Enrichment Planting for Forest Restoration
What is the difference between reforestation, enrichment planting, and forest restoration?
Reforestation is the process of planting trees where they do not exist. Enrichment planting is planting trees and other plants in areas with woody vegetation with the objective of increasing the species diversity of the site. Forest restoration is the process of planting trees and other vegetation in order to try to recreate the forest ecosystem and the interactions that maintain it. Forest restoration can also include the re-introduction of animals and other organisms and/or creating the conditions so that these organisms can re-colonize the site without the need for re-introduction. Enrichment planting is often a principal component of forest restoration so the two are not mutually exclusive.
Why bother with forest restoration?
Forest restoration is expensive and time consuming, so why bother planting trees instead of just letting the forest regenerate on its own? In tropical rainforests cleared land left abandoned is quickly covered with a dense cloak of vegetation with some trees growing as much as 5 m in the first year. In the reserve landscape we know that within 10 years, cleared land will support a forest with vine covered trees reaching 8+ m. We also know that in regenerating areas far from mature forest stands, that the species that occupy these sites are mostly pioneer species. In the reserve landscape we have observed that even 70 years after a clearing is abandoned the plant community remains dominated by pioneer species in state sometimes referred to as “arrested succession”. Approximately 600 ha of the reserve support rubber groves overgrown with pioneer forest in a state of arrested succession. What is wrong with a forest dominated by pioneer species and why don’t these forests develop into the mature forests of high conservation value?
While pioneer species play an important role in the ecosystem, when a forest supports only pioneer species it will be of limited conservation value. Pioneer species tend to grow quickly and produce abundant fruit crops, but the fruits that they produce are mostly of limited nutritional quality for highly frugivorous animals. The fruits tend to contain high sugar content but few other benefits for wildlife, so animals that depend mostly on a fruit diet suffer for lack of adequate nutrition. This means that when a forest is dominated by pioneer species it is unlikely to support the full complement of forest wildlife. Also, pioneer species are widespread and super-abundant, especially in hyper-disturbed biomes like the Atlantic Forest, which means that they are of lesser conservation concern than trees of the mature forest community and thus less desirable than mature forest trees from the perspective of reserve managers.
In order to understand why these pioneer forests do not develop into mature forests as they would if located adjacent to mature forest patches, it is necessary to understand several important aspects of tree recruitment ecology and the distribution of the mature forests in the reserve. Studies of tree recruitment ecology indicate that most seeds are dispersed short distances from the mother tree (mostly <100 m), especially in forests such as the Michelin Ecological Reserve where frugivores that disperse seeds further, such as the tapir (Tapirus terrestris) and howler monkeys (Alouatta guariba), were extirpated. Less than 2% of sunlight reaches the forest floor in tropical evergreen rainforests which means that trees in the understory tend to grow slowly and that canopy tree inter-generation times tend to be long. It may take decades before a tree grows sufficiently to produce fruit and this, coupled with the short dispersal distances, suggests that trees migrate very slowly across the landscape. Since most of the reserve rubber groves are not located near mature forest patches, it will take centuries for plants of high value for frugivorous wildlife and conservation objectives to colonize these areas. If the pioneer forests do not develop into mature forests, the reserve will not attain its full potential to sustain viable populations of species of high conservation value.
For the sake of illustration, let us consider the oiti tree (Licania salzmannii) – member of the mature forest community in our forests that produces a very large and nutritious fruit. The high physiological cost of producing its fruits means that the tree is unable to produce fruits every year and according to common folk knowledge the tree only produces crops at 7 year intervals (our studies indicate that this is not far off the mark). The fruits attract most large frugivorous animals, but because the seed is very large no animal is able to swallow it and dispersers only carry it short distances from the mother tree. With a large endosperm, the seeds germinate and grow quickly, but on reaching 1-2 m they have expended the energy stored in the seed and growth slows to an imperceptible pace. After years or decades of growth the tree reaches a size that enables it to reproduce, animals visit the tree, and disperse the seeds another 5-20 m, and the process begins anew. In the case of the oiti and many other mature forest trees, this means that it can take centuries for the species to migrate a distance of 1 km. This process is further delayed by hunters who reduce the populations of wildlife that disperse the seeds and hence the probability of a seed being dispersed; by the fact that many forest animals do not disperse seeds into the pioneer forests far from the forest edge because they do not frequent these food poor forests; and because the slow migration of seeds across the landscape means that even if animals disperse mature forest seeds into these areas, it will take a very long time for these seeds to reach most of the pioneer forests in the reserve because they are far from the forest edge. While the hunting problem has been largely eliminated in the reserve and seed dispersing wildlife populations are recovering, these animals have few incentives to frequent the distant pioneer forests (and hence are not dispersing the seeds into these areas) because they provide inadequate habitat. This means that if we allow the natural process of forest succession to take place without interfering, it will take centuries for the mature forest community to colonize the entire reserve and large areas will be dominated by pioneer forests for the foreseeable future. Following this logic, Michelin decided that in order to increase the carrying capacity of the reserve for populations of mature forest trees and highly frugivorous animals, that planting trees was necessary in order to accelerate the process of forest recovery.
Our restoration sites are open to scientific visits scheduled in advance according to the reserve protocols.