Although tropical secondary forest ecosystems are of enormous interest to science, with great potential for development, their ecology and biology are poorly understood. Available data are sketchy and often inadequate for planning and policy development. The taxonomy of many trees and shrubs remain weak, and many species are unnamed. Reference is made here to the knowledge of forests in general.
Evidence shows that the productivity of the tropical rain forest comes from good growing conditions, constituted by a combination of high temperatures, light and rainfall all year round, coupled with efficient nutrient recycling processes.
Ecological studies of tropical forests are concerned with how the biotic component (plant and animal species, or the living component) interacts with abiotic factors (the non-living component of the substrate, nutrients, moisture and climate). Sustainable management of secondary forests calls for a clear understanding of how species interact with the biotic and abiotic environment.
The biggest challenge facing forest management today is the development of strategies to adopt sustainable forest management practices. This is called multipurpose management, in which the overall capacity of forests to provide goods and services is not diminished. This calls for a firm knowledge and understanding of the resource base (resource inventory as a basis of stand history, structure, species composition, diversity and dynamics), and availability and implementation of environmentally sound forest harvesting practices.
However, some progress has been made through research to understand the ecology of the secondary forests, as summarized under the following categories:
The occurrence and distribution of the various forest types can largely be explained by geophysical characteristics, and the quantity and seasonality of rainfall. Forest composition and species population sizes are determined in part by species tolerance of prevailing environmental conditions, particularly rainfall and soils, and in part by local site history (Swaine and Hall, 1983).
The total area of secondary forests in most developing countries is largely unknown, as is the area per vegetation type. Where forest inventories are available, they are often outdated. The forests are often fragmented and patchy. The boundaries between one vegetation type and another may be difficult to discern. The nature of fragmentation itself leads to overall degradation of forests due to the large ratio of forest margin to forest area.
Information emerging from different fields such as tree architecture and canopy structure have shown that forests are much more dynamic than was considered earlier when long term stability was emphasized. The vegetation composition is therefore seen as a balance between the process of competition and interactions between plants and their biotic and physical environments. Trees display a wide spectrum of physiological and life history characteristics that suite them in different site conditions.
The moist forests hold over 100 different species on one ha but only a minority of which have known commercial value. Two or more resource-limited species having identical patterns of resource use cannot co-exist. One species will be better adapted and will out compete the others. It is the available space that dictates the number of trees that can be accommodated in any class and continual tree mortality permits further growth of surviving trees and recruitment of new ones. Growth rates of trees are highly variable, with large differences between species, tree sizes, sites, and even between the same sizes of individual trees of the same species growing in the same site. In contrast, growth of individual during successive periods is much less variable. Trees which are growing fast continue to do so while slow growing individuals remain slow. Felling guidelines are therefore given on tree diameter ranges as opposed to the age of the trees. It is crucial to understand how the different species interact through intra and inter-species competition within associated physical factors. It calls for accumulation of a comprehensive knowledge of the ecology of individual species, including phenology, seed biology, dispersal and germination requirements and physiology.
5.2.1 Regeneration of secondary forests
Considerable progress has been made in understanding the regeneration of secondary forests in the tropical moist forests. A regeneration system evolved in many natural clearings caused by river floods, storms, trees that die of age, and the like. A number of species developed characteristics that were advantageous in the rapid colonization of such clearings.
Swaine and Whitmore (1988) recognized two crude categories in this succession process: pioneer and secondary species. Pioneer species typically germinate, establish, grow and mature relatively quickly in the clearings and breaks created by the death of dominant plants. Many of the primary species regenerate by the seedlings and young plants already on the forest floor, root suckers and rhizomes, seed in the soil and seeds with a very short seed dormancy that happen to be in fruit during disturbance of the area (Richards, 1966; Gomez-Pompa, 1971).
Pioneer species establish only in highly disturbed environments. Their ecology is designed to exploit rapidly the resources made available by the death of dominant plants. Surrounding forests serve as seed sources. Early pioneers are often fast-growing, short-lived "weedy trees". Their ecology is well suited to plantation forestry. At the other extreme, many climax species are designed to tolerate resource scarcity. Seeds are able to germinate in the dark forest understorey and seedlings can tolerate canopy shade for long periods, until disturbance creates an opportunity for growth. Shade-tolerant species reach their peak rates of photosynthesis at much lower light levels than their counterparts (Riddock et al., 1991).
Eventually the late pioneers are replaced by late successional vegetation that is diverse in architectural form and long-lived. The mature forest is the ecological unit that has reached its maximum diversity and number of species by containing all stages of the forest mosaic.
5.2.2 Adaptation to light and temperature
Tropical forests have developed pronounced stratification created by different heights of the trees. Most light is blocked by the layers of tree crowns and the forest floor is dark. At the forest floor level the air is hot, humid and very still, limiting the number of plants. Some of the plants here are woody shrubs and trees of species of stunted growth forms. Others are young plants of trees and palm species that may grow over 48 m high. Herbaceous growth forms in the forest understorey are adapted to the low light conditions. For example, fern species adapted to these conditions tend to have large, broad, and very dark green leaves with extra pigments to make the best use of this dim light. Each leaf is set at the best angle to receive as much light as possible. The young leaves of the lower and upper canopy trees have one unusual feature that is only found in tropical plants. When they are first formed, they come in a range of brilliant colors, from red, purple, to blue and even white. The leaves do not turn green until they are older. Denslow (1980) proposed that different species of climax rain forest tree seedlings may be adapted for optimum growth at different levels of irradiance. Disturbances of the forest canopy are essentially haphazard and maintain a wide spectrum of light levels. Specialization would give competitive advantage in a disturbance of a specific suite of light conditions, but would be likely to involve adaptive compromises with restricted success where the light was at a different level.
5.2.3 Seed development and dispersal
Available information shows that different plants have evolved an intricate system for pollination and seed dispersal by different insects, birds and other animals. Animals disperse seeds from fleshy fruits. When eaten, animals' digestive juices induce germination. When finally dropped in the right habitat, they are ready to germinate. Other seeds are dispersed by water or wind.
5.2.4 Soils of tropical African rainforests
African tropical rainforest areas have been geologically stable for long periods of time and the soils have undergone intense weathering. Most soils are primarily oxisols and ultisols. Both these soil types are almost incapable of storing nutrients. In forest environments the nutrients are largely locked in the living vegetation. The warm, moist conditions cause a rapid decay of the large quantities of leaf-fall and other plant biomass supplied to the forest floor. The released nutrients are rapidly recycled directly from the decaying vegetation to the living plants without first being stored in the soil. In some circumstances tree roots even grow upward towards the soil surface, permeating the litter layer. It is this closed nutrient cycle through the veneer of partly decaying organic matter resting on the soil itself that must be preserved, if the forest is to regenerate. Once the forest is removed, the soil degrades rapidly as a result of microclimatic changes, and quickly loses its nutrient supply through leaching under the abundant rainfall.
Many micro-organisms take part in this decomposition process: termites, bacteria, fungi, various invertebrates and mycorrhizal fungi, which invade the roots of trees to obtain nourishment. These fungi gain carbon nourishment from the tree and benefit the tree by providing a vastly expanded nutrient gathering network in the soils.
The regeneration and ecology of dry forests are leas studied. These forests are characterized by lower rainfall. Annual precipitation decreases noticeably from the wet forest belt, and the dry season becomes longer and more pronounced. The dominant vegetation changes to broad-leaf deciduous forest, and gradually to tropical broadleaf deciduous woodland and the savannahs.
Many dryland plants have evolved specialized morphological, anatomical and physiological adaptive characters and mechanisms for these ecosystems. These unique adaptive features include root systems that spread out vertically and horizontally over vast distances, a chain of rain roots, deciduous habits, reduced leaf size, waxy or hairy leaf surface, increased albedo and succulence or tuberous features. Some exhibit seed dormancy until the right circumstances prevail, such as seed coats that need to be damaged by heat or passage through the gut of a browser before they can germinate. Many dry forest species are adapted to fire and heavy browsing, that either triggers seed germination, root coppicing or preferential development of one species above others.
Under normal conditions with regular rainfall pattern, C3 plants are widespread. With an increasingly irregular rainfall pattern, the C3 plants gradually thin out, and C4 plants (i.e. plants that are able to avoid heavy transpiration during the day, and absorb CO2 through their open stomata at night) take over (Adams, et al 1978). CAM (Crassulacean Acid Metabolism) plants form another category of dryland plants with specialized physiological attributes.
In practical terms, any activity, such as over-cropping or over-grazing, which precipitates the elimination of C3 plants from a site, will in due course promote the establishment of C4 and eventually CAM plants. Generally such plants are of no value in fodder terms. However, they are significant in that, once established, they can improve soil structure and stability by the binding action of their roots, and add to the soil's organic content, so assisting the return of the C4 and C3 plants on return of favourable conditions.
Logging operations with modern machinery, frequent fires and intensive grazing under current land use systems have introduced disturbance dimensions and impacts that are new to these forests. Forest management is therefore unable to cope using available tools to ensure sustainability of all forest components. By contrast, the elements of shifting cultivation blend well with those of natural disturbance and unless the cultivation phase is extended or the stands are opened up prior to maturity of the secondary forest cover, this practice remains sustainable.
5.4.1 Shifting cultivation
Traditional shifting cultivation favoured regeneration of secondary forests because the regenerative system was well adapted to the activities of the primitive man. The use of small pieces of land for agriculture and their abandonment after a decrease in crop production (two to four years of shifting agriculture) is similar to the occasional destruction of the forest by natural causes (Baur, 1968; Gomez-Pompa, 1971).
In modern times farmers have extended cultivation in the opened areas for long durations. With the rapid depletion of nutrients in the tropical soils this has been followed with a sharp decline in productivity, forcing the farmers to open up new areas in the forest. At the same time, extended cultivation prevents recolonisation of the cultivated areas. By this time the seeds of the majority of the rain forest species are not available for regeneration. Even if seeds are available, microclimatic alterations resulting from the removal of the forest make many areas no longer ideal environments for rain forest regeneration. Such areas tend to revert to bushes with no value.
5.4.2 Maintaining strict natural conditions under non-disturbance
Around the world, evidence shows that culminating disturbance can have adverse impacts on both biodiversity and forest productivity. For example, forest-fire suppression programmes and single-tree selection harvesting prescriptions are resulting in major disruptions of ecosystem processes and changes in forest composition in mahogany forests. Mahogany species for example, require substantial canopy openings for regeneration. Strict non-disturbance is therefore likely to derail the course of development of the secondary forest away from reaching management goals.
5.4.3 Fire management
Fire influences the environment directly by consuming organic matter, releasing nutrients in bulk, and killing intolerant species. Indirectly fire affects the environment by creating space for regeneration and modifying microclimates and the population of component flora and fauna (Frost and Robertson, 1987).
Early burning has been used widely in forest management as a tool for clearing debris and fire belts to ward off wild fires and minimize the ferocity of late dry season fires in forests. There are also situations where firing may aid regeneration of certain species such as the regeneration of mahogany and disruption is likely to upset ecosystem processes. But Swaine et al. (1987) have discouraged its use in dry semi-deciduous forest reserves in Ghana. This is because forest fires devastate forests during the dry seasons, particularly where there are heavy accumulations of debris from previous logging activities. In the woodlands and savannas, communities practice seasonal grazing. Periodic fires are used for pasture renewal and to kill browsing/predator vectors. Fires promote the germination and growth of fire-tolerant plant species, such as the Acacias, but suppress regeneration of fire-sensitive species. With the exception of communities with knowledge of the management of grass fires in resource management, wrong fire management in ecosystems that have evolved in different directions, can cause serious management bottlenecks.
5.4.4 Harvesting of timber and other products
Timber harvesting is the most important single disturbance factor in forests over which the managers have significant influence, and the most important issue regarding protective management. Logging has in recent decades been more intense in the semi-deciduous zones than in the evergreen due mainly to the greater densities of desirable timber trees.
Several silvicultural treatments have been tested to improve management. Notably among these are the improvement thinning, salvage felling and selective felling by diameter limits. In this letter system, a single tree or a group of trees is given to concessionaires for exploitation. Generally, trees selected are over-mature, with broken tops or diseased.
The amount of wood to be cut generally depends on the growth of the stand. Total growth of a stand is a function of stocking. Total growth is greater with a higher stocking, within the limits of diameter distribution. It has been suggested that a reserve stock should be maintained after every logging to optimise growth.
The system of logging is influenced by distribution of the species in demand and unit costs of bringing logs to points of loading. The latter is largely determined by the volume of timber removed per unit area. The greater quality of timber removed the lower the unit costs. Often over 80 tons of timber per hectare are needed on most commercial ventures. This is probably about four times more than what can be removed if the forest were able to regenerate itself without systematic replantings.
The majority of commercial species is patchily distributed which disperse timber extraction. To accumulate a significant volume of timber of any one species, an extensive area of forest has to be exploited to provide adequate material for haulage. Loggers have reverted to practices such as creaming, to overcome this constraint. But felling all saleable trees above a given size has led to marked damage to residual trees, biodiversity complement and even the sustainability of commercial species.
Logging can be tolerable or fatal from the point of view of forest regeneration and maintenance of a healthy mosaic. Where few trees (say, <1.5 per hectare in a long felling cycle) are removed, and the disturbance is well-dispersed, the long term effects on plant species composition are likely to be minimal. Loading areas and roads suffer particularly from soil erosion and lead to poor plant regeneration, but the effects of these can be contained in the context of forest recovery (Hawthorne, 1993).
If logging is appropriately managed then it need not be a serious threat to the integrity of the forest vegetation. The problem is that logging in recent decades has certainly not been managed appropriately, and this is the main reason for the degradation of many forest reserves.
5.4.5 Silviculture and management practices
Despite a long history of scientific forest management, logging practices impact on several issues related to silvicultural management of the forest. Ecologists now see vegetation composition as a balance between the processes of competition between plants and environmental disturbance, rather than seeing a plant community as a stable entity. Disturbance releases resources by killing off the dominant plants and provides opportunities for new ones to grow. The magnitude and frequency of disturbance exert a crucial influence over the types of plants that are likely to survive and grow.
The countries of SSA have a long history of forest management within protected areas dating from the 1900s. Typically a large number of different tree species grow in a small area of tropical moist forest but only a minority has commercial value. Some attempts have been made to maintain a good species mix and to spread the yield proportionally among all species present. This measure was not successful because loggers would not fell what they cannot sell. Only about 35 per cent of the allowable yields are actually felled by concessionaires (Parren and De Graaf 1995). Even where attempts have been made to extract timber based on total forest growth, the tendency has been to remove all the increment from only a few species. Consequently the population of such lesser-used species continues to build up while the number of desirable species decline. A marketing strategy that sells timber by wood categories instead of single species might help to correct the future species composition of the natural forests.
The low growth rate of trees in most tropical rain forests is another constraint to sustainable management of secondary forests. The economic value in terms of commercial timber per unit area as compared to plantation forests is therefore low.
Appropriately managed logging would not threaten the forest vegetation. However, extended logging practices have considerable impact on forest structure and species composition and may lead to loss and fragmentation of forests. Species diversity is lower in logged-over sites compared to mature and unlogged natural forests. Moreover, markets change between one phase of logging and the next - new species become marketable and current fashions may decline. So the level of "timber production" to be sustained cannot be the same from one cutting cycle to the next. Management must therefore plan and execute logging activities carefully, following results of research and field experiences, if sustainable forest management is to be achieved. The following are some of the priority activities to be considered in the development of strategies for sustainable exploitation: