Park to conserve its natural scenery, at least the biodiversity cannot change by any significant factor. It is, however, not claimed that this is enough for a park to conserve the natural scenery. It could be argued that other factors, like noise or horizon pollution, mean that the natural scenery is not conserved, but this is not discussed in this paper. Only the contribution of the biodiversity is taken into account.

Coming back to the recovery after a natural catastrophe, to say that a forest fire has a significant effect on the park's biodiversity is an understatement. Depending on the fire’s size, temperature, and the propagation speed it will wreak havoc over a large area of the national park which has disastrous consequences for all wildlife within the area. In order to get the park back to its original level of biodiversity, a certain degree of control is needed, as the original ratios of species should be established once again. Regaining this biodiversity is divided in both the animal and plant life, and can be done in multiple ways. This paper will be limited to the methods of regrowing the plant life, to be referred to as reforestation from now on. In order for reforestation to be effective, it needs to give all species a fair chance to return to their original population. This also requires to take into account the species-specific growth preferences, meaning factors such as temperature, nutrition, degrees of freedom, and exposure to sunlight. All these factors are heavily dependent on the plant's location, and thus on the location where the initial seed starts to sprout after the fire. Hence, the location of the seeds is of vital importance for reforestation. The three currently most used methods for reforestation are aerial, manual and natural reforestation, which will be further elaborated on below.

Current methods of reforestation

Natural Reforestation

One method of reforestation is natural reforestation. Natural reforestation relies on nature to return an area to forestland after the area is deforested. This returning of vegetation can happen through seeds that are carried by the wind, transported or buried by animals or that are dropped by mature trees (North Carolina Forestry Association, 2017) [4]. This already leads to the first constraint of natural reforestation; there must be adequate living trees and animals around to enable natural reforestation, i.e. a nearby ecosystem has to be present. If there are no trees in the entire environment, there is only a negligible probability that seeds will be transported to the area. However, this study is concerned about returning a forest after a wildfire in a National park, in most cases, the fire is eliminated after a while due to human interference and this results in sufficient living trees left in the neighborhood to provide seeds.

In contrast to artificial deforestation, natural reforestation happens without the help of humans or machines. Because natural reforestation happens without the interference of humans, there is absolutely no control over natural reforestation. This means that the most dominant species, or the species that have not been destroyed by the fire, will regrow on the devastated area whilst other species which were originally located in this area are either all destroyed by the fire or take much longer to regrow, endangering them to vanishing from the National park since they cannot spread their seeds anymore. As is stated in the introduction from this wiki page, in order to rehabilitate the Natural park the biodiversity must stay in its original state to the best possible extent. This may happen with natural reforestation when the National park only consisted of one or a few species. However, when the National park consisted of many different species, which is often the case to create a nice scenery, natural reforestation provides no control and thus no guarantee that the old ratio of species will regrow. It can be said that the natural reforestation will always be good since nature can do what she wants, this is however not the case for a National park. Some species will always be dominant over other species, e.g. weeds in your own backyard. These dominant species are however not the (only) species you want to have in a National park. In order to recreate the original ratios of species in the National park, some level of control is needed which cannot be created with natural reforestation.

The benefit of natural reforestation is that the costs are extremely low. Because natural reforestation happens without the interference of humans, technology or other materials, there is also no money invested in the reforestation.

In the introduction, it is also stated that in order to achieve the preferred ratios of species, the different seeds need to be planted at different depths in the soil. It is obvious that this situation cannot be achieved by means of natural reforestation. Because most natural reforestation happens with seeds that are dropped by mature trees, these seeds will all end up on the ground and none will be buried, unless they are taken away by local wildlife. The level of control that is thus needed to recreate a National park cannot be obtained with natural reforestation.

Another source confirms this conclusion and states that only 7.9% of reforestation is done with natural reforestation (Lukaszewicz, 2002)[5]. This number is so low because where natural methods of natural regeneration fail or are unrealistic, artificial planting ensures the attainment of the main goal - sustainability of forest ecosystems.

Artificial reforestation has certain important benefits as to why it is often preferred over natural reforestation. It provides better control over tree spacing, more control over the species present in the new forest, the opportunity to plant genetically improved seeds or seedlings, and a higher rate of tree survival (North Carolina Forestry Association, 2017) [4]. It can be summarized that when using artificial reforestation, the reforestation can be better managed than with natural reforestation, which is exactly what is necessary to recreate the National park as is stated in the introduction (United States Department of Agriculture, 2014) [6]. Two main methods of artificial reforestation are manual reforestation and aerial reforestation. These two methods will now be discussed.


Manual reforestation

Manual reforestation is the oldest and most well-known method of replanting areas devastated by wildfires and the overall replanting of trees in general. However, manual reforestation is also a very slow method. It requires a large workforce and costs can quickly rise beyond proportions. It is estimated that manual reforestation can cost up to 62.000 USD for the initial replanting of a mere 1 by 1 km lot (Engel and Parrotta, 2001) [7]. Maintenance is also required in the following years to make sure the forest grows as it is expected to be. This can cost up to 120.000 USD in the first 2 years only (Engel and Parrotta, 2001)[7]. This is a significant investment for national parks or other involved parties and other methods might be more cost efficient. More than 182.000 USD has to be spent on workforce and material. This is by far the most expensive and most labor intensive method and therefore not the most wanted solution. Manual reforestation is, however, preferable over methods for small areas.

This method of reforestation also poses significant health risks to the workforce (Finlay et al., 2012) [8]. Since our focus lies on wildfires in national parks, this implies high pollution rates for soil, air, and water in the target areas. Ashes and toxic residues are often found in and near the devastated areas. There is an increase in mortality rate and symptomes related to the lungs occur at a significantly higher rate (Finlay et al., 2012) [8] . This is perhaps the most important factor when deciding which method is to be used to replant an area after a wildfire.

Manual reforestation also has some benefits over the other primary replanting methods. This method is more precise compared to the other methods. Seeds can be planted at a more optimal depth and invasive or other unwanted species can be easily removed by the workforce on-site (Waldrop, 1998) [9]. This means that the destroyed forest can be changed to a more healthy ecosystem by having the workforce suppress and promote certain species. The germination rate is therefore also relatively high compared to the other primary replanting methods (Waldrop, 1998) [9]. However, this rate also depends on other factors like seed quality and differs per species. Seeds are also less prone to animals that use them as food as they are planted directly into the ground and are not easily accessible by animals. Seedlings and young trees can also be planted instead of seeds themselves to kickstart the growth of the forest. This method bypasses the inherent danger of the seed just lying on the ground. However, seedlings and saplings are more expensive and are fairly more difficult to move on-site when compared with the relatively small seeds. Recent advancements in seed quality have also made seed-only methods more beneficial, both in terms of costs and survival rate.

Manual reforestation is also already augmented by utilizing machines, by preparing the ground beforehand by means of subsoiling, machines can increase the growth rate and survival rate of the seeds (Waldrop, 1998) [9]. Subsoiling also provides the option for placing the seeds even deeper into the ground, which can be beneficial for certain species. However, planting is still a labor-intensive practice and these large machines for subsoiling, often used in agriculture, cannot be used in all areas in national parks. Without machines, manual reforestation can still reach hill and mountain like areas that are normally not easily accessible for large machines.

A small robot could significantly decrease overall costs while still being able to a reach difficult areas. There are also no extra health risks for the human workforce since they can perform their function off-site. If high precision can be achieved without a significant loss of speed and a high germination rate, then small robots might provide an elegant, cheap and fast solution to the given problem.


Aerial reforestation

Aerial seeding is perhaps the most novel method of reforestation among the other options, which have generally existed for many centuries. Its main premise is a reduction in labor, as seeds can be sown at a much higher rate than manual seeding could ever produce and time-effectiveness, as an airplane can easily cover an area of several hectares at a much quicker rate than manual seeding using volunteers. However, the question remains if this method is truly beneficial in terms of actual saplings it produces and the costs it inherently carries, considering an aircraft of several metric tons needs to be lifted in the air burning kerosine and enormous amounts of seeds are spread.
Contrary to intuitive belief aerial seeding is in most cases not a standalone method, in order to be effective more often than not some preliminary groundwork is required to prepare the area to be seeded (in terms of boosting the receptiveness of the ground to the dropped seeds) (Régnière, 1982) [10]. A very crude probabilistic model, taking into account two classes of possible areas (highly receptive due to site preparation or natural levels) and a constant occupation of highly receptive area per unit of area exists (Régnière, 1982) [10], which reveals that higher seeding rates do in general lead to more saplings, however the relationship is only linear in the case of pure natural occupation. If the site is prepared and occupation rates become higher, the relation between the number of saplings per unit area approaches a more or less square root relationship. This model further reveals that the variance of the pattern in saplings per unit area severely depends on the width between airplane runs, with lowest variance (corresponding to a uniformly seeded area, or in the case of multiple sown species a random pattern) only occurring at 1 meter distances. However, for each and every combination of width between airplane runs and seeding density a minimum invariance exists to create an optimal balance between the two. Using a purely random seed distribution (obtained by a uniform seeding density created by narrow spacing between airplane runs) as a measure for maximum obtainable sapling rate, it was found that the efficiency of a real process with a limited spacing decreases rapidly as the spacing becomes larger, although there is a compensating effect for higher seeding rates, albeit the amplitude of this compensation is much smaller than the amplitude of the decrease in efficiency at wider spacing. Very counterintuitive, for equal spacing between the airplane runs, the efficiency of the process first drops to a minimum of 88% after which it slowly increases for larger seeding drop densities, meaning that higher seeding rates do not necessarily mean better unless ridiculous amounts of seeds are used.

Furthermore, a second study in China comparing growth rates and carbon stock levels (equivalent for biomass) of both aerial seeded forests and naturally regenerated forests showed differences effectiveness between the two methods, with aerially seeded forests needing longer time to completely develop and hence always following behind naturally regenerated forests (Xiao et al., 2015) [11]. This study was performed using the same tree species in previously highly naturally degraded areas. In the early years (10-20 years) of the new forests, the naturally regenerated forests seem to do better in terms of overall carbon stock, with an eventual conversion of the carbon stock values for the aerially seeded forest to carbon stock values of the naturally regrown forests for elder forests (50+ years). This faster growth rate of naturally regenerated forests is most likely caused by the self-capacity of natural forests to sustain themselves, which can only kick in at a later stage for an artificially planted forest by means of aerial seeding and by a higher occupation of carbon stock of the forest floor litter layer in the natural forests which provide many nutrients for trees. Thus overall, aerially seeded forests will eventually over time reach a state where the difference in their carbon stock is no longer statistically significant, however this approach in carbon stock is always from below and will never exactly converge.

Hence taking into account the above findings (Régnière, 1982) [10] (Xiao et al., 2015) [11] it is safe to conclude that aerial seeding can possibly be an effective method for reforestation, however it is largely context dependent on the problem. Considering one needs a pre-prepared area to increase the survivability of seeds and the airplane needs to make very tightly spaced runs to create a somewhat universally spread forest, the process becomes very time intensive in execution, whereas the main appeal of aerial seeding would be the simplicity in saving time as compared to manual seeding. In a situation where natural deposits of seeds are scarce, aerial seeding would be an optimal solution, since it simply would take nature too long to naturally recover the forest, even though the growth rate of natural forests are higher. However, considering we are dealing with forests fires in national parks this last situation is not very likely, as most often due to human intervention national parks do not completely vanish due to wildfires, rendering plenty of natural seed deposit left. In terms of control, aerial seeding gives little opportunities unless one chooses to specifically seed only one species of plant, as for any other desired degree of control a mixture of seeds has to be spread, for which different survival rates exist. Albeit, if one desires empty patches of land in the forest this could be done by severely increasing the spacing of airplane runs.
In terms of biodiversity, a similar conclusion can be drawn. Some level of biodiversity can be reached by mixing seeds from different trees and plants together for the airplane to drop, however this increases the difficulty of pre-preparing the site for different types of trees/plants to coexist together as each seed will have a different optimal depth and nutritional needs, whereas such a situation would also eventually be reached by nature. In terms of costs however, a study of the Society of American Foresters conducted in 1948 revealed that the costs of aerial seeding would be $7.25 per hectare (Westveld, 1949) [12], so taking into account inflation this would yield a cost of $75.39 (Bureau of Labor Statistics, 2018) [13]. More recent sources report a cost of $60 per hectare for aerial seeding process only, with a whopping $630 per hectare if site preparations are taken into account as well (Canadian Silviculture, 2005) [14]. The discrepancy between the values obtained by means of inflation and the more recent value is most likely caused by technological improvements making the process cheaper and more efficient, thus countering the inflation.
A popular novel alternative method for aerial seeding is by the employment of drones. Although this technique has a lot of potential, in its current state it is rather limited; communication technologies only allow a maximum distance of a couple of hundred meters for commands to be received by the drone (Fortes, 2017) [15], and even if the drones are build to be autonomous to overcome this distance limitation due to communication problems, they are still severely limited by their battery capacity which usually allows operation for somewhere between 30-90 minutes (Anderson, 2016) [16] (Bustamante, 2015) [17].

All in all it can thus be concluded that in the case of a forest fire a robot can certainly be a proper solution as it gives a decent level of control if the resolution of the actuators is big enough and will certainly be an incentive for National Parks to switch to this novel technology if it can operate at a cost of less than $630 per hectare.

Conclusion

In the conclusion, a decision will be made on which current method of reforestation is most effective. This decision will be based on several factors which the research group considers important. The factors are:

  • Effectivity with respect to time
  • Costs
  • Labour intensity
  • Ability to restore biodiversity
  • Effectivity with respect to resource
  • Level of control
  • Imposed health risk

The three methods will each get one of the four rankings per factor (--, -, +, ++). The ranking they get will be based on the literature review that is done per reforestation method. The reforestation method with the best overall score is considered to be the best reforestation method in the case of a forest fire in a National park. However, as the primary factors determining the effectiveness of reforestation are the degree of biodiversity and level of control, these criteria are assigned a weight of 2, whereas all other criteria have a regular weight of 1.

Natural reforestation and Aerial reforestation both receive one + with respect to biodiversity, this is because they have the ability to regrow several different species but the more dominant species will take over and the natural scenery will not recover. Manual reforestation receives two + because, with manual reforestation, complete recovery of the natural scenery is possible. Manual reforestation receives one + with respect to the time effectivity, this is because of a forest recovers faster when seeds are planted. Aerial and natural reforestation get a - and a -- respectively because aerial reforestation is only beneficial with respect to time in a non-fertile area and natural reforestation is very slow. Natural reforestation receives two + with respect to effectivity with resources because natural reforestation uses no additional resources and thus also has no waste. Manual reforestation receives one + because different seeds can be planted in the correct environment and therefore only the minimal number of seeds are used. Aerial reforestation receives two - because the seed-tree ratio is extremely high. Because of this high ratio and much fuel costs aerial reforestation receives two - with respect to costs. Manual reforestation also gets one - for costs because manual labor is always very expensive. Natural reforestation is free and therefore gets two +. Because manual labor is so intensive, manual reforestation receives two - when looking at the factor of labor intensity. With natural reforestation no labor is needed, thus this method receives two +. Aerial reforestation involves labor, this is however less intensive than with manual reforestation, the area that can be covered in a small amount of time is very big and this method therefore receives one +. Nobody manages the reforestation when it is done with natural reforestation, this method scores two - when looking at the level of control that can be fulfilled. Because with manual reforestation the location of each species can be determined you can exert a good level of control and this method thus receives two +. Aerial reforestation receives one + because you can determine the area at which you want to plant trees but you cannot control it per centimeter. Manual reforestation receives two - with respect to imposed health risk since it involves sending people in polluted and hence mildly toxic areas. Natural reforestation is ranked with two + on this aspect since the only involved health risk is minor injuries which might occur during the upkeep of the forest in the years to come, but this is something all methods of reforestation have. Aerial seeding receives one + for imposed health risk. This is because pilots will be exposed to low level of kerosine vapor when refueling their planes, which is a toxic substance. However, these vapor concentrations are low as they are quickly diffused into the surrounding area. Furthermore, flying at an alleviation exposes the pilots to higher levels of radiation, however seeding heights are rarely above the cloud level and are still in the lower level of the atmosphere where most of the radiation has already been filtered out, thus not posing a significant extra dose when compared to groundwork.

Table 3: Evaluation of the reforestation methods
Natural reforestation Manual reforestation Aerial reforestation
Ability to restore biodiversity (x2) + ++ +
Effectivity with respect to time -- + -
Effectivity with respect to resources ++ + --
Costs ++ - --
Labour intensity ++ -- +
Level of control (x2) -- ++ +
Imposed health risk ++ -- +
Result +4 +5 +1


The result shows that natural reforestation and manual reforestation have a tight score of +4 and +5 points respectively. If the grating scale had a higher resolution (more than 4 options and/or less discrete weights) better distinction between the two could have been made. For now, they can thus be considered about equally good for reforestation after a forest fire with respect to the factors the group considers important, with the preference for manual reforestation arising from the evaluation matrix from table 3. Aerial reforestation, despite being commonly a used solution to a reforestation problem in the scope of a national park, is actually least favored. This is because in both important factors (biodiversity and level of control) it did not emerge victoriously.

This conclusion is interesting when a prototype of a robot which combats deforestation as a result of forest fires in National parks, is designed. This robot must follow the main design of manual reforestation since this method is considered most efficient. However, as can be seen in the table, there are also some improvement points for this method. Manual reforestation is very labor intensive and it is expensive as well. When a prototype for a reforestation robot is made, the robot must improve the way manual reforestation is done now with respect to labor intensiveness and costs, but the robot must be able to restore biodiversity and have a level of control at least to the same extent as manual reforestation methods have nowadays. A robot would, therefore, be a desirable way to improve manual reforestation since a robot is a good alternative to decrease the labor intensiveness of a job.

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