Composition Of Tropical Forests Research Articles

The Abundance of Large Ateline Monkeys is Positively Associated with the Diversity of Plants Regenerating in Neotropical Forests

Ateline monkeys, the largest primates in the Neotropics, may disperse more than one million seeds/km 2 /d at sites where they are abundant, but it is unclear whether a reduction in their populations can alter plant diversity patterns. The species richness and composition of regenerating plants as a proxy of future plant communities were studied by comparing 16 sites with different ateline abundance in three countries in northwestern South America. A total of 3658 plots included 94,340 regenerating plants, which were assigned to species or morphospecies. Paired t-tests comparing sites in the same region but with different densities of atelines, and regression analyses showed a consistent positive relationship between ateline density and plant diversity. These results were due to the larger number of stems per area and higher evenness at sites with more atelines, suggesting higher recruitment rates for dispersed seeds. Differences were also found in plant composition, as canopy, endozoochorous, and medium seed size plants were consistently more abundant in sites with more ateline monkeys than in sites with less atelines. The findings of this study suggest that these primates play a key role in plant regeneration. In order to maintain the diversity and plant composition of tropical forests for future generations, conservation of these large frugivores and other key game species is imperative.

Increasing liana abundance and biomass in tropical forests: emerging patterns and putative mechanisms

Tropical forests are experiencing large-scale structural changes, the most apparent of which may be the increase in liana (woody vine) abundance and biomass. Lianas permeate most lowland tropical forests, where they can have a huge effect on tree diversity, recruitment, growth and survival, which, in turn, can alter tree community composition, carbon storage and carbon, nutrient and water fluxes. Consequently, increasing liana abundance and biomass have potentially profound ramifications for tropical forest composition and functioning. Currently, eight studies support the pattern of increasing liana abundance and biomass in American tropical and subtropical forests, whereas two studies, both from Africa, do not. The putative mechanisms to explain increasing lianas include increasing evapotranspirative demand, increasing forest disturbance and turnover, changes in land use and fragmentation and elevated atmospheric CO₂. Each of these mechanisms probably contributes to the observed patterns of increasing liana abundance and biomass, and the mechanisms are likely to be interrelated and synergistic. To determine whether liana increases are occurring throughout the tropics and to determine the mechanisms responsible for the observed patterns, a widespread network of large-scale, long-term monitoring plots combined with observational and manipulative studies that more directly investigate the putative mechanisms are essential.

Plant–soil associations in a lower montane tropical forest: physiological acclimation and herbivore‐mediated responses to nitrogen addition

Summary 1. Soil nutrients influence plant productivity and community composition in tropical forests. In lower montane tropical forests in western Panama, the distribution of understory palm species over a scale of 1–20 km correlates with differences in soil nitrogen (N). We hypothesized that soil N determines seedling performance in the forest understory, and, may therefore influence species distributions along the soil N gradient. 2. We explored the potential for N availability to generate species‐habitat associations through species‐specific differences in biomass allocation, photosynthetic capacity, N use‐efficiency, and susceptibility to herbivory. Seedlings of nine palm species from two sub‐families and four habitat types were transplanted into N‐addition and control plots at a low N site. Growth, mortality, biomass allocation, photosynthesis, foliar N content and herbivory were measured over 21 months. 3. Foliar N increased for all species (15–68%) following N addition. Most species showed strong (20–200%) increases in photosynthetic rates with N addition except two species with marginal decreases in photosynthetic rates (5–15%). However, shifts in physiological traits did not increase relative growth rate or change in biomass allocation for any species or N treatment combination. Rather, increased leaf quality contributed to greater levels of herbivory in species associated with soils of intermediate and high inorganic N availability. 4. Thus, potential increases in overall growth with N addition were masked by herbivory, resulting in no apparent growth response with increased N. We suggest that for understory palms, and potentially other montane forest plants, distribution patterns are driven by a combination of physiological and herbivore‐mediated responses to soil nutrient availability.

Drought–mortality relationships for tropical forests

*The rich ecology of tropical forests is intimately tied to their moisture status. Multi-site syntheses can provide a macro-scale view of these linkages and their susceptibility to changing climates. Here, we report pan-tropical and regional-scale analyses of tree vulnerability to drought. *We assembled available data on tropical forest tree stem mortality before, during, and after recent drought events, from 119 monitoring plots in 10 countries concentrated in Amazonia and Borneo. *In most sites, larger trees are disproportionately at risk. At least within Amazonia, low wood density trees are also at greater risk of drought-associated mortality, independent of size. For comparable drought intensities, trees in Borneo are more vulnerable than trees in the Amazon. There is some evidence for lagged impacts of drought, with mortality rates remaining elevated 2 yr after the meteorological event is over. *These findings indicate that repeated droughts would shift the functional composition of tropical forests toward smaller, denser-wooded trees. At very high drought intensities, the linear relationship between tree mortality and moisture stress apparently breaks down, suggesting the existence of moisture stress thresholds beyond which some tropical forests would suffer catastrophic tree mortality.

DNA Barcoding—a Windfall for Tropical Biology?

'I AM STANDING IN A COSTA RICAN RAIN FOREST', writes tropical ecologist and conservationist Dan Janzen (2005). 'There are a thou sand species of plants within a long stone's throw. Nearly every one of them is a described species with a proper scientific name, a handle that you can plug into Google and come up with something.... And I cannot identify a single species. Imagine what it would do to any and all aspects of human interactions with wild plants if you could walk up to any plant anywhere-seedling, sapling, 40 m tree, grass, root, pressed leaf, or fallen log-and know in a few seconds its scientific name'. Janzen, one of tropical biology's more ardent advocates, po etically describes the taxonomic impediment that exists today for many ecologists and evolutionary biologists working in the field: determining the correct species identification for any plant sample in a fast and repeatable fashion. In fact Janzen knew when he wrote this passage that a rapid and accurate method is now being devel oped and refined for the quick identification of plant species based on extracting DNA sequence from a tiny tissue sample from any part of a plant. Appropriately called 'DNA barcoding', referring to the coded labels one finds on commercial products, DNA barcodes consist of a standardized short sequence of DNA between 400 and 800 base pairs long that can be easily generated and characterized for all species on the planet (Savolainen et al. 2005). These genetic barcodes will be stored in an open access digital library that can be used to compare the DNA barcode sequence of an unidentified sample from the field, garden, or market by matching it to a known sequence with an associated species name in the data base. DNA barcoding has the potential to greatly advance our access to the col lective knowledge of biodiversity and in turn our understanding of Nature. By harnessing advances in molecular genetics, sequencing technology, and bioinformatics, DNA barcoding will allow users to quickly and cheaply recognize known species and retrieve informa tion about them. It may also speed the discovery of the thousands of species yet to be named. Barcoding, if developed sufficiently, will be a vital new tool for appreciating and managing the Earth's immense and changing biodiversity (Cowan et al. 2006). And most of that diversity is in the tropics. Some of the basic questions about tropical ecosystems that biologists have been attempting to answer over the last century re quire an estimate of species diversity. Why do tropical biomes have greater species diversity than temperate zone habitats? How is this species richness maintained in time and space? What are the ecolog ical and evolutionary processes that influence species composition in tropical forests? Answers to some of these broad questions ini tially require accurate assessments of the level of species diversity in a particular habitat. These assessments must have accurate species identifications and, ideally, a uniform species definition. As tropical biologists, we all know how difficult it is to obtain accurate identifi cation of organisms in species-rich tropical forests. DNA barcoding will provide a new tool that will supplement traditional methods and allow us to more independently assess this overwhelming species diversity.

Cyclone damage sustained by riparian revegetation sites in the Tully‐Murray floodplain, Queensland, Australia

Abstract Cyclones have been instrumental in shaping the structural and floristic composition of tropical forests, including tropical rainforests of north Queensland, Australia. The response of tropical riparian rehabilitation sites to cyclonic wind damage, however, is currently unknown. This lack of knowledge may severely hamper long‐term success of riparian restoration efforts, particularly in light of predictions that cyclones in north Queensland may become less frequent but more severe. In this study, we examined the extent, type and magnitude of damage inflicted on revegetation works in the Tully‐Murray floodplain of north Queensland by Severe Tropical Cyclone Larry. We compared wind damaged in 20 paired revegetated and associated rainforest remnant sites, using (i) gross community damage scores, (ii) mean weighted damage scores, and (iii) type of damage sustained by individual plants. Overall, wind damage due to Severe Tropical Cyclone Larry was surprisingly similar in revegetated and remnant sites. Both gross community damage scores and mean weighted damage scores did not differ between paired revegetated and remnant sites. In contrast, the type of damage sustained by individual plants was not independent of site, with a larger proportion in revegetated sites sustaining severe damage compared with remnant sites. This larger proportion of severely damaged individuals in revegetated sites was at least in part due to the significantly higher proportion of pioneers at these sites. The pioneer species Homalanthus novoguineensis was particularly susceptible to wind damage. The potential effects of spatial differences, such as consistent bias due to size, shape or exposure between the remnants and revegetated sites, on our results are discussed. In light of our results, we recommend that future revegetation sites include fewer pioneer species that are highly susceptible to wind damage, more pioneer species that are resistant to wind damage, and alteration of pioneer species distribution within planting matrices.

The influence of climate on the spatial patterning of Neotropical plant families

AbstractAim Climate is recognized for the significant role it plays in the global distribution of plant species diversity. We test the extent to which two aspects of climate, namely temperature and precipitation, explain the spatial distribution of high taxonomic groupings (plant families) at a regional spatial resolution (the Neotropics). Our goal is to provide a quantitative and comparative framework for identifying the local effects of climate on the familial composition of tropical forests by identifying the influence of climate on the number of individuals and the number of species within a given family.Location One hundred and forty‐four 0.1‐ha forest transect sites from the Neotropics (19.8°N–27.0°S and 40.1°W–105.1°W). Data were originally collected by A.H. Gentry.Methods Spatial variability in the abundance (density) and species richness of 159 tropical plant families across a range of predominately lowland Neotropical landscapes were attributed to eight temperature and precipitation measures using the eigen analysis method of two‐field joint single‐value decomposition.Results Climate significantly affects the within‐clade diversity of several ecologically important Neotropical plant families. Intrafamily abundance and richness covary with temperature in some families (e.g. Fabaceae) and with precipitation in others (e.g. Bignoniaceae, Arecaceae), with differing climatic preferences observed even among co‐occurring families. In addition, the family‐level composition of Neotropical forests, in both abundance and richness, appears to be influenced more by temperature than by precipitation. Among lowland families, only Asteraceae increased in species richness with decreasing temperature, although several families, including Melastomataceae and Rubiaceae, are more abundant at lower temperatures.Main conclusions Although plant diversity is known to vary as a function of climate at the species level, we document clear climatic preferences even at the rank of family. Temperature plays a stronger role in governing the familial composition of tropical forests, particularly in the richness of families, than might be expected given its narrow annual and diurnal range in the tropics. Although other environmental or geographic variables that covary with temperature may be more causally linked to diversity differences than temperature itself, the results nonetheless identify the taxonomic components of tropical forest composition that may be most affected by future climatic changes.

Route-based travel and shared routes in sympatric spider and woolly monkeys: cognitive and evolutionary implications

Many wild primates occupy large home ranges and travel long distances each day. Navigating these ranges to find sufficient food presents a substantial cognitive challenge, but we are still far from understanding either how primates represent spatial information mentally or how they use this information to navigate under natural conditions. In the course of a long-term socioecological study, we investigated and compared the travel paths of sympatric spider monkeys (Ateles belzebuth) and woolly monkeys (Lagothrix poeppigii) in Amazonian Ecuador. During several field seasons spanning an 8-year period, we followed focal individuals or groups of both species continuously for periods of multiple days and mapped their travel paths in detail. We found that both primates typically traveled through their home ranges following repeatedly used paths, or "routes". Many of these routes were common to both species and were stable across study years. Several important routes appeared to be associated with distinct topographic features (e.g., ridgetops), which may constitute easily recognized landmarks useful for spatial navigation. The majority of all location records for both species fell along or near identified routes, as did most of the trees used for fruit feeding. Our results provide strong support for the idea that both woolly and spider monkey use route-based mental maps similar to those proposed by Poucet (Psychol Rev 100:163-182, 1993). We suggest that rather than remembering the specific locations of thousands of individual feeding trees and their phenological schedules, spider and woolly monkeys could nonetheless forage efficiently by committing to memory a series of route segments that, when followed, bring them into contact with many potential feeding sources for monitoring or visitation. Furthermore, because swallowed and defecated seeds are deposited in greater frequency along routes, the repeated use of particular travel paths over generations could profoundly influence the structure and composition of tropical forests, raising the intriguing possibility that these and other primate frugivores are active participants in constructing their own ecological niches. Building upon the insights of Byrne (Q J Exp Psychol 31:147-154, 1979, Normality and pathology in cognitive functions. Academic, London, pp 239-264, 1982) and Milton (The foraging strategy of howler monkeys: a study in primate economics. Columbia University Press, New York, 1980, On the move: how and why animals travel in groups. University of Chicago Press, Chicago, pp 375-417, 2000), our results highlight the likely general importance of route-based travel in the memory and foraging strategies of nonhuman primates.

Short dry spells in the wet season increase mortality of tropical pioneer seedlings

Variation in plant species performance in response to water availability offers a potential axis for temporal and spatial habitat partitioning and may therefore affect community composition in tropical forests. We hypothesized that short dry spells during the wet season are a significant source of mortality for the newly emerging seedlings of pioneer species that recruit in treefall gaps in tropical forests. An analysis of a 49-year rainfall record for three forests across a rainfall gradient in central Panama confirmed that dry spells of > or = 10 days during the wet season occur on average once a year in a deciduous forest, and once every other year in a semi-deciduous moist and an evergreen wet forest. The effect of wet season dry spells on the recruitment of pioneers was investigated by comparing seedling survival in rain-protected dry plots and irrigated control plots in four large artificially created treefall gaps in a semi-deciduous tropical forest. In rain-protected plots surface soil layers dried rapidly, leading to a strong gradient in water potential within the upper 10 cm of soil. Seedling survival for six pioneer species was significantly lower in rain-protected than in irrigated control plots after only 4 days. The strength of the irrigation effect differed among species, and first became apparent 3-10 days after treatments started. Root allocation patterns were significantly, or marginally significantly, different between species and between two groups of larger and smaller seeded species. However, they were not correlated with seedling drought sensitivity, suggesting allocation is not a key trait for drought sensitivity in pioneer seedlings. Our data provide strong evidence that short dry spells in the wet season differentially affect seedling survivorship of pioneer species, and may therefore have important implications to seedling demography and community dynamics.

Mapping the species richness and composition of tropical forests from remotely sensed data with neural networks

The understanding and management of biodiversity is often limited by a lack of data. Remote sensing has considerable potential as a source of data on biodiversity at spatial and temporal scales appropriate for biodiversity management. To-date, most remote sensing studies have focused on only one aspect of biodiversity, species richness, and have generally used conventional image analysis techniques that may not fully exploit the data's information content. Here, we report on a study that aimed to estimate biodiversity more fully from remotely sensed data with the aid of neural networks. Two neural network models, feedforward networks to estimate basic indices of biodiversity and Kohonen networks to provide information on species composition, were used. Biodiversity indices of species richness and evenness derived from the remotely sensed data were strongly correlated with those derived from field survey. For example, the predicted tree species richness was significantly correlated with that observed in the field ( r = 0.69, significant at the 95% level of confidence). In addition, there was a high degree of correspondence (∼83%) between the partitioning of the outputs from Kohonen networks applied to tree species and remotely sensed data sets that indicated the potential to map species composition. Combining the outputs of the two sets of neural network based analyses enabled a map of biodiversity to be produced.

The Utility of Spectral Indices from Landsat ETM+ for Measuring the Structure and Composition of Tropical Dry Forests1

ABSTRACTThere is a growing emphasis on developing methods for quantifying the structure and composition of tropical forests that can be applied over large landscapes, especially for tropical dry forests that are severely fragmented and have a high conservation priority. This study investigates the relationships between various measures of forest structure (annual woody increment, canopy closure, stand density, stand basal area) and composition (tree species diversity, tree community composition) measured in semi‐deciduous tropical dry forests on islands in Lago Guri, Venezuela and three spectral indices derived from Landsat ETM+: Normalized Difference Vegetation Index (NDVI), Infrared Index (IRI), and Mid‐Infrared Index (MIRI). Even though there were significant autocorrelations among spectral indices, there were significant differences in the relationships between spectral indices and forest attributes. IRI was not significantly correlated with any of the structural variables while MIRI was correlated with canopy closure and NDVI values were correlated with canopy closure as well as annual woody increment. NDVI and MIRI were both related to relative tree diversity and all three indices were associated with aspects of tree species composition. Based on the results of this study, it appears that spectral indices, and in particular NDVI, may be useful indicators of forest attributes in tropical dry forest habitats. Further research needs to be undertaken to identify if the results of this study can be applied to other tropical dry forests at a global spatial scale.

Links between tree species, symbiotic fungal diversity and ecosystem functioning in simplified tropical ecosystems

We studied the relationships among plant and arbuscular mycorrhizal (AM) fungal diversity, and their effects on ecosystem function, in a series of replicate tropical forestry plots in the La Selva Biological Station, Costa Rica. Forestry plots were 12 yr old and were either monocultures of three tree species, or polycultures of the tree species with two additional understory species. Relationships among the AM fungal spore community, host species, plant community diversity and ecosystem phosphorus-use efficiency (PUE) and net primary productivity (NPP) were assessed. Analysis of the relative abundance of AM fungal spores found that host tree species had a significant effect on the AM fungal community, as did host plant community diversity (monocultures vs polycultures). The Shannon diversity index of the AM fungal spore community differed significantly among the three host tree species, but was not significantly different between monoculture and polyculture plots. Over all the plots, significant positive relationships were found between AM fungal diversity and ecosystem NPP, and between AM fungal community evenness and PUE. Relative abundance of two of the dominant AM fungal species also showed significant correlations with NPP and PUE. We conclude that the AM fungal community composition in tropical forests is sensitive to host species, and provide evidence supporting the hypothesis that the diversity of AM fungi in tropical forests and ecosystem NPP covaries.

Fragment shape and tree species composition in tropical forests: a landscape level investigation

AbstractFragmentation of tropical forest alters community composition as a result of changes in forest shape. This paper uses 22 hypotheses to test the effect of fragment shape on tree species composition in Ghana, West Africa, within biological categories of regeneration guild, rarity, phenology and dispersal. For both regenerating and mature trees, relationships between species composition and the shape of forest fragments were complex; almost half were significant but many failed to support the established hypotheses. Irregular shaped fragments had high proportions of regenerating, light‐demanding pioneers and mature, animal‐dispersed species. Species common to Ghana formed the foundation of communities in fragments of all shapes. Investigation at the landscape level indicated broad patterns of species change. Rigorous hypothesis testing is needed, following extensive demographic work on the ground, before population dynamics within tropical forest fragments can be comprehended fully and applied to conservation management.

Variation in tropical forest growth rates: combined effects of functional group composition and resource availability

Abstract Rates of tree growth in tropical forests reflect variation in life history strategies, contribute to the determination of species' distributional limits, set limits to timber harvesting and control the carbon balance of the stands. Here, we review the resources that limit tree growth at different temporal and spatial scales, and the different growth rates and responses of functional groups defined on the basis of regeneration strategy, maximum size, and species' associations with particular edaphic and climatic conditions. Variation in soil water availability determines intra- and inter-annual patterns of growth within seasonal forests, whereas irradiance may have a more important role in aseasonal forests. Nutrient supply limits growth rates in montane forests and may determine spatial variation in growth of individual species in lowland forests. However, its role in determining spatial variation in stand-level growth rates is unclear. In terms of growth rate, we propose a functional classification of tropical tree species which contrasts inherently fast-growing, responsive species (pioneer, large-statured species), from slow-growing species that are less responsive to increasing resource availability (shade-bearers, small-statured species). In a semi-deciduous forest in Ghana, pioneers associated with high-rainfall forests with less fertile soils, had significantly lower growth rates than pioneers that are more abundant in low-rainfall forests with more fertile soils. These results match patterns found in seedling trials and suggest for pioneers that species' associations with particular environmental conditions are useful indicators of maximum growth rate. The effects of variation in resource availability and of inherent differences between species on stand-level patterns of growth will not be independent if the functional group composition of tropical forests varies along resource gradients. We find that there is increasing evidence of such spatial shifts at both small and large scales in tropical forests. Quantifying these gradients is important for understanding spatial patterns in forest growth rates.

Molecular diversity of arbuscular mycorrhizal fungi and patterns of host association over time and space in a tropical forest.

We have used molecular techniques to investigate the diversity and distribution of the arbuscular mycorrhizal (AM) fungi colonizing tree seedling roots in the tropical forest on Barro Colorado Island (BCI), Republic of Panama. In the first year, we sampled newly emergent seedlings of the understory treelet Faramea occidentalis and the canopy emergent Tetragastris panamensis, from mixed seedling carpets at each of two sites. The following year we sampled surviving seedlings from these cohorts. The roots of 48 plants were analysed using AM fungal-specific primers to amplify and clone partial small subunit (SSU) ribosomal RNA gene sequences. Over 1300 clones were screened for random fragment length polymorphism (RFLP) variation and 7% of these were sequenced. Compared with AM fungal communities sampled from temperate habitats using the same method, the overall diversity was high, with a total of 30 AM fungal types identified. Seventeen of these types have not been recorded previously, with the remainder being similar to types reported from temperate habitats. The tropical mycorrhizal population showed significant spatial heterogeneity and nonrandom associations with the different hosts. Moreover there was a strong shift in the mycorrhizal communities over time. AM fungal types that were dominant in the newly germinated seedlings were almost entirely replaced by previously rare types in the surviving seedlings the following year. The high diversity and huge variation detected across time points, sites and hosts, implies that the AM fungal types are ecologically distinct and thus may have the potential to influence recruitment and host composition in tropical forests.

The Importance of Where to Dump the Refuse: Seed Banks and Fine Roots in Nests of the Leaf‐Cutting Ants Atta cephalotes and A. colombica1

ABSTRACTThe location of the nutrient‐rich organic refuse produced by a leaf‐cutting ant colony varies among ant species. Atta cephalotes locate their organic refuse in subterranean chambers, whereas A. colombica place their organic refuse on the soil surface near the nest. We studied the effect of the absence or presence of external organic refuse on the abundance of fine roots and seed bank composition in the superficial horizons of ant nests. We sampled soils from ant nests or dumps and adjacent areas of 15 adult nests of A. cephalotes at La Selva (LS), Costa Rica, and of 15 of A. colombica nests on Barro Colorado Island (BCI), Panama. Soils from A. cephalotes nests did not differ from adjacent soils in abundance of fine‐root and seed diversity. In contrast, organic refuse from A. colombica nests was less diverse in seed composition (due to the great abundance of Miconia argentea) and had a greater abundance of fine roots than adjacent areas. Thus the external location of the ant‐nest organic refuse is potentially important in determining the different types of plant recolonization in abandoned or dead ant nests. The relative abundance of these Atta species may influence the structure and/or composition of tropical forests.

The Importance of Where to Dump the Refuse: Seed Banks and Fine Roots in Nests of the Leaf-Cutting Ants Atta cephalotes and A. colombica1

The location of the nutrient-rich organic refuse produced by a leaf-cutting ant colony varies among ant species. Atta cephalotes locate their organic refuse in subterranean chambers, whereas A. colombica place their organic refuse on the soil surface near the nest. We studied the effect of the absence or presence of external organic refuse on the abundance of fine roots and seed bank composition in the superficial horizons of ant nests. We sampled soils from ant nests or dumps and adjacent areas of 15 adult nests of A. cephalotes at La Selva (LS), Costa Rica, and of 15 of A. colombica nests on Barro Colorado Island (BCI), Panama. Soils from A. cephalotes nests did not differ from adjacent soils in abundance of fine-root and seed diversity. In contrast, organic refuse from A. colombica nests was less diverse in seed composition (due to the great abundance of Miconia argentea) and had a greater abundance of fine roots than adjacent areas. Thus the external location of the ant-nest organic refuse is potentially important in determining the different types of plant recolonization in abandoned or dead ant nests. The relative abundance of these Atta species may influence the structure and/or composition of tropical forests.

A classification and ordination of the tree community of Tikal National Park, Petén, Guatemala

We studied tropical forest composition and structure in relation to topographic and edaphic variation. with the goal of understanding the factors that determine species distributions and the degree to which species composition can be predicted by local environmental conditions. We sampled the tree community of Tikal National Park in Petén, Guatemala using 294 sample plots of 0.041 ha each, totaling 12.1 ha, placed systematically along topographic gradients. In addition to recording tree and sapling occurrences, we took data on environmental factors, vegetation structure, and disturbance history. Indirect and direct gradient analyses were performed (using DCA and CCA, respectively) to investigate variation along a predominant topographic/edaphic gradient and along gradients of disturbance and light availability. We used TWINSPAN analysis and ranked sorting of samples by environmental variables to produce a forest type classification practical for field use. Individual species distributions were examined with respect to edaphic conditions, understory light availability, and natural forest disturbance. Though topographic relief is moderate in the area, several edaphic factors varied strongly and predictably, though not monotonically, from hillcrests to low-lying depressions, and exerted a predominant influence on tree community composition. Most tree species distributions were strongly correlated with topography and associated edaphic conditions, and distribution patterns were consistent among widely separated topographic gradients, suggesting that environmental conditions rather than historical events, were largely responsible for these patterns. Many of the less shade-tolerant species also showed positive relationships with natural disturbance history and degree of canopy opening. Treefall gaps have different functional significance for regeneration at different points along the topographic/forest type continuum, due to correlated differences in canopy height canopy evenness, and light penetration. Hence, many species that appear to depend on treefall gaps for colonization and recruitment in upland portions of the continuum, are less confined in their exploitation of lower regions of the gradient, where canopy discontinuities may provide sufficient light for seedling persistence and recruitment. Thus many highly light-demanding species were not associated with treefall gaps, and appeared capable of colonizing relatively open lowland forests. Distribution patterns and disturbance/light responses are discussed for individual species, which are provisionally placed into ecological guilds based on apparent tolerances for light levels and edaphic and other environmental factors. In Tikal, much of the natural variation in forest composition and structure can be related to topography and edaphic conditions.

Altitudinal gradients in tropical forest composition, structure, and diversity in the Sierra de Manantlán

1 Data on the composition, structure and diversity of plant communities were gathered along a 1000‐m altitudinal transect from tropical seasonal dry forest to cloud forest on calcareous Cerro Grande in Jalisco, México. 2 A total of 470 species, 292 genera and 103 families of vascular plants occurred in 43 samples of 0.1 ha, stratified at 100‐m elevational intervals between 1500 and 2500 m a.s.1. There were 97 tree species, 76 shrubs, 70 vines, 181 terrestrial herbs, 39 epiphytes, 3 hemiparasites, 3 succulent rosette shrubs and 1 saprophyte. 3 Forest composition varied continuously with altitude, based on the Shipley & Keddy (1987) test, ordination via reciprocal averaging, and elevational trends in the Sorenson similarities of samples at adjacent altitudes. supporting the individualistic hypothesis of plant community organization. 4 Understorey herbs, shrubs and vines showed the greatest decline in species number with increasing altitude. This pattern is hypothesized to result from the more open, more frequently disturbed, and more completely deciduous canopies at lower, drier elevations. The proportion of evergreen woody plants was greater at higher altitudes, reflecting less seasonal aridity and greater soil leaching. The proportion of endozoochorous species increased with altitude, while the proportion of pterochorous and ectozoochorous species decreased, reflecting trends in the hypothesized efficacy of these mechanisms of seed dispersal. 5 Total basal area of woody plants > 2.5 cm d.b.h. and basal area per tree both increased roughly fourfold between 1500 and 2500 m. 6 Species richness decreased sharply with altitude, due mainly to decreases in terrestrial herbs, and (to a lesser extent) shrubs and vines. The average number of species per 0.1 ha declined from 134 at 1500 m to 43 at 2500 m. The numbers of species, genera and families per sample declined linearly with elevation. Species composition of samples within an altitudinal band showed greater horizontal turnover (β diversity) at lower elevations, showing that low‐elevation forests are not only locally more diverse, but spatially more patchy. Community composition varies roughly six times as rapidly with elevation as with the same distance horizontally. 7 A cascading series of effects of elevation on soil fertility, anti‐herbivore defences, and the level of density‐dependent mortality may account for the observed drop in diversity with elevation, and would be consistent with lower β diversity and greater basal area at higher elevations.

Stable carbon isotope variation in C3 and C4 plants along the Amazon River

ALL plants assimilate 12C in preference to 13C. As a result of this isotope fractionation, the tissues of subaerial plants have lower 13C/12C ratios than that of atmospheric CO2. By contrast, plant respiration and tissue decomposition are accompanied by little, if any, fractionation and hence release 13C-depleted biogenic CO2 back into the atmosphere. If this biogenic CO2 is reassimilated before it is thoroughly mixed into the atmosphere, a further depletion of 13C in the plant tissue will result. This recycling effect has been found most often in vertical variations of stable isotope composition in tropical forests where plant tissues near the forest floor are more depleted in 13C (refs 1–5). Here we show that the intensity of biogenic CO2 recycling in flood plain forests of the Amazon systematically increases inland, in the western Amazon basin. We also show that a similar recycling mechanism affects the 13C composition of semiaquatic grasses owing to evasion of biogenic CO2 from the Amazon river. But in this case the degree of recycling is more pronounced in the eastern basin because the flux of 13CO2 out of the river is smaller there. Our data indicate that significant spatial carbon isotope gradients can exist across the same general ecosystem, both between different species and also within a single species. Recycling effects therefore need to be taken into account in studies that try to relate plant carbon composition to animal and human diet, and in those attempting to determine the carbon isotope composition of the ancient atmosphere from preserved plant tissues.