Managing and Directing Natural Succession

Transcription

1 37 Managing and Directing Natural Succession Steve Whisenant Key Points to Retain Carefully designed silvicultural strategies can accelerate growth, influence the direction of succession, increase the goods and services provided, or enhance diversity. Directing natural processes toward land use goals requires an understanding of the processes driving succession. Tools for managing and directing natural succession should be used as an imitation of natural processes rather than as a substitute for them. 1. Background and Explanation of the Issue After regeneration begins on previously forested sites, carefully designed silvicultural strategies can accelerate growth, influence the direction of succession, increase the goods and services provided, or enhance diversity. 327 Selecting proper treatment options requires an understanding of the factors limiting successional change and increases in desired species. These treatments should be designed to assist natural processes rather than fight them. This is most likely to occur when forest restoration plans (1) consider and remove the underlying 327 Lamb and Gilmour, causes rather than the symptoms of degradation; (2) are based on an understanding of succession and threshold barriers that must be overcome through designed interventions; and (3) stimulate the desired successional behaviour with minimal interventions Consider Underlying Causes Halting Natural Succession Many forest restoration programmes fail because they do not address the underlying causes of degradation.a number of social, political, and economic factors are often the underlying cause of forest loss or degradation. It is equally important to identify the biophysical barriers to recovery through natural successional processes. For example, livestock may contribute to degradation in some situations but be an important part of the recovery plan in other circumstances. Forests limited by excessive fire and invasive grasses may benefit from cattle that reduce fuel loads until the tree canopy begins to close. In contrast, forests limited by livestock that consume high percentages of developing seedlings benefit more from livestock exclusion than from control of unsustainable harvest of wood and nonwood forest products Understand Natural Succession and Potential Threshold Barriers Having stimulated natural regeneration processes that establish forest species (see previous 257

2 258 S. Whisenant chapter), it is necessary to manage and direct succession processes toward the desired objectives. It is important to promote continued development of the vegetation to conserve soil, nutrient, and organic resources; restore fully functional hydrologic, nutrient cycling and energy flow processes; and create self-repairing landscapes that provide the goods and services necessary for biophysical and socioeconomic sustainability. Different stages of degradation require management actions that focus on different processes. Severely degraded sites require early repair of hydrologic, nutrient cycling, and energy capture and transfer processes. As the vegetation increases in biomass and stature, it reduces abiotic limitations of the site by improving soil and microenvironmental conditions. Directing natural processes toward land use goals requires an understanding of the processes driving succession. The rate and direction of succession is influenced by the availability of species, the availability of suitable sites, and by differential species performance. Previous land use has important and potentially long-lasting impacts on the rate and direction of natural succession. 328 Natural succession on abandoned farms and pastures is limited and directed by the available seed bank, sprouting ability of remaining stump and root systems, seed immigration, soil type and condition, and climatic conditions. 329 Natural recovery occurs most rapidly and completely following abandonment of pastures that were cleared by hand and received little weeding and light grazing. These areas benefit from diverse seed banks, nearness to seed sources, and sprouting from stumps and roots. Moderately grazed pastures are much less productive and diverse due to the loss of grazing intolerant species, diminished seed banks, and less organic matter in the upper soil horizons. Heavily grazed, mechanically cleared pastures are far more likely to remain dominated by grasses and forbs following abandonment, since they are completely dependent on seed immigration for successional development. Frequent burning prior to abandonment 328 Uhl et al, Kammesheidt, reduces the density of tree seed and sprouts. Large treeless areas are unattractive to most birds and bats that disperse small seeds. Monkeys and ground-dwelling mammals that disperse large-seed, late successional species are even more prone to avoid open areas. Thus, perching sites provided by isolated trees can accelerate succession Design Minimal Interventions to Achieve Goals Will the site recover within an acceptable time frame in the absence of active restoration efforts? If so, will it provide the desired combination of goods and services? Answers to these key questions may be found by examining two types of reference sites. Selecting reference sites that have not been damaged provides an approximation of the potential goods and services. Reference sites that have been similarly damaged and allowed to recover naturally for different periods of time provide important information on the presence or absence of barriers to recovery. This provides critically important information about the passive intervention option. Active management interventions may be required where invasive species, damaged ecosystem processes, or other limitations halt natural recovery. If the site is not seriously degraded and seed sources are adequate, the first few years of succession will be dominated by herbaceous vegetation and shrubs. This will typically be followed by early succession tree species and midsuccessional tree species will gradually become more dominant. In lowland humid forests, biomass peaks of early successional species occur at around 10 years. Mid-successional species may reach their peak biomass at 15 to 30 years, but remain dominant for many decades. These successional changes occur more slowly in less humid or very degraded environments. Improving the management of ecosystem consumption (timber or wood harvest) is usually effective on relatively intact sites. Sites dominated by grasses may require vegetation control of the existing vegetation. This may be done with fire, herbicides, or mechanical or biological control methods. It may be necessary to

3 add some species through seeding or transplanting. Denuded or depauperate sites that can neither stabilise nor achieve management objectives require enrichment plantings. 37. Managing and Directing Natural Succession Examples 2.1. Restoring Dry Tropical Forests to Anthropogenic Grasslands in Guanacaste National Park, Costa Rica Anthropogenic fire converted the dry tropical forest of Costa Rica to grasslands that continued to burn frequently. A programme begun in the 1980s effectively stopped fire and allowed the natural reforestation by trees. The initial forests, of species with wind-blown seed, rapidly covered the landscape. As these trees grew larger, seed-dispersing birds and mammals increasingly moved through the site and added new species to the developing forest. 330 This is an excellent example of removing barriers to natural succession and then allowing natural processes to operate over many decades to return an increasingly diverse forest to the landscape Plantation Trees as Nurse Plants to Increase Regeneration of Native Species Tree plantations can sometimes facilitate the return of native vegetation. In Puerto Rico, tree plantations improved soil and microenvironmental conditions enough to facilitate the natural immigration of native species. 331 The plantation also accelerated the return of native species by attracting animals that brought additional seed. Tree plantations in the moist and wet tropics do not remain monocultures because native trees invade the understorey and penetrate the canopy of the exotic species. Unless site damage is extreme, native forests eventually dominate. Where damage is more severe, the resulting forests are likely to 330 Janzen, Aide et al, Figure Previously mined site in Hungary that has undergone natural regeneration for about 30 years. (Photo Steve Whisenant.) become a combination of native and exotic species Spontaneous Regeneration of a Mine Site in Hungary Mining is a drastic alteration of site conditions and processes. Planting trees on these sites is expensive and risky, thus they are often abandoned to natural processes. A mine site in Hungary received no active replanting, but 30 years following the cessation of mining, it shows numerous signs of spontaneous regeneration of herbaceous and woody vegetation (Fig. 37.1). The abundance of natural vegetation in the surrounding landscape provides seed sources. This site will take many more decades for recovery, but natural processes are operating in the absence of new disturbances. 3. Outline of Tools Tools for managing and directing natural succession should be used as an imitation of natural processes rather than as a substitute for them. The tools described in the previous chapter focus on influencing natural regeneration. They remain appropriate throughout succession, but here is a list of tools for manipulating existing vegetation:

4 260 S. Whisenant Patience: Time can be used as a tool. Wait for signs and expression of successional trajectory. Understanding what drives and limits succession will make it easier to recognise the probable direction of successional change and the potential vegetation for that area. Knowledge of potential successional pathways: Understanding how forest vegetation recovers following disturbances is a critical aspect of directing natural successional processes. Know what prevents improvement and remove that limiting factor. Fencing: Where livestock delay, limit, or prevent successional development, fences that restrict livestock entry are one method for increasing seedling development. This may only be necessary until the seedlings grow out of reach of the livestock (or fences may also be more permanent for continued benefits). Direct removal of invasive species: Invasive species may be killed or removed with herbicides, mechanical treatments, or hand removal to release native species. These tools may be expensive or very labour intensive, so their practicality is often limited to small or high priority sites. Reducing invasive species with shade: Shadeintolerant invasive species are most effectively managed with tree species and management strategies that accelerate the occurrence of closed canopies. For example, establishing forests on fire-prone grasslands requires the prevention of fires until the forest canopy effectively excludes the grasses. Thinning to reduce density or alter species composition: Selective thinning may be used to provide products and income while increasing growth rates of the remaining trees. It may also be used to encourage regeneration and growth of certain desired species while reducing the abundance of more common species. Enrichment plantings: Sites with no regeneration of shade-requiring late successional species may necessitate enrichment plantings under the canopy of earlier successional species. Enrichment plantings add species to sites where they are unlikely to enter through natural processes. They are most useful where the desired species, or suite of species, are neither present nor found in adjacent forests. 4. Future Needs Priority areas for further development are: Policies that encourage the development of natural, diverse forests: Government policies can accelerate destruction of natural forests or they can be crafted to encourage the development of natural and managed forests that combine production and conservation functions and reduce pressures on natural forests of high conservation value. Improved understanding of successional processes and barriers to natural recovery: There are numerous gaps in our knowledge of succession and ways in which we might encourage and direct those processes. Many factors drive succession and similar impacts may have dramatically different results in different ecosystems. A more mechanistic understanding of the factors limiting or accelerating succession would greatly improve our predictive ability in new situations. Novel strategies for payment of landscape forest restoration: New ways to fund forest restoration are essential. Programmes to plant trees are more easily funded than those designed to encourage and manage natural regeneration. This is unfortunate because natural succession often occurs more rapidly and at less risk than artificially planted forests. References Aide, T.M., Zimmerman, J.K., Pascarella, J.B., Rivera, L., and Marcano-Vega, H Forest regeneration in a chronosequence of tropical abandoned pastures: implications for restoration ecology. Restoration Ecology 8(4): Janzen, D.H Tropical ecological and biocultural restoration. Science 239: Kammesheidt, L Perspectives on secondary forest management in tropical humid lowland America. Ambio 31:

5 37. Managing and Directing Natural Succession 261 Lamb, D., and Gilmour, D Rehabilitation and Restoration of Degraded Forests. IUCN, Gland, Switzerland, and Cambridge, UK, and WWF, Gland, Switzerland. Uhl, C., Buschbacher, R., and Serrao, E.A.S Abandoned pastures in Eastern Amazonia. I. Patterns of plant succession. Journal of Ecology 76: Additional reading Feyera, S., Beck, E., and Lüttge, U Exotic trees as nurse-trees for the regeneration of natural tropical forests. Trees 16: Parrotta, J.A Influence of overstory composition on understory colonization by native species in plantations on a degraded tropical site. Journal of Vegetation Science 6: Whisenant, S Repairing Damaged Wildlands: A Process-Oriented, Landscape-Scale Approach. Cambridge University Press, Cambridge, UK. Ashton, M.S Regeneration methods for dipterocarp forests of wet tropical Asia. Forestry Chronicle 79: