Human actions are causing declines in plant biodiversity, increases in atmospheric CO2 concentrations and increases in nitrogen deposition; however, the interactive effects of these factors on ecosystem processes are unknown. Reduced biodiversity has raised numerous concerns, including the possibility that ecosystem functioning may be affected negatively, which might be particularly important in the face of other global changes. Here we present results of a grassland field experiment in Minnesota, USA, that tests the hypothesis that plant diversity and composition influence the enhancement of biomass and carbon acquisition in ecosystems subjected to elevated atmospheric CO2 concentrations and nitrogen deposition. The study experimentally controlled plant diversity (1, 4, 9 or 16 species), soil nitrogen (unamended versus deposition of 4 g of nitrogen per m2 per yr) and atmospheric CO2 concentrations using free-air CO2 enrichment (ambient, 368 micromol mol-1, versus elevated, 560 micromol mol-1). We found that the enhanced biomass accumulation in response to elevated levels of CO2 or nitrogen, or their combination, is less in species-poor than in species-rich assemblages.

Summary 0We measured competition intensity "CI# between herbaceous vegetation and tree seedlings "Quercus macrocarpa and Q[ ellipsoidalis# along an experimental moistureÐ light gradient[ Contrasting theories were tested by comparing variation in competition intensity to changes in neighbour biomass and resource supply and demand[ 1 CI based on survival was inversely correlated with net soil water supply "gross supply minus demand by herbaceous vegetation#[ CI was not positively correlated with either gross resource supply or neighbour biomass\ contrary to predictions of Grime|s triangular model for plant strategies[ 2 Many of the inconsistencies and con~icting results that have characterized the recent literature on plant competition could be eliminated if changes in competition intensity along a resource gradient are compared with changes in net resource supply rather than changes in productivity or neighbour biomass[ 3 Tree seedling success in savannas and grasslands may be strongly in~uenced by the intensity of competition from herbaceous vegetation[ Factors that reduce soil water content are likely to increase competition intensity "and reduce seedling success# in these environments\ while factors that increase soil water content will favour seedling success through decreased competition for water with herbaceous vegetation[ Keywords] competition\ grassland\ Quercus\ resource demand\ resource gradients\ resource supply\ savanna\ tree regeneration Journal of Ecology "0887# 75\ 541Ð550

Summary• We tested the hypothesis that biological trait-based plant functional groups provide sufficient differentiation of species to enable generalization about a variety of plant ecophysiological traits or responses to nitrogen (N).• Seedlings of 34 North American grassland and savanna species, representing 5 functional groups, were grown in a glasshouse in an infertile soil with or without N fertilization.• Forbs, C 3 and C 4 grasses, on average, had similar relative growth rates (RGR), followed in declining order by legumes and oaks, but RGR varied greatly among species within functional groups. All measured attributes differed significantly among functional groups, of these, only RGR and photosynthesis differed among functional groups in response to N. All groups, except the legumes, had significantly greater photosynthetic and respiration rates at elevated N supply. Principal components analyses and cluster analyses yielded groupings that corresponded only moderately well to the biologically based a priori functional groupings.• Variation in RGR among species and treatments was positively related to net CO 2 exchange (photosynthesis and respiration) and net assimilation rate, but unrelated to leaf area ratio. Photosynthetic and respiration rates were related to tissue %N among treatments and species. Our data indicate that RGR and related traits differ among the functional groups in significant ways, but in a complex pattern that does not yield simple generalizations about relative performance, controls on RGR, or response to resource supply rate. IntroductionThe potential utility of considering plant species within differing functional groups or types (hereafter used interchangeably) has been increasingly examined (Grime, 1979;Pearcy & Ehleringer, 1984;Garnier, 1992;Smith et al ., 1996;Lavorel et al ., 1997;Reich et al ., 1998b;Campbell et al ., 1999;Diaz et al ., 1999; Wand et al ., 1999;Craine et al ., 2002). If the use of functional types allows us to more easily characterize the attributes or responses of vegetation, it will enable higher order conceptual and more complex quantitative models at a range of scales. It is common sense to assume that functional groups will only be of use if the members of one group differ consistently on average from those of another group with respect to a single or set of target traits or responses. As yet, however, there is no definite set of rules that allows us to judge when functional types will be useful and when not.In this paper, we will refer to both traditional a priori groupings based largely on single biological traits of species and to posthoc classification schemes (e.g. plant functional types) that attempt to group plant species based on their responses to specific environmental factors (Lavorel et al ., 1997). The traditional a priori groupings are typically defined by discrete and measurable biological trait differences (e.g. whether a plant fixes nitrogen (N) or not; has perennial woody tissues or not; has a given photosynthetic pathway or not). Thus,...