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

Abstract

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, temperatezone 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.complementarity ͉ diversity ͉ ecosystem functioning ͉ fertile soil ͉ plant competition D esign of sustainable ecosystems, whether for biodiversity conservation, economic gain, or restoration of ecosystem services, involves the assembly of plant communities comprising species of high ecological combining ability (1, 2). That combining ability is determined by interspecific interactions and refers to the ways and degree to which common resources, particularly light, water, and mineral nutrients, are shared. Does the nature of plant interactions remain relatively constant over time? If not, prediction of combining ability will require either case-specific long-term observation or thorough understanding of the mechanisms involved.To examine the stability of plant interactions, and to reveal mechanisms driving any changes observed, we constrained the number of perennial life-form groups in simple experimental communities while varying the specific identity of 1 of them. The study site, a young alluvial terrace at La Selva Biological Station in the humid tropical lowlands of Costa Rica, was chosen because of its exceptionally fertile soil and warm, wet climate. By conducting experiments in an environment where plants experienced few abiotic constraints on growth we achieved results quickly and avoided the delays intrinsic to environments where plant growth is slow or where the ecological clock is reset annually.The life forms selected are among those most common in mature forest at La Selva (3) and character...

No evidence of spatial root system segregation and elevated fine root biomass in multi-species temperate broad-leaved forests

“…For example, roots of certain tree species may penetrate into lower mineral soil layers, thereby exploring additional soil volume, or they might specialise on preferential rooting in the organic layer which may not be explored in a comparable intensity by other tree species (e.g. Loreau 1998;Grierson and Adams 1999;Pate and Bell 1999;Ewel and Mazzarino 2008). A higher fine root biomass in multi-species compared to monospecific stands resembles the 'overyielding' phenomenon, which is observed e.g.…”

“…Menalled et al 1998;Ewel and Mazzarino 2008), a number of authors have emphasized the importance of below-ground complementarity of species for their coexistence and for diversity effects on ecosystem functioning (e.g. Hooper and Vitousek 1997;Loreau 1998;Pate and Bell 1999;Ewel and Mazzarino 2008).…”

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

“…[85] is therefore key to predicting and managing the longterm effects of selective logging and enrichment planting on the structure of the forest and on the services that it provides. Predicting which species provide the most complementary community of species therefore remains a key challenge [43]-especially when multiple forest ecosystem functions are considered. We hope the Sabah Biodiversity Experiment will contribute towards achieving this goal for lowland dipterocarp forests.…”

“…Overall, a review of monoculture and polyculture plantations found that mixed species stands tend to be more productive [41]. Apart from these reviews comparing mixedand single-species plantations, the small literature chiefly comprises work from the Neotropics by Ewel and co-worker [42,43] on predicting complementary mixtures of species and a pioneering biodiversity experiment by Potvin and co-workers [44 -46].…”

The relative importance of above- versus belowground competition for tree growth during early succession of a tropical moist forest

“…Combined, these points suggest that a high density of smaller neighbors correlates positively with crown depth, causing competition for light between overlapping crowns of focal trees and their smaller neighbors (Ewel and Mazzarino 2008). An additional and unrelated indication for a relatively limited role of belowground competition comes from the above mentioned allometry study: Averaged across 260 saplings of 12 species, the projected root area was 3.0 (±0.2 SE) times smaller than the projected crown area (M. van Breugel unpublished data).…”