David S. Hammond

Seed Size of Woody Plants in Relation to Disturbance, Dispersal, Soil Type in Wet Neotropical Forests

Fresh and dry masses of seeds of woody species collected at Mabura Hill, Guyana were evaluated according to dispersal mode, gap size preference, and soil type. Data published for woody species from Manu, Peru and Barro Colorado Island (BCI), Panama were analyzed by dispersal mode and gap size preference for comparison. Larger seeds at the three sites were significantly associated with canopy or emergent trees or lianas that are dispersed by mammals or gravity and recruit best in small gaps. Variation in seed size, after accounting for the effects of plant height and life-form, could be explained by dispersal mode (Mabura Hill : 22%, Manu : 14%, BCI : 26%) and gap size preference (Mabura Hill : 9%, Manu : 7%, BCI : 11%). Soil type preference, analyzed only for Mabura Hill, was not associated with seed size. Mean seed size at Mabura Hill was significantly larger than at Manu or BCI. Short-term successional processes may explain the variation of seed size within sites ; dispersal mode is particularly important in defining seed size selection during the successional process. However, short-term successional processes cannot explain the variation of seed size among sites. It is suggested that this variation may be attributed to differences in the effects of larger-scale variations in soils and disturbance on seed size among the three sites.

CHARACTER CONVERGENCE, DIVERSITY, AND DISTURBANCE IN TROPICAL RAIN FOREST IN GUYANA

The level of tree diversity varies greatly between sites in Guyana and decreases along a gradient from south to north. We conducted a study to understand what controls this gradient of diversity using data from country-wide forest inventories. Analysis of tree diversity on the basis of soil and rainfall effects in an area of 15 × 106 ha showed that soil exerts a strong local effect on tree diversity, whereas rainfall, potential evapotranspiration, and length of the dry season do not. Community averages of disturbance-sensitive attributes of trees, such as seed mass or wood density, were not correlated with variation in either rainfall or soil, but were strongly correlated with tree diversity. As such, the forests in central Guyana were characterized by generally a high average wood density, large seeds, and rodent or unassisted dispersal. These forests also displayed the highest single dominance, thus lowest diversity and low percentages of pioneers in the community. The more highly diverse forests in sou...

A compilation of known Guianan timber trees and the significance of their dispersal mode, seed size and taxonomic affinity to tropical rain forest management

Abstract Information available concerning all but the most common timber species in the Guianas is scattered and difficult to obtain. Very little is known of the ecological requirements of these timber species. Over 170 tree species in Guyana, Suriname and French Guiana were found to have useable timber properties. Three-quarters of these are currently harvested in one of the three countries and less than 20% account for greater than 90% of production. Most timber species are mammal-dispersed (51%), followed by bird and wind dispersal (21 and 20%, respectively). The seeds of less than 8% of the species are transported through unassisted modes, though several of these are important timber trees. The largest seeds are associated with mammal dispersed and unassisted species. Most bird and wind dispersed species have seed sizes below the median for all timber species. The seeds of timber species in Guyana were significantly larger than non-timber species. Dispersal is important in maintaining timber tree population structures and avoiding inbreeding depression. Maintaining adequate dispersal of timber species in the Guianas depends largely on the conservation of mammals. Many species can not be dispersed by smaller animals because their seeds are too large. Wind and bird dispersed species are at a relative advantage since their seeds are smaller and can be effectively dispersed by wind and mammals in the absence of their primary agents. Available data suggest that disproportionate harvesting of mammal-dispersed species relative to their contribution to total estimated standing volumes occurs in Guyana, while bird and wind dispersed species are the most disproportionately harvested in Suriname and French Guiana, respectively. Groups of species with the same dispersal mode should be exploited in relative proportion to their availability to avoid altering resources for animals. Maintaining the widest spectrum of resources available to animals will increase the likelihood that they continue to act as effective dispersal agents of valuable timber trees in the Guianas.

Basic and Applied Research for Sound Rain Forest Management in Guyana

Five years of research have increased our understanding of the effects of disturbances on some forest processes and given directions for forest management in Guyana. At present most logging concentrates on Chlorocardium rodiei (greenheart) and current practices are damaging, but they may have little effect on the overall water balance and nutrient cycle if some simple rules are adhered to. The basic research results do not lead to a fully documented sustainable forest management system, as translation to prac- tical management is not always simple. In general it would appear that applied research may give fast results in an often quick and dirty approach. Basic research, if directed at the right forest processes, may provide answers when the quick and dirty approach does not fulfil its objectives. As such they are complementary. In tropical forest management we cannot wait until basic research provides all answers leading to proper forest man- agement, and a combination of the two would probably provide most answers in the long term.

Considering Background Condition Effects in Tailoring Tropical Forest Management Systems for Sustainability

Systems devised for managing tropical forests sustainably have yet to prove successful. In many instances, they have fallen short of initial prospects, but the reasons for these shortfalls are often not apparent. Here, we explore factors that can shape the likelihood of success, collectively referred to as background conditions, which are not always adequately considered prior to selecting a suitable management system. We examine the ability of one background condition, geologic terrane, to explain crude spatial variation in a number of trailing indicators of varying forest land use. Forest areas on Precambrian and Phanerozoic terranes show significant differences in production of fossil hydrocarbons, gold, and tropical roundwood, among other indicators, even after considering regional effects. Background conditions are considered to be factors capable of quantifying spatial variation in the likelihood of achieving management success given predefined benchmarks of sustainability. A number of avenues for further exploring and discriminating spatial variation of background conditions are discussed.