Ray Dybzinski

From selection to complementarity: shifts in the causes of biodiversity-productivity relationships in a long-term biodiversity experiment.

In a 10-year (1996–2005) biodiversity experiment, the mechanisms underlying the increasingly positive effect of biodiversity on plant biomass production shifted from sampling to complementarity over time. The effect of diversity on plant biomass was associated primarily with the accumulation of higher total plant nitrogen pools (N g m−2) and secondarily with more efficient N use at higher diversity. The accumulation of N in living plant biomass was significantly increased by the presence of legumes, C4 grasses, and their combined presence. Thus, these results provide clear evidence for the increasing effects of complementarity through time and suggest a mechanism whereby diversity increases complementarity through the increased input and retention of N, a commonly limiting nutrient.

Modeling carbon allocation in trees: a search for principles

We review approaches to predicting carbon and nitrogen allocation in forest models in terms of their underlying assumptions and their resulting strengths and limitations. Empirical and allometric methods are easily developed and computationally efficient, but lack the power of evolution-based approaches to explain and predict multifaceted effects of environmental variability and climate change. In evolution-based methods, allocation is usually determined by maximization of a fitness proxy, either in a fixed environment, which we call optimal response (OR) models, or including the feedback of an individuals strategy on its environment (game-theoretical optimization, GTO). Optimal response models can predict allocation in single trees and stands when there is significant competition only for one resource. Game-theoretical optimization can be used to account for additional dimensions of competition, e.g., when strong root competition boosts root allocation at the expense of wood production. However, we demonstrate that an OR model predicts similar allocation to a GTO model under the root-competitive conditions reported in free-air carbon dioxide enrichment (FACE) experiments. The most evolutionarily realistic approach is adaptive dynamics (AD) where the allocation strategy arises from eco-evolutionary dynamics of populations instead of a fitness proxy. We also discuss emerging entropy-based approaches that offer an alternative thermodynamic perspective on allocation, in which fitness proxies are replaced by entropy or entropy production. To help develop allocation models further, the value of wide-ranging datasets, such as FLUXNET, could be greatly enhanced by ancillary measurements of driving variables, such as water and soil nitrogen availability.

Evolutionarily Stable Strategy Carbon Allocation to Foliage, Wood, and Fine Roots in Trees Competing for Light and Nitrogen: An Analytically Tractable, Individual-Based Model and Quantitative Comparisons to Data

We present a model that scales from the physiological and structural traits of individual trees competing for light and nitrogen across a gradient of soil nitrogen to their community-level consequences. The model predicts the most competitive (i.e., the evolutionarily stable strategy [ESS]) allocations to foliage, wood, and fine roots for canopy and understory stages of trees growing in old-growth forests. The ESS allocations, revealed as analytical functions of commonly measured physiological parameters, depend not on simple root-shoot relations but rather on diminishing returns of carbon investment that ensure any alternate strategy will underperform an ESS in monoculture because of the competitive environment that the ESS creates. As such, ESS allocations do not maximize nitrogen-limited growth rates in monoculture, highlighting the underappreciated idea that the most competitive strategy is not necessarily the “best,” but rather that which creates conditions in which all others are “worse.” Data from 152 stands support the model’s surprising prediction that the dominant structural trade-off is between fine roots and wood, not foliage, suggesting the “root-shoot” trade-off is more precisely a “root-stem” trade-off for long-lived trees. Assuming other resources are abundant, the model predicts that forests are limited by both nitrogen and light, or nearly so.

Mechanisms of plant competition for nutrients, water and light

Summary Competition for resources has long been considered a prevalent force in structuring plant communities and natural selection, yet our understanding of the mechanisms that underlie resource competition is still developing. The complexity of resource competition is derived not only from the variability of resource limitation in space and time and among species, but also from the complexity of the resources themselves. Nutrients, water and light each differ in their properties, which generates unique ways that plants compete for these resources. Here, we discuss the roles of supply pre-emption and availability reduction in competition for the three resources when supplied evenly in space and time. Plants compete for nutrients by pre-empting nutrient supplies from coming into contact with neighbours, which requires maximizing root length. Although water is also a soil resource, competition for water is generally considered to occur by availability reduction, favouring plants that can withstand the lowest water potential. Because light is supplied from above plants, individuals that situate their leaves above those of neighbours benefit directly from increased photosynthetic rates and indirectly by reducing the growth of those neighbours via shade. In communities where juveniles recruit in the shade of adults, traits of the most competitive species are biased towards those that confer greater survivorship and growth at the juvenile stage, even if those traits come at the expense of adult performance. Understanding the mechanisms of competition also reveals how competition has influenced the evolution of plant species. For example, nutrient competition has selected for plants to maintain higher root length and light competition plants that are taller, with deeper, flatter canopies than would be optimal in the absence of competition. In all, while more research is needed on competition for heterogeneous resource supplies as well as for water, understanding the mechanisms of competition increases the predictability of interspecific interactions and reveals how competition has altered the evolution of plants.

Soil fertility increases with plant species diversity in a long-term biodiversity experiment

Most explanations for the positive effect of plant species diversity on productivity have focused on the efficiency of resource use, implicitly assuming that resource supply is constant. To test this assumption, we grew seedlings of Echinacea purpurea in soil collected beneath 10-year-old, experimental plant communities containing one, two, four, eight, or 16 native grassland species. The results of this greenhouse bioassay challenge the assumption of constant resource supply; we found that bioassay seedlings grown in soil collected from experimental communities containing 16 plant species produced 70% more biomass than seedlings grown in soil collected beneath monocultures. This increase was likely attributable to greater soil N availability, which had increased in higher diversity communities over the 10-year-duration of the experiment. In a distinction akin to the selection/complementarity partition commonly made in studies of diversity and productivity, we further determined whether the additive effects of functional groups or the interactive effects of functional groups explained the increase in fertility with diversity. The increase in bioassay seedling biomass with diversity was largely explained by a concomitant increase in N-fixer, C4 grass, forb, and C3 grass biomass with diversity, suggesting that the additive effects of these four functional groups at higher diversity contributed to enhance N availability and retention. Nevertheless, diversity still explained a significant amount of the residual variation in bioassay seedling biomass after functional group biomass was included in a multiple regression, suggesting that interactions also increased fertility in diverse communities. Our results suggest a mechanism, the fertility effect, by which increased plant species diversity may increase community productivity over time by increasing the supply of nutrients via both greater inputs and greater retention.

DOES METABOLIC THEORY APPLY TO COMMUNITY ECOLOGY? IT'S A MATTER OF SCALE

al. (2004), there were moments of insight, of enthusiastic consensus, and of strongly divergent opinion. We agreed that the empirical relations and scaling theory of Brown et al. (2004) hold great appeal because of their power to abstract and simplify some of the complexity of nature. The earth harbors several million species, each having unique aspects to its morphology, physiology, and life history. A fundamental goal of science is to simplify and explain such complexity. Brown et al. (2004) do just this. They have documented robust patterns relating the body size and temperature of species to their basal metabolic rate; plotted on loglog scales, these empirical functions are well fit by straight lines. Moreover, they have used these scaling relations to make numerous predictions about other patterns and processes, thus greatly extending an approach that already had been shown to have considerable power (e.g., Huxley 1932, McMahon and Bonner 1983, Peters 1983, May 1986). One question that generated considerable debate among us was whether metabolic scaling theory represents a fundamental mechanism that has shaped life on earth, or whether it is a description of correlated patterns of as yet poorly known causes. Brown et al. (2004) hypothesize that scaling relations have a fundamental basis that comes from the universality of metabolic activation energy and of the fractal branching networks that determine resource distribution within

Resource Use Patterns Predict Long‐Term Outcomes of Plant Competition for Nutrients and Light

An 11‐year competition experiment among combinations of six prairie perennial plant species showed that resource competition theory generally predicted the long‐term outcome of competition. We grew each species in replicated monocultures to determine its requirements for soil nitrate ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Resource limitation in a competitive context determines complex plant responses to experimental resource additions

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