Overstory #27 - Foster Ecosystems
One of the biggest challenges for tropical agroforesters is the restoration and revegetation of lands that have been degraded or cleared. Past issues of The Overstory have addressed pioneering degraded lands (#22), succession plantings to make use of the stages of forest plantings (#10), and ways to improve habitat and attract wildlife to agroforestry plantings (#21). In this issue, guest author Imants Pone of the University of Minnesota introduces the concept of "foster ecosystems"--using tree plantations to facilitate the natural regeneration of native species in their understories. Through findings from Puerto Rico, Amazonia, and the Philippines, Imants shows how plantations can be used to foster the reintroduction of native species.
Creating Foster Ecosystems to Accelerate Tropical Forest Regeneration
Moist tropical forests are some of the most productive areas on Earth, but since most nutrients and biomass are stored above ground, cleared land quickly loses its productivity. What is left are open fields dominated by shrubs and grasses in which the forest is slow to regenerate. The methods described in this paper are designed to accelerate natural regeneration in cleared tropical forests through the use of plantations as "foster ecosystems."
Factors affecting regeneration
The creation of foster ecosystems focuses on improving conditions for natural succession and overcoming barriers to regeneration. When land is degraded for a period of time, natural processes are disturbed and barriers form which block the natural pathways of forest succession. Barriers to be taken into account are low availability of native seeds and other propagules on-site, seed and seedling predation, seasonal drought, root competition, and poor soil conditions. These barriers need to be ameliorated before a restoration project can be attempted.
Minimizing barriers to seed survival, germination, and growth
A major barrier to natural regeneration of forests is that conditions in open lands are unfavorable to seed survival, germination, and growth of tropical forest species. Abandoned clearings are quickly overgrown by invasive grasses and shrubs, which in turn provide excellent habitat for seed predating ants and rodents. A study by Nepstad (1991) in the Paragominas region of Amazonia found that predation of native seeds placed in abandoned clearings is very high, as is predation on germinants. This same study also found that root competition in such cleared areas is a serious deterrent to the development of seedlings. Young tree seedlings growing in open areas suffered dramatically stunted growth, mainly due to root competition from thick grasses, while he same kinds of seedlings planted in treefall gaps grew almost four times as quickly.
Plantation trees can be planted to alleviate the problems of seed predation and competition. Although tropical systems are complex and different situations may require different approaches, Albizia lebbeck, Pinus caribaea, and Swietenia macrophylla have worked well as "foster" trees in Puerto Rico. Established trees shade out invasive grasses and shrubs under their canopies. Ant and rodent habitat and thus predation is decreased. Because the ground vegetation has been reduced, root competition for emerging seedlings is also greatly reduced. This makes conditions for seed survival, germination and growth more favorable under the canopies of plantation trees.
Accelerating soil improvement and natural seed dispersal
Most of the nutrients and organic matter of a tropical rainforest are stored in above-ground biomass. When the trees are removed from a site which is then grazed or farmed, soil conditions can become degraded. Soil conditions can be improved by using plantations of selected tree species. Leguminous, nitrogen-fixing species grow well under difficult conditions and produce nutrient rich litter. In a plantation, soil nutrients and organic matter can be increased by nitrogen fixation and litter fall. In addition, trees can help by reducing the bulk density of soil with their roots, conserving moisture in their shade, and providing conditions favorable to a healthy population of soil microorganisms.
The last and probably most important factor in a successful regeneration is the availability and dispersal of seeds and other propagules of the native tree species. Many tropical trees can propagate by roots, by residual seeds, or by seeds disseminated from an adjacent forest. In the open conditions of a clearing, much of the residual propagative material is destroyed or is unable to germinate and survive. Rather than giving humans the work of disseminating native seeds, the presence of plantation trees can attract seed disseminating bird and mammal species to do the job of dispersing seeds naturally. Appropriate plantation species should provide perches and/or food for disseminating birds and bats. Fruit-eating birds and bats will be attracted to the tree to perch or eat its fruit. In the process, they disseminate seed from previously eaten fruit. Nepstad (1991) found 400 times more seeds under tree crowns than in open field conditions.
Pastures and abandoned croplands become open grass and shrub fields when pathways of tree invasion are blocked. Because they reduce the barriers to tree invasion, plantation trees emerging in the open fields improve the probability that other plant species will invade. A study by Parrotta (1993) in Puerto Rico on an Albizia lebbeck plantation found that after 4.5 years, 42 different species of trees, shrubs, vines, and other plants were present on the site. By comparison, a site left to natural processes and not planted with trees contained only one species of vine, fifteen species of grasses and forbs, and no trees or shrubs.
When creating a foster plantation, steps may be taken to improve the speed and efficiency of native forest regeneration. The pattern in which trees are planted is important. The ideal method would be to plant trees evenly throughout the field, allowing disseminators to penetrate deep into the field from the forest edge. If funds are limited, forested strips (corridors) or islands may be effective and more economically feasible. Planted forest strips should extend from the edges of natural forest stands at intervals easily transversed by disseminators (20-30m). Plantation islands are trees planted together in groups. Islands should also be planted so they are 20-30m from forest edges and other islands. For both the strip and island methods, it is expected that as these systems develop, they will spread outwards and eventually overlap, creating a continuous forest.
Because of the complexity of tropical forest ecosystems, different sites require different restoration techniques. Restoration managers should have a good understanding of plant and animal life histories, seed ecology and dispersal characteristics, germination microhabitat requirements, and habitat requirements of potential seed dispersing birds and mammals. Managers then have the ability to adapt their techniques to changing conditions. All researchers agree that a greater diversity of plantation species improves chances for a successful regeneration. This also allows natural selection processes to decide on the best species composition on a site.
Contrary to management intensive restorations, restorations using foster ecosystems rely on natural regeneration processes. Refining "foster ecosystem" techniques help regenerate large areas of deforested land. It will be important that such efforts are documented so that others can learn and adapt their efforts for their specific sites. Without proper documentation, it will be difficult to develop this promising technique.
1) Lugo, A.E. and Peter G. Murphy, 1986. Nutrient dynamics of a Puerto Rican subtropical dry forest. Journal of Tropical Ecology, 2: 55-72.
2) Lugo, A.E., 1988. The future of the forest. Environment, 30(7): 17-20, 41-44.
3) Nepstad, Daniel C., et. al., 1991. Recuperation of a degraded Amazonian landscape: Forest recovery and agricultural restoration. Ambio, 20(6): 248-255.
4) Parrotta, John A., 1993. Secondary forest regeneration on degraded tropical lands. In: H. Lieth and M. Lohmann, Restoration of Tropical Forest Ecosystems, Kluwer Academic Publishers, 69-73.
5) Parrotta, John A., 1992. The role of plantation forests in rehabilitating degraded tropical ecosystems. Agriculture, Ecosystems and Environment, 41: 115-133.
6) Rabor, Dioscoro S., 1981. The role of wildlife in forest regeneration in Southeast Asia. Biotrop Special Publication, 13:187-200.
7) Godt, M.C. and Malcolm Hadley, 1993. Ecosystem rehabilitation and forest regeneration in the humid tropics: Case studies and management insights. In: H. Lieth and M. Lohmann, Restoration of Tropical Forest Ecosystems, Kluwer Academic Publishers, 25-36.
Other references on this subject
- Alternatives to Deforestation: Steps Toward Sustainable Use of the Amazon Rain Forest by Anthony B. Anderson (Editor). 1992. Columbia Univ Pr.
- Restoration of Tropical Forest Ecosystems: Proceedings of the Symposium by Helmut Lieth, Martina Lohmann (Editor). 1993. Kluwer Academic Pub.
- The Once and Future Forest: A Guide to Forest Restoration Strategies by Leslie Jones Sauer, I. McHarg. 1998. Island Press.
- Trees on the Treeless Plains - Revegetation Guide to Central Victoria by David Holmgren. 1993. Order through Permaculture International Journal, PO Box 6039, South Lismore, NSW 2480, Australia, Tel: +61 66 220020, Fax: +61 66 220579.
Related Editions to The Overstory
- The Overstory #78--Reforestation of Degraded Lands
- The Overstory #70--Rhizosphere
- The Overstory #61--Effects of Trees on Soils
- The Overstory #42--Improved Fallow
- The Overstory #20--Five Fertility Principles
- The Overstory #22--Pioneering
- The Overstory #17--Microcatchment
- The Overstory #8--Mycorrhizae