David F. R. P. Burslem

The interpretation and misinterpretation of mortality rate measures

Ecologists frequently measure and compare mortality rates and other count-dependent rates of change. The simplest measures employ mortality counts for predetermined populations over a defined census interval (e.g. Harper 1977; Putz & Milton 1983; Connell et al. 1984; Hubbell & Foster 1990; Turner 1990; Osunkoya et al. 1992). More complex formulations are required to allow comparison over varying time periods, because these measures require a knowledge or assumption of how probabilities of death change over time. In many ecological applications this probability is taken to be constant and can therefore be used to define a rate. In this paper we examine some potentially misleading discrepancies that occur in the recent ecological literature. In its simplest form a constant mortality is modelled by exponential population decline:

Disturbing hypotheses in tropical forests

The intermediate disturbance hypothesis (IDH) is a controversial explanation for the maintenance of tropical forest tree diversity, but empirical tests of it are rare. Two data-intensive evaluations have recently yielded contradictory outcomes: one for and one against the IDH. We propose that the explanation for these results lies in the subtleties of divergent interpretations and approaches, and in the different characteristics of the study sites. The apparent simplicity of the IDH is deceptive, because a range of distinct phenomena is involved, each of which can be defined and examined. Recent developments offer exciting opportunities for a deeper comprehension of how disturbance influences forest diversity.

GERMINATION ECOLOGY OF NEOTROPICAL PIONEERS: INTERACTING EFFECTS OF ENVIRONMENTAL CONDITIONS AND SEED SIZE

Germination provides many potentially unrecognized sources of variation in the regeneration niche. In this study we relate germination requirements and seed size for 16 species of pioneer trees to microclimatic conditions present in gaps in semi-deciduous rain forest in Panama. We found that, whereas increased duration of direct irradiance can be an effective indicator of the presence of a canopy gap across all scales of canopy openness, diel fluctuations in soil temperature effectively discriminate both understory sites and small gaps (25 m2) from larger gaps. Germination response was significantly related to seed size. Small-seeded species (seed mass 2 mg) germinated equally in light and darkness (wit...

Biotic interactions in the tropics: their role in the maintenance of species diversity.

To understand how tropical ecosystems function we need to appreciate not only what plants, animals and microbes they contain, but how they interact with each other. This volume synthesizes the current state of knowledge of tropical biotic interaction, with chapters providing reviews or case studies drawn from research conducted in both Old and New World tropics, including interactions among taxa at all levels. An underlying theme of the volume is revealing the importance of the maintenance of high diversity in tropical regions.

Topographic position affects the water regime in a semideciduous tropical forest in Panamá

The effects of topographic position on water regime in a semideciduous tropical forest on Barro Colorado Island in Panamá were assessed by measuring soil matric potential using the filter paper technique and by using measured soil water release characteristics to convert a long-term (20 years) gravimetric water content data-set to matric potential. These were also compared against predictions from a simple water balance model. Soil matric potentials on slope sites were significantly higher than on plateau sites throughout the measurement interval and slopes experienced a shorter duration of drought during the annual dry-season. Measured values of matric potential agreed with those predicted from converting the gravimetric measurements using water release characteristics. Annual duration of drought predicted by the simple water balance model agreed with values determined from the converted long term water content data-set and was able to predict the annual duration of drought on plateau sites. On slope sites, the water balance systematically and significantly overestimated the duration of drought obtained from the water content data-set, suggesting that slope sites were supplied with water from upslope. Predictions of annual drought duration from sites with higher annual rainfall than Barro Colorado Island (BCI), suggest that while plateau sites on BCI experience a water regime consistent with annual rainfall, slopes experience a water regime more similar to that of forests with much higher rainfall. We conclude that such large variations in water regime over small spatial scales may play a role in maintaining high species richness through providing opportunities for niche specialisation and by buffering slopes against possible climate change.

Species–habitat associations in a Sri Lankan dipterocarp forest

Forest structure and species distribution patterns were examined among eight topographically defined habitats for the 205 species with stems≥ 1 cm dbh inhabiting a 25-ha plot in the Sinharaja rain forest, Sri Lanka. The habitats were steep spurs, less-steep spurs, steep gullies and less-steep gullies, all at either lower or upper elevations. Mean stem density was significantly greater on the upper spurs than in the lower, less-steep gullies. Stem density was also higher on spurs than in gullies within each elevation category and in each upper-elevation habitat than in its corresponding lower-elevation habitat. Basal area varied less among habitats, but followed similar trends to stem density. Species richness and Fisher’s alpha were lower in the upper-elevation habitats than in the lower-elevation habitats. These differences appeared to be related to the abundances of the dominant species. Of the 125 species subjected to torus-translation tests, 99 species (abundant and less abundant and those in different strata) showed at least one positive or negative association to one or more of the habitats. Species associations were relatively more frequent with the lower-elevation gullies. These and the previous findings on seedling ecophysiology, morphology and anatomy of some of the habitat specialists suggest that edaphic and hydrological variation related to topography, accompanied by canopy disturbances of varying intensity, type and extent along the catenal landscape, plays a major role in habitat partitioning in this forest.

NEIGHBORHOOD AND COMMUNITY INTERACTIONS DETERMINE THE SPATIAL PATTERN OF TROPICAL TREE SEEDLING SURVIVAL

Factors affecting survival and recruitment of 3531 individually mapped seedlings of Myristicaceae were examined over three years in a highly diverse neotropical rain forest, at spatial scales of 1-9 m and 25 ha. We found convincing evidence of a community compensatory trend (CCT) in seedling survival (i.e., more abundant species had higher seedling mortality at the 25-ha scale), which suggests that density-dependent mortality may contribute to the spatial dynamics of seedling recruitment. Unlike previous studies, we demonstrate that the CCT was not caused by differences in microhabitat preferences or life history strategy among the study species. In local neighborhood analyses, the spatial autocorrelation of seedling survival was important at small spatial scales (1-5 m) but decayed rapidly with increasing distance. Relative seedling height had the greatest effect on seedling survival. Conspecific seedling density had a more negative effect on survival than heterospecific seedling density and was stronger and extended farther in rare species than in common species. Taken together, the CCT and neighborhood analyses suggest that seedling mortality is coupled more strongly to the landscape-scale abundance of conspecific large trees in common species and the local density of conspecific seedlings in rare species. We conclude that negative density dependence could promote species coexistence in this rain forest community but that the scale dependence of interactions differs between rare and common species.

Species diversity, susceptibility to disturbance and tree population dynamics in tropical rain forest

. In 1964 a census of all trees > 9.7 cm diameter at breast height (DBH) was conducted on 22 plots totalling 13.2 ha in lowland tropical evergreen rain forest on Kolombangara, Solomon Islands. Over the following 30 yr (1964–1994), populations of all individuals > 4.85 cm DBH of the 12 most common tree species and amounts of disturbance have been monitored on a declining number of these plots (in 1994, nine plots totalling 5.4 ha were still being recorded). Between November 1967 and April 1970, Kolombangara was struck by four cyclones, although only two of these caused substantial amounts of damage to the canopy structure. Multivariate analysis has identified six forest types on Kolombangara (Greig-Smith et al. 1967). The species richness and diversity of trees in the 1964 census, turnover rates of the populations monitored over 1964–1975, and the amount of disturbance sustained during a cyclone in 1970, were all positively correlated across five of the forest types. The sixth forest type was a consistent outlier in these analyses and is believed to have been seriously disturbed by humans about a century ago. The floristics, turnover and disturbance data support Connells intermediate disturbance hypothesis. The most species-rich forest types contained a higher proportion of fast-growing individuals and species that are early-successional and which have low density timber. Properties of these species rendered them more susceptible to damage when struck by the 1970 cyclone. They showed higher turnover rates because disturbance-dependent species are also characterised by higher mortality and recruitment rates. Thus, periodic cyclones appear to favour the maintenance of differences in species diversity and composition between forest types.

EDAPHIC SPECIALIZATION IN TROPICAL TREES: PHYSIOLOGICAL CORRELATES AND RESPONSES TO RECIPROCAL TRANSPLANTATION

Recent research has documented the importance of edaphic factors in de- termining the habitat associations of tree species in many tropical rain forests, but the underlying mechanisms for edaphic associations are unclear. At Sepilok Forest Reserve, Sabah, Malaysian Borneo, two main soil types derived from sandstone (ridges) and alluvium (valleys) differ in nutrient and water availability and are characterized by forests differing markedly in species composition, structure, and understory light availability. We use both survey and reciprocal transplants to examine physiological adaptations to differences in light, nutrient, and water availability between these soil types, and test for the importance of resource-use efficiency in determining edaphic specialization. Photosynthetic surveys for congeneric and confamilial pairs (one species per soil type) of edaphic specialists and for generalists common to both soil types show that species specializing on sandstone- derived soil had lower stomatal conductance at a given assimilation rate than those occurring on alluvial soil and also had greater instantaneous and integrated water-use efficiencies. Foliar dark respiration rates per unit photosynthesis were higher for sandstone ridge than alluvial lowland specialists. We suggest that these higher respiration rates are likely due to increases in photosynthetic enzyme concentrations to compensate for lower internal CO 2 concentrations resulting from increased stomatal closure. This is supported by lower pho- tosynthetic nitrogen-use efficiencies in the sandstone ridge specialists. Generalist species had lower water-use efficiencies than sandstone ridge specialists when growing on the drier, sandy ridgetops, but their nitrogen-use efficiencies did not differ from the species specialized to the more resource-rich alluvial valleys. We varied light environment and soil nutrient availability in a reciprocal transplant experiment involving two specialist species from each soil type. Edaphic specialist species, when grown on the soil type for which they were not specialized, were not capable of acclimatory shifts to achieve similar resource-use effi- ciencies as species specialized to that soil type. We conclude that divergent water-use strategies are an important mechanism underlying differences in edaphic associations and thus contributing to maintenance of high local tree species diversity in Bornean rain forests.

Associations between tree growth, soil fertility and water availability at local and regional scales in Ghanaian tropical rain forest

Relating patterns of species abundance to the processes that determine them at local and regional spatial scales is one of the central aims of ecological research. To examine the relationship between distribution and growth in tropical rain forest, seasonal and annual changes in tree diameter of two tree species with contrasting distribution patterns, Celtis mildbraedii and Strombosia glaucescens, were monitored across topographic gradients in moist semi-deciduous and moist evergreen forest in Ghana over 2 years, 1997-9. Concurrent measurements were made of soil water availability, and during 1997/8, of rainfall, nutrient availability and photosynthetically active radiation (PAR). The two sites received similar, low, total rainfall during 1997/8, but soil water availability in the dry season at the evergreen site exceeded that at the semi-deciduous forest site. Soil water availability was higher during 1998/9 at both sites. During 1997/8, PAR was similar at the two sites. The evergreen site had soils of lower pH, available P, Mg, Ca and K concentrations than the semi-deciduous forest site. Water availability and the sand content of the soil increased and concentrations of total N and C decreased, from summit to valley positions at both sites. Celtis mildbraedii had significantly faster growth rates than S. glaucescens, and growth of both species was greater at the semi-deciduous than the evergreen forest site during the wet year (1998/9) but not during the dry year (1997/8). Celtis mildbraedii in semi-deciduous forest grew less in valley than summit and slope positions. We conclude that in the absence of a severe dry season, growth is higher in semi-deciduous than evergreen forest, and is related to the higher soil fertility in more seasonal forest. The patterns are consistent with the hypothesis that concentrations of available P and/or cations in the soil limit growth in evergreen forest, while concentrations of N in valley soils limit growth of C. mildbraedii in semi-deciduous forest. There was no evidence that variation in PAR influenced growth rates in these forests. A reduction in growth rate on less fertile soils may be a factor determining the distribution limit of dry forest species, such as C. mildbraedii, in evergreen forest.