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.

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.