No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecological Monographs.Abstract. Seed germination in the seasonal tropical forest on Barro Colorado Island, Panama, was studied at the community level to (1) determine the seasonal timing of germination of the community, (2) identify primary selective factors controlling timing of germination, (3) determine the relative importance of dormancy vs. timing of seed dispersal as mechanisms controlling timing of germination, (4) examine inter-and intraspecific components of variance in length of dormancy, and (5) identify major seed germination syndromes.Three community-level measures of seedling emergence indicated that there was a unimodal community peak in germination within the first 2 mo of the 8-mo-long rainy season. Of the = 185 dicot, mostly woody, species germinating each year, the median time of emergence of 75% of the species occurred within the 1st 3 mo. There was a unimodal peak in germination in pioneer tree species, lianas, canopy trees, wind-and animal-dispersed species, and seedlings with and without persistent seed reserves. In contrast, there was no distinct peak period of emergence in understory species and shade-tolerant tree species. Initial seedling height of species emerging sequentially throughout the rainy season did not increase.The early emergence of most species, including three-fourths of the 35 species fruiting in the late rainy season, indicated that the early rainy season was the optimal time to emerge. Maximizing the length of the first growing season was not an important factor selecting for early emergence, because later-emerging species did not compensate for the shorter growing season by being larger at germination or by having seed reserves for rapid growth. Seedling-seedling competition is a primary biotic factor selecting for early emergence in pioneer species growing in the intensely competitive light-gap habitat. In shaded understory habitats, where competition among seedlings is much less intense, understory and shade-tolerant tree species emerged throughout the rainy season.Mean length of dormancy (MLD), the time between sowing and germination, of 157 woody dicot species on Barro Colorado Island ranged from 2 to 370 d. In over half the species, MLD exceeded 4 wk; hence, delays in germination are common. The season in which seeds were dispersed and the dispersal mechanism explained small but significant portions of the variance in MLD among species; life form explained none; but differences among three germination syndromes explained two-thirds of this variance.In the delayed-rainy syndrome (18% of all species) seeds were dispersed in the rainy...

Summary 1.We used cross-species and phylogenetic analyses to compare seed traits of 36 species with desiccation-sensitive and 189 with desiccation-tolerant seeds from a semi-deciduous forest in Panamá. 2. When correcting for phylogenetic dependence between taxa, the desiccation-sensitive seeds were significantly larger than desiccation-tolerant seeds (3383 vs 283 mg) and typically shed during the wet (as opposed to dry) season. Both traits presumably reduce the rate of seed drying and hence the risk of desiccation-induced mortality for the desiccation-sensitive species. 3. Growing-house germination trials in simulated understorey and canopy gap environments revealed that the desiccation-sensitive species germinated most rapidly. Additionally, on a proportion basis, the desiccation-sensitive seeds allocated significantly less resources to seed physical defences (endocarp and /or testa) which may partially facilitate rapid germination. Both relationships were significant when correcting for phylogenetic dependence and seed mass. 4. Our results suggest that, for large-seeded species which will dry slowly, desiccation sensitivity may be advantageous. Rapid germination may reduce the duration of seed exposure to predation, and the low investment in physical defence means that, per unit mass, desiccation-sensitive seeds are a more efficient use of resources in seed provisioning.

Summary 1.Theoretical models predict that large-seeded species should germinate more rapidly than smallseeded species, since large seeds are more likely to have higher post-dispersal seed predation than small seeds. A prompt germination strategy would therefore enable large seeds to reduce risks of mortality. 2. To assess this predicted relationship between seed mass and mean time to germination (MTG), we used a meta-analysis of published data sources. Our data base contained information for these two traits for 1037 tree species from five tropical areas worldwide (Brazil, India, Ivory Coast, Malaysia and Panama). Both cross-species analyses and phylogenetically independent contrasts (PIC) were conducted on the log-transformed values of seed mass and MTG. 3. Log-seed mass was a significantly phylogenetically conserved trait in all five data sets. Log-MTG was significantly phylogenetically conserved in all sites except for Malaysia and India. 4. Log-MTG and log-seed mass were significantly positively correlated in all sites except for Malaysia. PIC analyses showed a significantly positive relationship in Brazil, India and Ivory Coast but not in Malaysia and Panama. When all sites were combined, PIC analyses indicated a significant positive relationship between these two traits. 5. Our findings do not support the hypothesis that large seeds germinate faster than small seeds, but rather that small seeds germinate faster. We interpret our results in light of phylogenetic and biophysical constraints. We propose four alternative mechanisms that could account for the observed pattern, including developmental constraints, water absorption and investment to physical defences.

This model may have application as a decision-making tool in the handling of species of unknown seed storage behaviour in species from three disparate habitats.

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.

Seasonal changes in the densities of dormant seeds in the soil around eight pioneer trees in the 50-ha Forest Dynamics Plot, on Barro Colorado Island, Panamá were studied, and how seed dispersal and seed dormancy influenced patterns of seed abundance and distribution were examined. Twenty-four, 3-cm-deep soil samples were collected on 30 m transects radiating out from each of the trees in each of four time-intervals through the year, and four 21-cm-deep samples were collected beneath the focal tree crowns. In the surface 0–3 cm of soil, germinable seed densities of all species combined declined from a peak of 1090 seeds m−2 in the mid-wet season in August, to 330 seeds m−2 by the end of the wet season in November. In contrast, at soil depths >3 cm, there was little variation in soil seed bank density through the year. Some variation in soil seed bank density for individual species could be accounted for by distance to reproductive conspecifics. Among species, abundance in the soil was negatively correlated with seed size. Seed persistence varied greatly among species at this site; after 1 y of burial in mesh bags, seed germinability of four species was near zero, while four other species showed no consistent decline in seed germinability after >2 y of burial. For at least one species, Trema micrantha, prolonged seed dormancy was also possible under natural conditions. Twenty-five percent of Trema seeds extracted from the soil at a site occupied by an isolated Trema tree that died between 1982 and 1985 were still germinable in 1994.

The tropical forests of Borneo and Amazonia may each contain more tree species diversity in half a square kilometre than do all the temperate forests of Europe, North America, and Asia combined. Biologists have long been fascinated by this disparity, using it to investigate potential drivers of biodiversity. Latitudinal variation in many of these drivers is expected to create geographic differences in ecological and evolutionary processes, and evidence increasingly shows that tropical ecosystems have higher rates of diversification, clade origination, and clade dispersal. However, there is currently no evidence to link gradients in ecological processes within communities at a local scale directly to the geographic gradient in biodiversity. Here, we show geographic variation in the storage effect, an ecological mechanism that reduces the potential for competitive exclusion more strongly in the tropics than it does in temperate and boreal zones, decreasing the ratio of interspecific-to-intraspecific competition by 0.25% for each degree of latitude that an ecosystem is located closer to the Equator. Additionally, we find evidence that latitudinal variation in climate underpins these differences; longer growing seasons in the tropics reduce constraints on the seasonal timing of reproduction, permitting lower recruitment synchrony between species and thereby enhancing niche partitioning through the storage effect. Our results demonstrate that the strength of the storage effect, and therefore its impact on diversity within communities, varies latitudinally in association with climate. This finding highlights the importance of biotic interactions in shaping geographic diversity patterns, and emphasizes the need to understand the mechanisms underpinning ecological processes in greater detail than has previously been appreciated.