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Seed dispersal is a short-term phenomenon with long-term consequences for population survival and spread. Physiological mechanisms that target the release of seeds to particular sets of environmental conditions that maximize the probability of long-distance dispersal should evolve if long dispersal distance enhances fitness., In this study, we use high-frequency censuses of seeds actually dispersed, high-frequency within-canopy meteorological observations and long-term measurements of above-canopy wind to investigate the environmental conditions that control the timing of seed abscission at different time-scales for a wind-dispersed tropical tree, Luehea seemannii., We show that seed abscission follows a typical seasonal pattern, is rare at night and is most prevalent during periods of prolonged updrafts, higher temperature, with negative feedback when the heat canopy flux is relatively high., We use phenomenological (super- WALD) and mechanistic (coupled Eulerian-Lagrangian closure) models to estimate the relative effects of the timing of seed release at different subannual temporal scales (seconds-hours) on the resulting long-term (season-decade) dispersal kernels. We find that periods of high wind speed increase the probability of long-distance dispersal between 100-1000 m, but decrease the probability at distances further than 1000 m relative to unbiased environmental conditions. We also find abscission during updrafts to increase the probability of long-distance dispersal at distances greater than 100 m., Synthesis: We observe preferential abscission during updrafts in a tropical wind-dispersed tree. We use mechanistic models and long-term wind statistics to estimate the dispersal consequences of preferential seed release in specific environmental conditions. We find that the timing of the dispersal season may be influenced by wind conditions that maximize long-distance dispersal; however, there are likely other environmental factors essential for their determination. Our approach provides a method to bridge between small turbulence scales and large ecosystem scales to predict dispersal kernels. These findings shed light on the evolutionary processes that drive optimization of the timing of seed abscission and may be incorporated into plant population movement models to increase their accuracy and predictive power. [ABSTRACT FROM AUTHOR]

Modern global temperature and land cover and projected future temperatures suggest that tropical forest species will be particularly sensitive to global warming. Given a moderate greenhouse gas emissions scenario, fully 75% of the tropical forests present in 2000 will experience mean annual temperatures in 2100 that are greater than the highest mean annual temperature that supports closed-canopy forest today. Temperature-sensitive species might extend their ranges to cool refuges, defined here as areas where temperatures projected for 2100 match 1960s temperatures in the modern range. Distances to such cool refuges are greatest for equatorial species and are particularly large for key tropical forest areas including the Amazon and Congo River Basins, West Africa, and the upper elevations of many tropical mountains. In sum, tropical species are likely to be particularly sensitive to global warming because they are adapted to limited geographic and seasonal variation in temperature, already lived at or near the highest temperatures on Earth before global warming began, and are often isolated from cool refuges. To illustrate these three points, we examined the distributions and habitat associations of all extant mammal species. The distance to the nearest cool refuge exceeded 1000 km for more than 20% of the tropical and less than 4% of the extratropical species with small ranges. The biological impact of global warming is likely to be as severe in the tropics as at temperate and boreal latitudes. [ABSTRACT FROM AUTHOR]

In closed-canopy forests, plant morphology and physiology determine shade tolerance and potential growth and mortality rates; potential vital rates and ongoing gap dependence determine realized vital rates; and realized vital rates determine individual size distributions. This hypothesis was evaluated for the 73 most abundant canopy tree species from Barro Colorado Island, Panama. The percentage of recruits located in tree-fall gaps (P), sapling growth (G), and mortality (M) rates, and the coefficient of skewness of size distributions (g1) were determined from censuses of all individuals >10 mm dbh in a 50- ha plot. Seven qualitative, bivariate predictions relating g[sub1], G, M, P, and wood density (W) were evaluated. Six of the seven predictions were substantiated in pairwise analyses. A path analysis integrated all seven predictions and explained 51% of the interspecific variation in g[sub1]. Size distributions with many large individuals and a long tail of relatively rare, small individuals (g[sub1] < 0) characterized gap-dependent species with large fecundity, seed mortality, seedling mortality, G, M, and P. Size distributions with many small individuals and a long tail of relatively rare, large individuals (g[sub1] > 0) characterized shade-tolerant species with the opposite traits. The percentage of tropical tree species that require tree- fall gaps to regenerate has been estimated to range from <20% to >70% for old-growth forests. [ABSTRACT FROM AUTHOR]

Despite the potential for changes during transit or preservation, the physicochemical properties of leaves are typically measured in a laboratory setting. A suite of laboratory methods adapted for use in the field is described here. The equipment is portable and operable in remote environments. Each technique has been validated against laboratory standards and has been tested throughout the tropics in a variety of ecological contexts. The properties of canopy and understorey leaves from Central Panama are reported here. The results show clear differences between leaves growing in different light regimes. Canopy leaves are tougher and possess greater concentrations of protein, phenols, and tannins. The implication of these results to the resource availability hypothesis, which proposes trade‐offs between physiology and defences against herbivory, is discussed. [ABSTRACT FROM PUBLISHER]