Latest News on tropical forest : Apr 2022

The Future of Tropical Forest Species

Deforestation and habitat loss are widely expected to precipitate an extinction crisis among tropical forest species. Humans cause deforestation, and humans living in rural settings have the greatest impact on extant forest area in the tropics. Current human demographic trends, including slowing population growth and intense urbanization, give reason to hope that deforestation will slow, natural forest regeneration through secondary succession will accelerate, and the widely anticipated mass extinction of tropical forest species will be avoided. Here, we show that the proportion of potential forest cover remaining is closely correlated with human population density among countries, in both the tropics and the temperate zone. We use United Nations population projections and continent-specific relationships between both total and rural population density and forest remaining today to project future tropical forest cover. Our projections suggest that deforestation rates will decrease as population growth slows, and that a much larger area will continue to be forested than previous studies suggest. Tropical forests retracted to smaller areas during repeated Pleistocene glacial events in Africa and more recently in selected areas that supported large prehistoric human populations. Despite many caveats, these projections and observations provide hope that many tropical forest species will be able to survive the current wave of deforestation and human population growth. A strategy to preserve tropical biodiversity might include policies to improve conditions in tropical urban settings to hasten urbanization and preemptive conservation efforts in countries with large areas of extant forest and large projected rates of future human population growth. We hope that this first attempt inspires others to produce better models of future tropical forest cover and associated policy recommendations.[1]

The dynamics of tree populations in tropical forest: a review

Published work on the dynamics of forest tree recruitment, growth and mortality in natural tropical forest is reviewed. In most forests studied, annual mortality is between 1% and 2% and is independent of size class in trees >10 cm dbh; mortality is negatively correlated with growth rate and crown illumination; growth rate is highly variable between individual trees, but shows strong autocorrelation between successive measurements on the same tree.

Differences in the rate of dynamic processes can be detected between some species at a site, but data are presently insufficient to determine whether these differences are preserved at other sites where the species occur. None of the studies discussed are of sufficient duration to permit us to draw any conclusions about the equilibrium or non-equilibrium of floristic composition.[2]

Restoration of Degraded Tropical Forest Landscapes

The current scale of deforestation in tropical regions and the large areas of degraded lands now present underscore the urgent need for interventions to restore biodiversity, ecological functioning, and the supply of goods and ecological services previously used by poor rural communities. Traditional timber plantations have supplied some goods but have made only minor contributions to fulfilling most of these other objectives. New approaches to reforestation are now emerging, with potential for both overcoming forest degradation and addressing rural poverty.[3]

Research in large, long-term tropical forest plots

The past 15 years has seen the creation oflarge (>/16 ha) permanent inventory plots in each of the major tropical forest formations of the world. Currently, six such plots have been fully mapped, and five more and under way. A standardized methodology is used at all sites — a complete census of all trees and saplings down to 1 cm in diameter — thus assuring strict comparability between sites and allowing the development of general models for the dynamics of tropical forests. The inventories aim to gather demographic information on individual tree species, to provide long-term information on forest composition so that future changes can be detected, to estimate the economic value of forest resources, to generate models of sustainable extraction, and to provide data on underused native species for use in reforestation or plantation forestry. The plots also provide data from undisturbed forest to serve as a control for anthropological and management studies of harvested forests.[4]

Dioecism in Tropical Forest Trees

In tropical lowland forests, one fourth to one half of all species have unisexual flowers, and a majority of such species are dioecious. Almost all dioecious species have relatively small (≤ 1.0 cm in length and breadth) pale yellow to pale green flowers. Field observations of selected species indicate that a large number of dioecious species are pollinated by small bees such as various species of Anthophoridae, Halictidae, Megachilidae and Meliponinae, while a small number is pollinated by moths. In the light of the high incidence of dioecism and the prevalence of entomophily in tropical trees, it is suggested that as far as tropical trees are concerned, anemophily cannot be invoked to explain the origin of dioecism (and monoecism) as has been done for temperate plants. In order to explain the evolution of dioecism in tropical trees, it is postulated that, with the exception of a few taxa where dioecism has evolved from heterostyly, in a majority of investigated species dioecism has probably evolved from a self-compatible breeding system in response to selective pressure for outcrossing. The main question on which attention is focused is: When selective pressure for outcrossing arises, what favors the evolution of dioecy over self-incompatibility? Four specific hypotheses are proposed which provide possible answers to the question. These are based on the assumptions of relative ease with which dioecism can arise, increased pollination success allowed by dioecism under certain conditions, enhanced escape from seed predators under a dioecious breeding system and the ability of dioecious species to exploit a wider variety of microhabitats than hermaphroditic species.[5]


Reference

[1] Wright, S.J. and Muller‐Landau, H.C., 2006. The Future of Tropical Forest Species 1. Biotropica: The Journal of Biology and Conservation, 38(3), pp.287-301.

[2] Swaine, M.D., Lieberman, D. and Putz, F.E., 1987. The dynamics of tree populations in tropical forest: a review. Journal of tropical Ecology, 3(4), pp.359-366.

[3] Lamb, D., Erskine, P.D. and Parrotta, J.A., 2005. Restoration of degraded tropical forest landscapes. Science, 310(5754), pp.1628-1632.

[4] Condit, R., 1995. Research in large, long-term tropical forest plots. Trends in Ecology & Evolution, 10(1), pp.18-22.

[5] Bawa, K.S. and Opler, P.A., 1975. Dioecism in tropical forest trees. Evolution, pp.167-179.

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