Forest Canopy Tree Research Articles

Effect of selective logging on canopy and stand structure and tree species composition in a lowland dipterocarp forest in peninsular Malaysia

The authors compared tropical rain forest canopy structure and tree species composition in two forests southeast of Kuala Lumpur, Malaysia: a primary forest and a regenerating forest that was selectively logged in 1958. For each of the forests, the study plots were set out and all trees of ≥1 cm in DBH (diameter at breast height) were mapped and measured. Canopy heights were measured in the two study plots based upon aerial triangulation using aerial photographs taken over the forests in 1997. Using this data, digital elevation models of the canopy were then constructed. The mean canopy height was greater in the primary forest (27.4 m versus 24.8 m), as was the variance in height and the number of emergent canopy trees >40 m height. The mean canopy surface area in the primary forest was nearly 1.5 times the value in the regenerating forest, and the mean crown size of canopy layer trees in the primary forest was more than twice that in the regenerating forest. The species diversity index (Fisher’s α) differed for the two forests, indicating that tree species diversity had been affected by the logging. Both forests had the same five families with the greatest stem density (stems ha −1), but the 50 most abundant species, in terms of both stem density and basal area, differed greatly between the two forests. Stem densities and basal areas were similar, but the number of stems per hectare and the basal areas of medium-sized trees (10–30 cm in DBH) were distinctly higher in the regenerating forest. These results suggest that average basal area and stem density in the regenerating forest that had been selectively logged 41 years earlier had recovered to levels similar to those in the primary forest; however, the regenerating forest had a more monotonic canopy structure comprised of medium-sized trees growing at high density. These findings also imply that structural development takes a long time to manifest in a regenerating forest as a result of the time taken for the development of emergent and canopy trees and the formation of gaps; structural development might also be delayed by the high density of medium-sized trees in the canopy layer.

FOREST STRUCTURE, CANOPY ARCHITECTURE, AND LIGHT TRANSMITTANCE IN TROPICAL WET FORESTS

Identifying factors that influence variation in light availability within forested ecosystems represents an important component in our understanding of the complex determinants of tree seedling regeneration. We assessed the influence of forest structure and canopy tree architecture on spatial heterogeneity of understory light availability in three old-growth and three second-growth forests in lowland Costa Rica. Forest structure and understory light availability were measured within forest types using contiguous 10 × 10 m quadrats along three 130–160 m transects in each stand. Two 20 × 60 m plots in each forest type were sampled more intensively, including vertical profiles of light availability from 1 to 9 m height. Mean diffuse light transmittance increased from 2% at 1 m height to over 10% at 9 m height and did not differ significantly between forest types at any height. However, the relationships among height classes differed between forest types. Second-growth plots showed a negative spatial autocorr...

Breeding biology of the Cape Parrot, Poicephalus robustus

Breeding biology and nesting requirements of Cape Parrots in their natural habitat and in captivity were investigated. Few nests were found, suggesting that nest-sites are limiting, and that the parrots have specific nesting requirements. Nests were secondary cavities in dead Podocarpus spp. (branches), high up in forest canopy trees. Breeding usually occurred from August to February, but was observed in other months, particularly in captive birds. Clutch size varied from 2–5, incubation was by the female and lasted 28–30 days, with fledging a further 55–79 days. In captive birds the ovaries are mature at 2.5 years, but age at first breeding is usually at 4–6 years.

Does Rhododendron maximum L. (Ericaceae) Reduce the Availability of Resources Above and Belowground for Canopy Tree Seedlings?

Abstract Subcanopy shrubs and perennial herbs inhibit recruitment of canopy trees in forests around the world. Although this phenomenon is widespread, and can have significant effects on community dynamics, the mechanisms of inhibition are not well understood. In the southern Appalachian region, Rhododendron maximum inhibits the recruitment of canopy trees in forests of northern red oak (Quercus rubra). We have shown, in previous research, that processes occurring before canopy tree seed germination are not responsible for this inhibition. Therefore, post-germination processes, such as competition for resources are most important. In this study we show that the presence of a thicket of R. maximum in the understory reduced the availability of light by 80%, the frequency and duration of sunflecks by 96%, the availability of water by 20% and the availability of several soil nutrients (particularly cations) by variable amounts. Moreover, the survival of Q. rubra seedlings in the understory over 3 y was signif...

Resource Partitioning and Interspecific Competition Among Coexisting Species of Guans and Toucans in Se Brazil

Between 1987 and 1998 fruit consumption and habitat use of six syntopic species of guans and toucans were studied in two Atlantic rainforest areas of southeastern Brazil: the Paranapiacaba forest fragment (140,000 ha) and the Parque Estadual da Ilha do Cardoso (22,500 ha). These two areas cover an altitudinal gradient of 0 to 1,095 m. Pipile jacutinga and the larger species of toucans (Ramphastos vitellinus and R. dicolorus) inhabit the upper levels of the forest (canopy and emergent trees), whereas Penelope obscura and the smaller toucanets (Baillonius bailloni and Selenidera maculirostris) mostly inhabit middle levels (subcanopy) and understorey. Diet composition (fruit) differed among guans and toucans and also among guan species. P. jacutinga and P. obscura tended to forage more on the fleshy fruits of Lauraceae and Myrtaceae trees, respectively, while toucans (especially B. bailloni) fed largely on palm fruits (Arecaceae). Potential congeneric competitors in the study areas are Ramphastos vitellinus ariel and R. dicolorus. We suggest that R. v. ariel recently colonized (end of the Quaternary period) the coastal Atlantic rainforest of Brazil, coming from the Amazonian rainforest.

Regulation of water flux through tropical forest canopy trees: do universal rules apply?

Tropical moist forests are notable for their richness in tree species. The presence of such a diverse tree flora presents potential problems for scaling up estimates of water use from individual trees to entire stands and for drawing generalizations about physiological regulation of water use in tropical trees. We measured sapwood area or sap flow, or both, in 27 co-occurring canopy species in a Panamanian forest to determine the extent to which relationships between tree size, sapwood area and sap flow were species-specific, or whether they were constrained by universal functional relationships between tree size, conducting xylem area, and water use. For the 24 species in which active xylem area was estimated over a range of size classes, diameter at breast height (DBH) accounted for 98% of the variation in sapwood area and 67% of the variation in sapwood depth when data for all species were combined. The DBH alone also accounted for > or = 90% of the variation in both maximum and total daily sap flux density in the outermost 2 cm of sapwood for all species taken together. Maximum sap flux density measured near the base of the tree occurred at about 1,400 h in the largest trees and 1,130 h in the smallest trees studied, and DBH accounted for 93% of the variation in the time of day at which maximum sap flow occurred. The shared relationship between tree size and time of maximum sap flow at the base of the tree suggests that a common relationship between diurnal stem water storage capacity and tree size existed. These results are consistent with a recent hypothesis that allometric scaling of plant vascular systems, and therefore water use, is universal.

Long-Term Survivorship and Crown Area Dynamics of Tropical Rain Forest Canopy Trees

Long-Term Survivorship and Crown Area Dynamics of Tropical Rain Forest Canopy Trees

ASSESSING THE GROWTH OF TROPICAL RAIN FOREST TREES: ISSUES FOR FOREST MODELING AND MANAGEMENT

Growth performance was assessed for a diverse suite of canopy and emergent tree species in a lowland neotropical rain forest (the La Selva Biological Station, northeastern Costa Rica). Species were evaluated based on annual diameter measurements of large samples of individuals in all post-seedling size classes, over a 12-yr period. The study species were seven non-pioneers (Minquartia guianensis, Lecythis ampla, Hymenolobium mesoamericanum, Simarouba amara, Dipteryx panamensis, Pithecellobium elegans, and Hyeronima alchorneoides) and two pioneers (Cecropia obtusifolia and C. insignis). For each species, inherent growth capacity through ontogeny was estimated as the mean of the five largest annual increments (from different individuals) in each juvenile and adult size class. At all tree sizes, species differed highly significantly in this measure. In the small sapling sizes (≤4-cm diameter), the two pioneers showed markedly higher maximum growth than all the non-pioneers, in line with current understanding. This growth advantage, however, was quickly lost with increasing tree size. For eight of the nine species, the ontogenetic patterns of maximal growth closely corresponded to species groupings previously based on juvenile microsites (forest stage, light levels). Average growth rates, however, which are the combined result of environmental and biotic factors, did not show this correspondence. The groups based on microsite occupancy and growth potential provide a starting point for treating the great numbers of species of tropical rain forest trees as many fewer functional types, a sine qua non for modeling these complex forests. The exceptional growth capacity of one of our focal non-pioneer species (Simarouba) suggests that it represents a hitherto unrecognized functional type among tropical rain forest trees. The data from this long-term study indicate that several assumptions of current forest-dynamics models are not applicable to trees of this biome. For most species (1) diameter growth during regeneration was not correlated with tree size; (2) adult sizes were not characterized by a constant maximal basal-area increment; and (3) growth capacity did not decline toward zero as trees approached maximum size. Evaluation of overall increment distributions revealed a broad adult size range of exclusively high growth in the emergent Hymenolobium; for two additional species, sectors of the adult size range showed an absence of very low or negative growth. At most sizes, however, all nine species displayed the full range of growth increments, from slightly negative to observed maxima. Describing the performance of tropical rain forest canopy trees will require similar analysis of growth data obtained from many individuals and intervals, and stratified across juvenile and adult life stages. More realistic forest-dynamics models will depend on such evaluations of many tree species, in diverse tropical rain forests. Such models are greatly needed, both for designing management for this biome and for predicting its responses to changing climate and atmospheric conditions.

Distribution of twelve moist forest canopy tree species in Liberia and Côte d'Ivoire: response curves to a climatic gradient

Abstract. The occurrence and abundance of 12 canopy tree species from the moist tropical forests of West Africa have been studied in relation to a climatic gradient. We focused on environmental factors related to water availability: annual amount of rainfall, the length of the dry season, and cumulative water deficit. Species occurrence and abundance data are used for 39 forest sites in Liberia and southwest Côte d'Ivoire. Species responses are modelled using a set of five increasingly complex models, ranging from a no‐trend model to a skewed bell‐shaped response curve.The study species show different distribution patterns. Most of them suggest a close relationship to climatic conditions. Fitting of species occurrence data to each of the three climatic factors results in most cases in simple models. In only one out of 36 cases a bell‐shaped response curve is needed to describe the data. Four of the 12 species show no response to the climatic factors when only occurrence is evaluated.When abundance data are used, in 33 of the 36 cases significant response models are found. In general these are much more complex than in the cases of species occurrence data: in 10 of the 36 cases a bell‐shaped response model is found to describe the data best. This is in contrast with the widespread belief that species response curves are bell‐shaped: within the forest zone in the area studied this is not generally the case.The importance of the three climatic factors for the distribution of the species is evaluated: for four species mean annual rainfall is the most important variable, for four species the length of the dry period, and for one species cumulative water deficit. Consequently, the assumption that mean annual rainfall is the most important factor determining tree species distribution in West African forests is not correct.Species response models to climatic factors show where species have their geographical optima. Implications for forest management are briefly discussed.

Airborne Digital Camera Image Semivariance for Evaluation of Forest Structural Damage at an Acid Mine Site

A forest downstream of a heavy metal acid tailings area at the KamKotia mine site near Timmins, Ontario shows visible signs of damage which include varied leaf size, leaf discoloration, dead branches, and increased individual tree crown and forest canopy openness. High resolution remote sensing has potential for providing means to spatially and temporally evaluate such damage. In particular, image texture can be used in modeling forest and individual tree structural variations which may result from stress. In this study, an airborne multispectral digital camera system was used to acquire imagery with ground pixel spacing of 0.25 m, 0.5 m, and 1.0 m. Relations of image semivariance measures with field forest structure and health measures were determined. Semivariograms were derived using two sampling techniques: transects in two perpendicular directions, and omnidirectional sampling within pixel matrices. Sampling was conducted over the forest canopy as well as within individual tree crowns. The principal objective of the study was to determine the types of forest canopy and individual tree crown structure and health information captured by semivariograms at the three spatial resolutions. In canopy scale sampling, the 1 m pixel semivariograms were best related to forest canopy closure, stem density, and a visually derived tree stress index. The 0.5 m pixel semivariograms related better to tree crown size and tree height. The transect technique was more sensitive to tree height and the matrix technique produced stronger relations with tree stress. In individual tree crown samples, the 0.25 m pixel semivariograms were well related to tree crown closure. This suggests that semivariograms extracted from the 1 m and 0.5 m pixel images are suitable for mapping structural and textural information related to forest damage at the canopy level while the semivariograms extracted from the 0.25 m pixel images depict information at the tree crown level.

Partitioning of soil water among canopy trees in a seasonally dry tropical forest.

Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θv), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about -15‰ at 0.1 m to -50 to -55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of -60‰. θv increased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θv dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θv remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season.

Vagile but inbred: patterns of inbreeding and the genetic structure within populations of the monsoon rain forest tree Syzygium nervosum (Myrtaceae) in northern Australia

Patterns of gene flow within 21 northern Australian populations of Syzygium nervosum a dominant, mass-flowering, monsoon rain forest canopy tree were investigated using 10 isozyme loci. S. nervosum was found to have relatively high genetic diversity within populations (He = 0.307, AP = 3.7, P = 65) but also to have significantly lower frequencies of heterozygotes than expected (Ho = 0.126) and high allelic fixation (F = 0.512). Heterozygosity and allelic fixation were not correlated with measures of genetic diversity within populations, nor were they correlated with rain forest patch size, plant size or population isolation. Within populations, trees, of the same genotype (at each loci tested) were significantly clumped at short distances (c. 20 m), whereas trees of unlike genotypes were negatively associated. S. nervosum trees however, were not clonal in origin and had unique multilocus genotypes. The results suggest that the high levels of homozygosity recorded are the result of restricted pollination, primarily among flowers within individual trees or among closely related neighbouring trees, rather than rectricted seed dispersal. High homozygosity, the large fruit crop produced by trees of this species and the lack of association between heterozygosity and plant size, indicate that, S. nervosum is self-compatible, its fecundity does not appear to be impaired by inbreeding depression.

DIET SELECTION AND RESOURCE USE BY FLYING FOXES (GENUSPTEROPUS)

Flying foxes of the genus Pteropus (Pteropodidae: Chiroptera) play important roles as pollinators and seed dispersers in oceanic-island forest communities. This research examined general theories of diet breadth, diet selection, and the evolution of feeding strategies in bats in light of information from members of the genus Pteropus that inhabit oceanic islands. The feeding ecology of two species of flying fox, Pteropus samoensis and Pteropus tonganus on the Samoan archipelago, was examined in detail by direct observation and by examining feeding refuse. P. samoensis and P. tonganus fed on over 78 plant species from 39 families throughout their range and on over 69 plant species in Samoa alone. Flying foxes interacted with 59% of the forest tree species in Amalau Valley for fruit or flower resources. Twenty-eight percent of the forest trees were commonly used, and 79% of forest canopy trees were used. Broad overlap in resource use was noted between P. samoensis and P. tonganus with over 22 shared plant species. Seasonal variation in fruit resource use was quantified with a preference index. Flying foxes ate a taxonomically nonrandom subset of fruit species and generally preferred primary forest fruits to those in the secondary forest. Six plant families (Sapotaceae, Mrytaceae, Moraceae, Combretaceae, Fabaceae, and Sapindaceae) were particularly important to flying foxes in Samoa. However, a set of core plant taxa does not exist for Pteropus spp. in Samoa. Differences in the evolution of feeding strategies between mainland fruit-eating chiroptera and island species likely reflect differences in the spatio-temporal availability of resources in the two systems.

RESOURCE CONGRUENCE AND FOREST REGENERATION FOLLOWING AN EXPERIMENTAL HURRICANE BLOWDOWN

Catastrophic uprooting of forest canopy trees creates mounds, pits, and other microsites that provide opportunities for regeneration of particular species. We measured environmental factors on five types of microsites created by simulated blowdown of a mixed deciduous forest in central New England, United States. We then estimated spatial variation in resource levels and quantified congruence among different resources at each site. Effects of simulated blowdown on light levels and CO2 concentrations were more pronounced after three years than effects on nitrogen availability and other soil resources. Spatial heterogeneity in light levels and net nitrification rates was greater in the blowdown, but heterogeneities of soil organic matter concentration and net mineralization rates were greater in the undisturbed forest. Availability of nitrate, a limiting resource in most New England forests, was low on mounds and in pits, but high on the vertical portion of forest floor resulting from uprooting of canopy trees. At a spatial scale relevant to seedlings, resource congruence was greater in the undisturbed forest than in the experimental blowdown, primarily because of the effects of blowdown on light levels. Congruence in the blowdown increased with an increase in spatial scale, but congruence in the undisturbed forest was similar at both spatial scales. Seedling growth of two birch species was correlated with light levels and with congruence among soil resources. This study shows that immediate disturbance effects on microtopography and light levels determine recruitment patterns of colonizing species, with changes in soil resource levels influencing later community development. Furthermore, some species appear to respond to resource congruence, which may provide an additional dimension to the regeneration niche.

Distribution of primary forest plant species in a 19-year old secondary forest in French Guiana

Secondary succession in a 19-y old tropical rainforest of French Guiana was investigated through the distribution pattern of primary forest species. The experimental plot of 25 ha was logged to simulate conditions of a paper pulp cut. Enumeration of all plants of height ≥ 50 cm in 52 subplots 10 m × 10 m indicated the number of species and individuals in five species categories; short-lived pioneer species, pioneer species that persist in mature forest, primary forest species capable of germinating in shade, primary forest species that indicate a preference for clearings (small gaps) in mature forest and understorey primary forest species. Change in proportion of the most abundant species (≥ 1 cm dbh) over the last 15 y indicated a decrease in short-lived pioneer species and an increase in pioneers that can persist in the mature forest. Proportion of species categories per height class indicated that primary forest species were mainly found in the lower height class in this forest which is largely dominated by secondary forest tree species forming a canopy that rarely exceeds 20 m. Individuals of primary forest species were encountered on ARBOCEL as sprouts from large stumps, sprouts from juveniles or as untraumatised saplings or juveniles. The distribution pattern in a 4-ha area of 11 selected primary forest canopy tree species revealed an overall trend in recolonisation of this species category in the secondary forest. Considered individually, the clustered distribution of stems of Eperua grandiflora showed the importance of regeneration through sprouting or remaining individuals. The gradient distribution, oriented from the edge to the centre of the studied plot, and displayed by the stem densities of Virola michelii, suggested a recent progressive dispersal of seeds. A more dynamic recolonisation front was observed for the primary forest understorey species Cassipourea guianensis and was probably linked to both the initial logging conditions and the fact that this species fruits when relatively young.

Stem water storage and diurnal patterns of water use in tropical forest canopy trees

Stem water storage capacity and diurnal patterns of water use were studied in five canopy trees of a seasonal tropical forest in Panama. Sap flow was measured simultaneously at the top and at the base of each tree using constant energy input thermal probes inserted in the sapwood. The daily stem storage capacity was calculated by comparing the diurnal patterns of basal and crown sap flow. The amount of water withdrawn from storage and subsequently replaced daily ranged from 4 kg d–1 in a 0·20‐m‐diameter individual of Cecropia longipes to 54 kg d–1 in a 1·02‐m‐diameter individual of Anacardium excelsum, representing 9–15% of the total daily water loss, respectively. Ficus insipida, Luehea seemannii and Spondias mombin had intermediate diurnal water storage capacities. Trees with greater storage capacity maintained maximum rates of transpiration for a substantially longer fraction of the day than trees with smaller water storage capacity. All five trees conformed to a common linear relationship between diurnal storage capacity and basal sapwood area, suggesting that this relationship was species‐independent and size‐specific for trees at the study site. According to this relationship there was an increment of 10 kg of diurnal water storage capacity for every 0·1 m2 increase in basal sapwood area. The diurnal withdrawal of water from, and refill of, internal stores was a dynamic process, tightly coupled to fluctuations in environmental conditions. The variations in basal and crown sap flow were more synchronized after 1100 h when internal reserves were mostly depleted. Stem water storage may partially compensate for increases in axial hydraulic resistance with tree size and thus play an important role in regulating the water status of leaves exposed to the large diurnal variations in evaporative demand that occur in the upper canopy of seasonal lowland tropical forests.

The Blackwood Group System: its relevance for sustainable forest management in the southern Cape

SYNOPSIS The invasion status of Australian blackwood, Acacia melanoxylon, is assessed against its value as a commercial tree in the southern Cape forests. The species was introduced from Australian rain forests and planted extensively in the southern Cape forests since 1909. It has since become well established in the forests, and invaded open areas outside the forest. However, it is also a useful and valuable timber tree which contributes more than 60% of both the timber volume and revenue obtained from the forest. A Blackwood Group System was established in the forest since 1968 in which the tree was planted in pure stands in the forest gaps caused by the felling of large trees. This study comprised various investigations to assess the invasion status of the species in the forests and to determine under what conditions it poses a threat to the forest. The results show that A. melanoxylon occurs in all the main forests throughout the southern Cape. It has the characteristics of an aggressive invader plant, but it does not aggressively invade closed, evergreen forest. In dosed forest its population diameter distribution is bell-shaped, a curve typical of pioneering populations in maturing vegetation. Outside the forest it is a serious invader of open and disturbed sites, as is shown by its negatively exponential diameter distribution along rivers and forest margins, i.e. a curve typical of expanding populations. In the forest it requires large gaps to grow fast. In the larger logging gaps (but generally small gaps) mean diameter growth was 0,9 cm/a over 22 years. In the larger gaps (l 200 to 1 500 m2) of the Blackwood Group System, diameter growth averaged 1,6 to 1,9 cm and the largest tree of 58 cm averaged 2,6 cm/a diameter growth over 22 years. Volume growth was 10,5 to 13,4 m3/ha per annum. The surrounding forest had a suppressive effect on the diameter and height growth of A. melanoxylon trees on the gap margin. A wide range of indigenous forest canopy tree species, including the commercially valuable Ocotea bullata, Podocarpus falcatus and P. latifolius, regenerated and became established in the understorey of A. melanoxylon in the Blackwood Groups. Crown volume is significantly related to DBH and total tree height. The large emergent crown and very shallow root system make the large trees very susceptible to strong gusty winds. It is concluded that the ecological characteristics and requirements of the shade-intolerant blackwood should be considered in relation to the ecological processes of disturbance and recovery of the forest to reconcile its production from the forest as timber tree with control of its invasion potential. Recommendations are made to consider the species as a useful, fast-growing, shade-intolerant pioneer tree which requires large gaps for optimum timber production, but whose spread can be controlled by keeping canopy gaps to a minimum.

Regeneration of canopy tree species at five sites in West African moist forest

Population structures were drawn for selected West African rain forest canopy tree species to evaluate whether regeneration was present. Regional variability was studied for five sites near the border between Liberia and Côte d'Ivoire.Population structures were highly variable. Three major types of population structures are recognised: a decrease in number of individuals with size, the typically inverse J-shaped curve indicating sufficient regeneration; an increase in number of individuals with size, indicating an absent or sparse regeneration; variable, consisting of strongly fluctuating patterns, in most cases many small individuals, no intermediate and many large ones. At a regional scale, most species show variable population structures. Population structures are static representations of a population composition at a certain moment in time. More long term information on population dynamics is needed to be able to interpret population structures correctly.To sustain the yield from tropical forest a minimum of juvenile trees of commercial species is required to compensate for logging of mature trees. The level of this minimum is hard to indicate without data on population dynamics. Regional variation in population structures warrants the use of local information as a basis for selection criteria of individual trees.

Patterns of Tree Species Richness in Rain Forests of the Middle Caqueta Area, Colombia, NW Amazonia

Tree species, genus, and family richness (DBH 2 10 cm) is recorded in 95 plots of 0.1 ha, situated in most physiographic units in the middle Caqueta area, east of Araracuara, in the Amazon lowlands of Colombia. A positive correlation is present between forest canopy height and tree species density. Highest tree taxon density is recorded in upland (non-flooded) plots with well-drained soils. Lowest tree taxon density is found in poorly drained swamps, while plots on podzolized (white sand) soils show intermediate levels. In well-drained flood plains tree taxon density is lower in comparison with well-drained uplands, but this difference becomes smaller when stem densities are taken into account. Patterns of cumulative numbers of tree taxa in series of plots combined are similar. The results suggest that general patterns of tree taxon richness are mostly related to habitat hostility and environmental stress factors. Fluvial dynamics, between plot soil heterogeneity and beta diversity, play only a subordinate role in the maintenance of the high tree species diversity in the well-drained uplands. Here, tree species density in proportion to stem density is slightly higher in forests on the less nutrient poor, more dayey and reddish soils with thinner humus forms compared to forests on very poor, loamy, and yellowish soils with thicker humus forms. These results may be explained in terms of habitat hostility (stress) or forest dynamics. Gap phase dynamics could be important in maintaining high levels of tree species alpha diversity in well-drained upland forests. Extinction of tree species related to cydic forest disappearances on a regional scale has probably been relatively unimportant.

Effect of Forest Fragmentation on Genetic Diversity and Mating System in a Tropical Tree, Pithecellobium elegans

Genetic diversity, population differentiation, and temporal variation in outcrossing rates were examined for Pithecellobium elegans, a Neotropical rain forest canopy tree. Several forest fragments and a large reserve (1500 ha) were compared for several population genetic parameters. For eight populations sampled on the Atlantic coastal plain of Costa Rica, allozyme heterozygosity (0.13), polymorphism (35%), and effective number of alleles (1.24) were similar to values reported for other tropical tree species that occur at similar densities of less than one individual per hectare. These measures of genetic variation were lowest in populations of the smallest size, farthest from the reserve, and more isolated from other populations. Differentiation among samples collected in small forest fragments and the reserve population accounted for 10% of the total genetic variation observed. There was a positive relationship between the level of differentiation of populations from the reserve population and their distance from the reserve. Though predominantly an annually flowering species, the number of trees in flower at any one time varied from 80% of observed trees to only 6%. Outcrossing rates did not differ for two episodes in which the proportions of flowering trees were 33% and 80%. But periods of low density of flowering adults resulted in poor seed crops or failure to set fruit for many individuals. Population size at many sites will be effectively decreased because of the variation in flowering. Fragmentation of what was once a large, continuous forested area is resulting in genetic erosion of small, isolated populations of Pithecellobium elegans.