Corine Vriesendorp

Conspecific density dependence in seedlings varies with species shade tolerance in a wet tropical forest

Density-dependent seedling mortality could increase with a species relative abundance, thereby promoting species coexistence. Differences among species in light-dependent mortality also could enhance coexistence via resource partitioning. These compatible ideas rarely have been considered simultaneously. We developed models of mortality as functions of irradiance and local conspecific density (LCD) for seedlings of 53 tropical woody species. Species varied in mortality responses to these factors, but mortality consistently increased with shading and LCD. Across species, density-dependent mortality on a per-neighbour basis was inversely related to species community abundance, but higher LCD in more common species resulted in a weak relationship between species abundance and density-dependent mortality scaled to species maximum LCD. Species mortality responses to shading and maximum LCD were strongly and positively correlated. Our results suggest that species differences in density-dependent mortality are more strongly related to physiologically based life-history traits than biotic feedbacks related to community abundance.

Seasonal drought limits tree species across the Neotropics

Within the tropics, the species richness of tree communities is strongly and positively associated with precipitation. Previous research has suggested that this macroecological pattern is driven by the negative effect of water-stress on the physiological processes of most tree species. This process implies that the range limits of taxa are defined by their ability to occur under dry conditions, and thus in terms of species distributions it predicts a nested pattern of taxa distribution from wet to dry areas. However, this ‘dry-tolerance’ hypothesis has yet to be adequately tested at large spatial and taxonomic scales. Here, using a dataset of 531 inventory plots of closed canopy forest distributed across the Western Neotropics we investigated how precipitation, evaluated both as mean annual precipitation and as the maximum climatological water deficit, influences the distribution of tropical tree species, genera and families. We find that the distributions of tree taxa are indeed nested along precipitation gradients in the western Neotropics. Taxa tolerant to seasonal drought are disproportionally widespread across the precipitation gradient, with most reaching even the wettest climates sampled; however, most taxa analysed are restricted to wet areas. Our results suggest that the ‘dry tolerance’ hypothesis has broad applicability in the worlds most species-rich forests. In addition, the large number of species restricted to wetter conditions strongly indicates that an increased frequency of drought could severely threaten biodiversity in this region. Overall, this study establishes a baseline for exploring how tropical forest tree composition may change in response to current and future environmental changes in this region.

Size of sampling unit strongly influences detection of seedling limitation in a wet tropical forest.

Seedling limitation could structure communities, but often is evaluated with sampling units that are orders of magnitude smaller than mature plants. We censused seedlings for 5.5 years in five 1 x 200-m transects in a wet Neotropical forest. For 106 common species (> or = 10 seedlings in a transect), we calculated prevalence (occurrence of > or = 1 newly emerged seedlings per sampling unit) at 1 m(2) and at 1 m x mature crown diameter units by aggregating adjacent quadrats. For most species, prevalence was 2-25% at 1 m(2), but 20-92% at mature crown scales. Increased prevalence arose from broadly distributed seedlings within transects, with unoccupied segments generally shorter than crown diameters. At the landscape scale, 69% of 301 species were locally rare (< 10 seedlings) and only 16% were represented in all transects (maximally separated by 2.4 km). Nonetheless, for more common species, much lower estimates of seedling limitation at mature crown scales suggest weaker influence of seedling limitation on community dynamics than previously assumed.

Threats to intact tropical peatlands and opportunities for their conservation

Abstract Large, intact areas of tropical peatland are highly threatened at a global scale by the expansion of commercial agriculture and other forms of economic development. Conserving peatlands on a landscape scale, with their hydrology intact, is of international conservation importance to preserve their distinctive biodiversity and ecosystem services and maintain their resilience to future environmental change. We explored threats to and opportunities for conserving remaining intact tropical peatlands; thus, we excluded peatlands of Indonesia and Malaysia, where extensive deforestation, drainage, and conversion to plantations means conservation in this region can protect only small fragments of the original ecosystem. We focused on a case study, the Pastaza‐Marañón Foreland Basin (PMFB) in Peru, which is among the largest known intact tropical peatland landscapes in the world and is representative of peatland vulnerability. Maintenance of the hydrological conditions critical for carbon storage and ecosystem function of peatlands is, in the PMFB, primarily threatened by expansion of commercial agriculture linked to new transport infrastructure that is facilitating access to remote areas. There remain opportunities in the PMFB and elsewhere to develop alternative, more sustainable land‐use practices. Although some of the peatlands in the PMFB fall within existing legally protected areas, this protection does not include the most carbon‐dense (domed pole forest) areas. New carbon‐based conservation instruments (e.g., REDD+, Green Climate Fund), developing markets for sustainable peatland products, transferring land title to local communities, and expanding protected areas offer pathways to increased protection for intact tropical peatlands in Amazonia and elsewhere, such as those in New Guinea and Central Africa which remain, for the moment, broadly beyond the frontier of commercial development.

Seedling survival responses to conspecific density, soil nutrients, and irradiance vary with age in a tropical forest.

Understanding processes that promote species coexistence is integral to diversity maintenance. In hyperdiverse tropical forests, local conspecific density (LCD) and light are influential to woody seedling recruitment and soil nutrients are often limiting, yet the simultaneous effects of these factors on seedling survival across time remain unknown. We fit species- and age-specific models to census and resource data of seedlings of 68 woody species from a Costa Rican wet tropical forest. In decreasing order of prevalence, seedling survivorship was related to LCD, soil base cations, irradiance, nitrogen, and phosphorus. Species-specific responses to factors did not covary, providing evidence that species life history strategies have not converged to one continuum of high-surviving stress tolerant to low-surviving stress intolerant species. Survival responses to all factors varied over the average seedlings lifetime, indicating seedling requirements change with age and conclusions drawn about processes important to species coexistence depend on temporal resolution.