R.J. Zagt

Long-term effect of timber harvesting in the Bartica Triangle, Central Guyana

Abstract In one of the longest time series available, we present a comparison between forest inventories of 1924, 1964 and 1999 of an 800xa0km 2 forest-harvesting area in Central Guyana. Harvesting mainly targeted one species Chlorocardium rodiei (Greenheart), of which the population declined by 63% over the 75 years of harvesting. The decline of the population was not only detectable in the harvested size classes (>30xa0cm) but also among the smaller size classes. In 1999, however, Greenheart still ranked third in abundance. There was little change in the overall species composition, proportion of disperser groups or proportion of functional groups in the tree community. There were also no detectable changes in tree diversity. The conservation value of these unique forests can still be considered high. Thus, while the long-term sustainability of Greenheart harvesting is low, other ecological criteria of sustainability are met. Short-term recovery of Greenheart is not to be expected due to low population growth and small numbers of seed trees. Strategic choices will have to be made with regard to harvesting the remaining Greenheart stands—three scenarios are discussed.

Spatial and temporal patterns of seed attack and germination in a large-seeded neotropical tree species

Abstract It has long been argued that seed dispersal enhances recruitment in tropical trees by allowing offspring to `escape strong density/distance-dependent attack by insects, pathogens and rodents. Here we examined the effects of canopy openness and parent-offspring distance upon the frequency and timing of Chlorocardium rodiei seed attack and germination within a 15-ha plot of Guyanan tropical rain forest. Seeds were artificially dispersed beneath parent trees, in the understorey away from trees and in gaps. Analysing our data from an 85-week period of regular monitoring, we found that the main spatial gradients, canopy openness and distance to nearest adult conspecific, do not lead to differences in the final number of seeds attacked by infesting scolytid beetles or rodents. The timing of beetle attack, however, varied along the distance gradient and this difference affords seeds at further distances a `window in which to germinate and produce a seedling before attack. Canopy openness was not a good predictor of rooting success, but distance was strongly associated with root and shoot formation success and the mean time to shoot formation. There was a strong negative effect of distance on the likelihood of a seed being colonised by scolytid beetles prior to removal by rodents and shoot failure was strongly associated with prior infestation. We believe these results bring a key point to bear on the well-established notion of distance-dependent attack on seeds in tropical rainforests, viz. that seed characteristics (size, germination syndrome) and the timing of attack may be more important in explaining patterns of early seedling recruitment than distance. Our studies suggest that advantages accrued through dispersal in species like Chlorocardium will depend heavily on the `race between seed germination and attack. In the case of Chlorocardium, the `race can be lost at considerable distances due to its prolonged dormancy and the temporal fluctuations in fruitfall and rainfall which influence attack and germination. The results presented here suggest that the lag between seed attack and germination in tropical trees can regulate the influence of parent-offspring distance on cohort recruitment at this life history stage.

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.

Pre-dispersal and early post-dispersal demography, and reproductive litter production, in the tropical tree Dicymbe altsonii in Guyana

The demographic history of a flower-cohort of the tropical tree Dicymbe altsonii (Caesalpiniaceae) was studied by collecting litter from flowers, fruits and seeds, and by monitoring seedling survival. Flower production was estimated at 137 to 172 m −2 in a plot of 1 ha. Four percent of the flowers yielded a pod, which contained an average of 2.0 seeds. Post-dispersal mortality over 3 mo was 39% of the initial number of dispersed seeds. Most flowers and fruits were aborted. Insect and vertebrate predation in the tree and on the ground was very low. Reproductive litter production of this species was estimated to be equal to leaf litter production. The amount of phosphorus lost in litter from fruits and flowers was much larger than in leaf litter. This implies that the impact of the biennial flowering events on the nutrient dynamics of the trees and of the ecosystem may be large. It is argued that the reproductive cycle and the pre-dispersal demography of Dicymbe seem to be determined by a scarcity of nutrient resources, and possibly by pollinator limitation.

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.