From the results of the general literature review and the extended literature review, found respectively at General Literature Review and Extended Literature Review, it was revealed that forest fires can be either advantageous or detrimental for a forest, however the effects mainly depend on burn severity and fire intensity. The most basic consequence of a forest fire is the incineration of organic matter, however, even such a simple consequence brings about changes in the chemical, physical and biological characteristics of the forest soil, which forms the basis of the forest ecosystem. Nonetheless, forest fires form an important component in the upkeep of forests by altering the composition of a forest. Secondary effects of a forest fire can be found in the animal kingdom since most animals require a specific ecosystem to optimally sustain themselves and thus a forest fire can severely impact the population dynamics of animals living in these areas. To combat the negative effects of the aftermath of a wildfire, several methods exist, including but not limited to: natural reforestation, manual reforestation, and aerial reforestation. However, each of these methods come with their own advantages and drawbacks.
To gain better insights in what strategies are used in real life reforestation plans and which pitfalls come along with them, some case studies are investigated. These case studies are considered with the goal of revealing crucial elements from reforestation strategies of the past which determine the success or failure of a method. Since the conclusion of the extended literature revealed that a robotic solution could be desirable if it can cut down on the labor intensity and costs of manual reforestation, which was deemed as the best currently available option, additionally, these case studies are also carried out with the hope of revealing what a robotic solution for reforestation should be able to do and what it definitively shouldn't do in order to achieve this.
The general information about the project can be found at PRE2017 4 Groep6.
Case study: the Mediterranean
The Mediterranean region has been plagued by forest fires for centuries. Countries like Spain, Portugal, and Italy experience a multitude of wildfires of various sizes and intensities every year. While the Mediterranean region was already prone to wildfires, climate changes across the region have lead to an increasing frequency and intensity of wildfires. An irregular weather pattern also contributes to devastating wildfires. Drought periods can last up to 6 months at a time, significantly disturbing the flora and fauna with increased risks of wildfires.
Dependant on the type of wildfire, a significant shift of the scenery can be found after a natural restoration of the forest. While a large interval between fires promotes tree plant communities, a shorter interval primarily promotes shrubs and bushes (Gonçalves and Sousa, 2017) . This can result in an imbalance of the biological hierarchy within the forest, meaning that other plants and animals will become a dominating factor. The Mediterranean region is especially vulnerable to this. A wildfire almost always causes the biological composition of the forest to change dramatically after natural restoration.
Fires in these regions do not only affect the landscape and composition of the forest but can also have devastating economic effects for the surrounding areas. While small fires often do not pose a significant threat, the few fires that destroy large areas filled with flora and fauna can affect a multitude of sectors, most importantly the tourism sector.
The Mediterranean region consists of countries that have relatively large tourism sectors. Countries like Spain, Portugal, and Italy facilitate a significant amount of national and international tourists. National parks and the general scenery of the area attract a gigantic amount of tourists throughout the year. Larger wildfires can affect this sector in three ways:
- Evacuation of surrounding areas. When a large fire occurs, the first and foremost priority is the safety of people. A significant area around the fire will be evacuated based on the intensity, wind direction, and size. This can severely disrupt tourism in the region for the duration of the fire and during the aftermath. Depending on the size, intensity and material loss after the fire, the region could have long-lasting negative effects in the tourism sector and on a social level.
- Transportation and health risks, a large fire generates a lot of dangerous particles in the air. Both dangerous for humans as for machines. Nano-particles in the air can pose a serious threat for airplanes. This often results in a lockdown of the surrounding airspace and various airports. Large fires can thus disrupt a significant amount of international tourism in an extremely large area. Other particles can also pose significant health risks for people in the area. Governments often advise citizens and tourists to stay indoors and keep the windows shut.
- Loss of biodiversity. Tourists are attracted to the region by the gorgeous scenery and wildlife, but large wildfires can severely change the composition of the forest. This can result in areas where the flora and fauna are severely damaged. If a national park or forest has sustained a significant amount of damage, the area could transform into a barren wasteland for a long time. It is in the interest of national parks to restore the original and healthy ecosystem since the original ecosystem attracted a lot of tourism.
Of the Southern countries, Portugal faces the most wildfires. However, in terms of area lost to wildfires, Spain has lost significantly more land compared to Portugal (JRC-IES, 2006)  . This is partly due to an aged legislation on fire prevention and suppression (El País, 2018) . This ineffective legislation has even caused fires to be abandoned or being suppressed at later times due to regulations preventing the fire department from working efficiently.
Wildfires do not discriminate between forests and national parks, however, national parks often have greater financial reserves and their income depends on the fitness of the forest the park encapsulates. Last year, wildfires ravaged forests and national parks around Rome, Naples and the national park around mount Vesuvius (New York Times, 2017) This resulted in a significantly reduced revenue, due to severe damage to the national parks and tourists staying away. Robotics could provide a relatively cheap solution to these national parks. While monitoring and suppressing wildfires is still done by fire departments, robots could help the forests and national parks with the restoration of the original ecosystem. Robots would enable national parks to restore the original biodiversity, which is not always an option with natural reforestation or other conventional methods. It is therefore in the interest of the national parks to look at solutions robotics can provide, which has a number of benefits over conventional reforestation methods.
Case study: South Korea
South Korea is an interesting topic for a case study since the country has a rich history of reforestation management techniques due to the revolution of socio-economical events in the 20th century. Furthermore, South Korea hosts a wide variety of national parks, ranging from mountainous parks, to marine and coastal parks and even a historical national park, totaling at 22 parks covering 6.7% of the nation’s area (Korea National Park Service, 2009) . Special care is given to the specific case of reforestation campaigns which have been launched due to the consequences of raging wildfires. This segment primarily adapts a review of the historical developments of South Korean forest management strategies and the development of a new post-fire restoration plan for sustainable forests conducted last year (Ryu et al., 2017) .
At the beginning of the 20th-century forests on the Korean peninsula were deprived under the Japanese colonial rule, lasting from 1910-1945. After the first Korean government was established, some policies were adopted to counter illegal logging and combat further deprivation of the Korean forests, however due to the Korean War during 1950-1953 hardly any of these policies were adhered to and the war was also a source of additional deforestation. At the end of the war, South Korea merely had 4.1 million hectares of forested land (Korea Forest Service, 2012)  as opposed to the 16.1 million hectares that were present at the end of the Japanese rule (National Archives of Korea, 2010) . Only after the 1960’s when socio-political conditions were finally calmed down some real measures were taken to protect and restore the forests and artificial planting methods were adapted. The responsibilities for these measures were put at the local governments to ensure a minimization of human impacts on the forests by locals. The economic revolution of the 1970’s severely reduced the nation’s wood consumption as fossil fuels were now primarily used as an energy source. Due to efforts of artificial reforestation, 83% of the total forest land in South Korea was indeed covered by trees again in 2000 (Korea Forest Service, 2012) .
South Korea not only has to deal with the problem of regular forest fires, but also with the problems of extreme climate conditions, such as the windy season which can cause landslides and soil erosion in barren patches of land which result from a forest fire, and a dry season which facilitates conditions for forest fires to occur.
In recent years, however, some larger forest fires have emerged, the most noteworthy being the Dongehaean forest fire in 2000 which burned an area of almost 24 thousand hectares (Krasny and Tidball, 2015) . These larger fires are primarily the results of poor forest management strategies during the earlier reforestation projects which made these replanted forests more susceptible to forest fires due to the high fuel load of the planted trees.
Armed with this experience from the past, new post-fire forest management plans take into account the economic, ecological and social values of the forest as well as the six fundamental functions of the forest; timber production, water conservation, disaster prevention, natural environment conservation, ecosystem conservation, and recreation. Additionally, in each reforestation plan the original land use laws are adhered to if the problem permits this degree of freedom. The goal of new reforestation plans is to create a sustainable long-term forest management trajectory, however if an area is deemed at risk of soil erosion and debris flow an urgent intervention is taken to prevent any secondary damage. These long-term plans allow for the maximization of the inherent economic, ecological, scenic and environmental values of the forest. Overall these plans include many elements involving the six fundamental functions, however we will limit our discussion only to the topics which are relevant to our problem, which constitutes of reforestation in national parks.
Forests which are targeted for water conservation are given a reforestation program which involves the planting of both deep-rooted and shallow-rooted hardwood to create a 'natural alloy' of roots for optimal water balancing to create a water reservoir, which enables natural waterways to maintain their shape. Forests which are targeted for natural environment conservation are treated with a plan for passive restoration which prioritizes the regeneration of remaining seeds and sprouts to assist national reforestation. Forests which are targeted for ecosystem conservation and recreational use are restored by a combination of passive reforestation, erosion control, and active reforestation, which includes the introduction of new species and careful planning of positioning, allowing the improvement of the six forest functions and gives room for local priorities. In South Korea, for example, pine mushrooms form a valuable resource for the local population, which gives rise to the local priority of restoring as much of the pine forest as possible (Ryu et al., 2017) .
Overall 2 different planting strategies exist: row planting and group planting. In row planting trees are planted in straight lines with some distance between them to create a more or less uniform and homogeneous distribution, resulting in a crown layer (the top layer of leaves) which is beneficial for natural pruning. In group planting, larger seedlings and saplings are planted with wider spacing than in row plating, with some preassigned number of additional trees which serve to control ground vegetation and can thus prevent dormant unwanted tree seedlings to start growing which originally survived the forest fire. Overall group planting is preferable over row planting for an ensemble of reasons; due to the forest fires and erosion nutrients tend to be harder to find for plants resulting in lower growth rates for trees, the turnover fraction of trees which can be harvested is relatively low for row planting, due to the homogeneity almost no promotion for the growth of other tree species exist for row planting, whereas group planting allows for the natural regeneration of extraneous tree species in between the planted groups which results in greater biodiversity.
To measure the success of restoration the recovery rate of the 6 major functions of the forest can be monitored. However, success rate measurements alone are not enough, a restored site needs continuous monitoring to evaluate the efficiency of the forest management process as well as collecting data about possible mistakes for future forest management programs. If it turns out an area is poorly restored, auxiliary restoration operations will be launched. An additional way to measure the success of the reforestation process is by counting the number of certain beetle species, as members of the Curculionidae family of beetles are predominantly present in burned forests where they attack weaker trees, whereas a healthy forest will house more beetles of the Collembola family which only feed on topsoil vegetation (Ahn et al., 2014) .
In general two species of trees are considered for planting: pine trees and oak trees. Pine trees have the advantage of producing high-quality timber and giving rise to the valuable pine mushrooms. However pine trees have big disadvantages: they are very susceptible to fire, therefore making reforestation with pure pine stands an undesirable result. Oak trees, on the other hand, are a natural candidate for reforestation: they can regrow from leftover intact floor level vegetation, making them an ideal candidate when tree mortality rates in the reforestation process are estimated to be high and oak trees have low heat yield making them a natural fire break. Furthermore, oak trees are a deep-rooted species which gives them soil stabilizing features. Nevertheless, an option of a forest purely consisting of species 'x' is not a desirable result either. In the new plan it is therefore recommended to make use of group planting with mixed groups of oak and pine trees. In this way, a fire barrier can be made with oak trees to protect the more valuable pine trees. With the inclusion of oak trees natural regeneration processes for other species is also promoted resulting in a more diverse forest. Albeit the exact mix of oak and pine trees will be a resulting factor of the natural fire regime of a certain area; fire regime parameters include fire frequency, fire intensity, and fuel consumption patterns.
In areas which have a high probability for the occurrence of landslides, swift restoration is of critical importance to prevent secondary damages to the forest. The probability for landslides to occur is directly related to the topological configuration of the area, making this a topic which might need assessment in a reforestation plan for a national park, depending on the national park of interest. Burned down forests can surprisingly be beneficial to undertake countermeasures for landslides. Burned or fallen trees can be used to build terraces on a hill, creating a cascade of barriers to stop landslides. After a forest fire, there will most likely be an abundance of trees to use for this countermeasure, therefore eliminating the need for additional resources which allows for immediate emergency restoration activities to commence as soon as an area has been cleared by the fire department. Another contingency reforestation method is seed spraying: in this method small seeds are sprayed onto a slope from which fast-growing grasses can emerge to provide slope stability.
From this case study it can be concluded that post-fire management for a reforestation process includes a wide range of stakeholders and evaluation parameters. Because the way in which restoration is executed and the environmental context have a substantial influence on the success rate of the reforestation, a thorough understanding of these parameters is needed before a reforestation plan can be constructed. To assure that reforestation programs do not interfere with the local land usage laws and do not impair the economical position of the local population any reforestation project has to be carried out in synergy between the local and central governing bodies. Special attention should be given to fire prevention in the new forest after the reforestation process is done by means of timely pruning of the crown layer and incorporation of oak trees to prevent unnecessary damage from future wildfires.
Conclusions from case studies
The above case studies revealed several elements which are critical successfulness of a reforestation operation which remained uncovered in the extended literature review. In summary these can be characterized as;
- Strong interplay between involved parties. During a reforestation operation it is important that all involved parties are cooperating and their interests are well represented. From the Mediterranean case it became clear that the central government played an inhibiting role due to dated legislation, whereas the South Korea case revealed that the local community of a forest required pine mushrooms to thrive, thus influencing the decision of which trees need to be planted. All in all, this will lead to a different reforestation plan each particular case, since the context for each particular case will differ.
- Not only an adequate reforestation plan is required, but this progress also needs to be adequately monitored. The case studies revealed that some initially good reforestation plans can turn out to be not as good as expected or carried out poorly. In such cases usually auxiliary methods need to be employed to restore or correct these plans, which introduce unnecessary costs and time loss.
- The planting method used significantly affects the outcome. The row planting method will result in a forest with higher quality timber wood, however this will also produce a forest more susceptible to disasters such as forest fires and diseases because of the homogeneous conditions of the resulting forest. Group planting methods, on the other hand, result in a mixed species forest which promotes biodiversity and can stimulate the growth of other species in the patches in between these groups.
- Certain tree species can increase fire resistance. Oak trees can form a natural fire break because of their low heat yield, which means that the planting of a cluster of oak trees near the edges of a forest area could either help to quarantine a fire originating from this area or could help to protect this area from external fire.
In terms of a potential robotic solution this means that a robot ought to: respect the interest of all involved parties, e.g. if a certain plant species is deemed of paramount importance for the local population the activities of the robot should stimulate the existence and growth of this species and definitively not be maleficent to this species; the robot should be able to handle the planting of multiple plant species both for the sake of biodiversity, as well as incorporating group planting and for the sake of creating a more fire resistant forest; have an incorporated mechanism to measure the progress and success of the reforestation operation.
If a robotic solution can be made which satisfies these additional conditions to the ones already present from the extended literature review (have lower costs and labor intensity w.r.t. manual reforestation) then this solution would be beneficial and desirable for national parks to have. Hence the case for designing a robotic solution for reforestation is made solid.
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