Extended Literature Review

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 methods, costs of current methods. General information about the project can be found at PRE2017 4 Groep6.

Current methods of reforestation

Natural reforestation vs Artificial 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. 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. 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. 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. 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.

Aerial seeding

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) . 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 , 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) . 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.

Hence taking into account the findings of both Régnière and Xiao et al. 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.

Biodiversity

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) 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. 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) 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.

Need for control

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.. 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.