Species and rotation frequency influence soil nitrogen in simplified tropical plant communities

Rhizosphere bacterial communities associated with long-lived perennial prairie plants vary in diversity, composition, and structure

The goal of this research was to investigate the variation in rhizosphere microbial community composition, diversity, and structure among individual Andropogon gerardii Vitman (big bluestem) and Lespedeza capitata Michx. (bush clover). Bacterial communities from the rhizosphere of 10 plants of each species (n = 20 plants total) were explored using a culture-independent pipeline. Microbial communities associated with both host plants had high bacterial diversity within individual plant rhizosphere and taxa unique to individual rhizospheres. Bacterial communities associated with the rhizosphere of A. gerardii were consistently more diverse than those associated with L. capitata, and there were significant differences between plant species in rhizosphere bacterial community composition. Differences included microbial taxa with no known functional relationship with their preferred host species, including sulfide-methylating obligate anaerobes (Holophaga), complete denitrifiers (Rhodoplanes), sludge inhabitants (Ktedonobacter), and nitrate oxidizers (Nitrospira). These results suggest the potential for plant species to have significant impacts on a broad array of ecosystem functions (e.g., cycling of carbon, nitrogen sulfurs, metals, and trace elements) via their selective impacts on soil microbes. However, sequence-based community analysis and the corresponding lack of intact microbial cultures limits understanding of the potential influences of enriched microbial taxa on plant hosts and their roles in ecosystem functioning.

The Sabah Biodiversity Experiment: a long-term test of the role of tree diversity in restoring tropical forest structure and functioning

Relatively, little is known about the relationship between biodiversity and ecosystem functioning in forests, especially in the tropics. We describe the Sabah Biodiversity Experiment: a large-scale, long-term field study on the island of Borneo. The project aims at understanding the relationship between tree species diversity and the functioning of lowland dipterocarp rainforest during restoration following selective logging. The experiment is planned to run for several decades (from seed to adult tree), so here we focus on introducing the project and its experimental design and on assessing initial conditions and the potential for restoration of the structure and functioning of the study system, the Malua Forest Reserve. We estimate residual impacts 22 years after selective logging by comparison with an appropriate neighbouring area of primary forest in Danum Valley of similar conditions. There was no difference in the alpha or beta species diversity of transect plots in the two forest types, probably owing to the selective nature of the logging and potential effects of competitive release. However, despite equal total stem density, forest structure differed as expected with a deficit of large trees and a surfeit of saplings in selectively logged areas. These impacts on structure have the potential to influence ecosystem functioning. In particular, above-ground biomass and carbon pools in selectively logged areas were only 60 per cent of those in the primary forest even after 22 years of recovery. Our results establish the initial conditions for the Sabah Biodiversity Experiment and confirm the potential to accelerate restoration by using enrichment planting of dipterocarps to overcome recruitment limitation. What role dipterocarp diversity plays in restoration only will become clear with long-term results.

Tree species control rates of free-living nitrogen fixation in a tropical rain forest

Tropical rain forests represent some of the most diverse ecosystems on earth, yet mechanistic links between tree species identity and ecosystem function in these forests remains poorly understood. Here, using free-living nitrogen (N) fixation as a model, we explore the idea that interspecies variation in canopy nutrient concentrations may drive significant local-scale variation in biogeochemical processes. Biological N fixation is the largest "natural" source of newly available N to terrestrial ecosystems, and estimates suggest the highest such inputs occur in tropical ecosystems. While patterns of and controls over N fixation in these systems remain poorly known, the data we do have suggest that chemical differences among tree species canopies could affect free-living N fixation rates. In a diverse lowland rain forest in Costa Rica, we established a series of vertical, canopy-to-soil profiles for six common canopy tree species, and we measured free-living N fixation rates and multiple aspects of chemistry of live canopy leaves, senesced canopy leaves, bulk leaf litter, and soil for eight individuals of each tree species. Free-living N fixation rates varied significantly among tree species for all four components, and independent of species identity, rates of N fixation ranged by orders of magnitude along the vertical profile. Our data suggest that variations in phosphorus (P) concentration drove a significant fraction of the observed species-specific variation in free-living N fixation rates within each layer of the vertical profile. Furthermore, our data suggest significant links between canopy and forest floor nutrient concentrations; canopy P was correlated with bulk leaf litter P below individual tree crowns. Thus, canopy chemistry may affect a suite of ecosystem processes not only within the canopy itself, but at and beneath the forest floor as well.

Competition from below for light and nutrients shifts productivity among tropical species

Chance events such as seed dispersal determine the potential composition of plant communities, but the eventual assemblage is determined in large part by subsequent interactions among species. Postcolonization sorting also affects the ultimate composition of communities assembled by people for restoration, horticulture, or conservation. Thus, knowledge of the mechanisms controlling interspecific interactions in plant communities is important for explaining patterns observed in nature and predicting success or failure of utilitarian combinations. Relationships among species, especially those from studies of biological diversity and ecosystem functioning, are largely based on studies of short-lived, temperate-zone plants. Extrapolation to perennial plants in the humid tropics is risky because functional relationships among large-stature species change with time. Shifts in competitive relationships among 3 life forms--trees, palms, and perennial herbs--occurred during 13 yr in experimental tropical ecosystems. In 2 cases the novel competitive mechanism responsible for the shift was reduction in crown volume, and therefore light-capturing capability, of overtopping deciduous trees by intrusive growth from below a palm. In a third case, complementary resource use developed between 2 evergreen life forms (overstory tree and palm), probably because of differential nutrient acquisition. Species-level traits and adequate time for shifts in interspecific relationships to emerge are crucial for predicting community trajectories.