On the statistical significance of functional diversity effects

Changes in biodiversity can affect ecosystem processes through a variety of pathways, such as changes in community structure, loss of a keystone or changes in resource use patterns among species. The latter, also known as resource use complementarity, is an established mechanistic link between species and ecosystems. At present, functional group richness is the dominant measure of the extent of resource use complementarity and has been manipulated in several experiments. These groups are constructed a priori using information about differences between species and a statistically significant effect is typically identified by standard parametric tests. These tests implicitly assume that the a priori functional groups are correct. Avoiding this assumption requires a randomization (bootstrap) test of statistical significance that accounts for the effects of grouping per se. This test compares the observed test statistic to the distribution of the test statistic resulting from random assignment of species to groups. Re-analyses of experimental manipulations of plant functional diversity by bootstrapping the critical significance value changed the ecological interpretation of results in nearly half of the experiments. This occurred because random assignment of species to functional groups frequently creates a strong relationship between functional diversity and ecosystem functioning. The significant bootstrapped results that were found perhaps represent some of the most convincing evidence that functional diversity is an important determinant of local-scale ecosystem functionin

Nitrogen pools and fluxes in grassland soils sequestering carbon

Carbon sequestration in agricultural, forest, and grassland soils has been promoted as a means by which substantial amounts of CO2 may be removed from the atmosphere, but few studies have evaluated the associated impacts on changes in soil N or net global warming potential (GWP). The purpose of this research was to ( 1) review the literature to examine how changes in grassland management that affect soil C also impact soil N, ( 2) assess the impact of different types of grassland management on changes in soil N and rates of change, and (3) evaluate changes in N2O fluxes from differently managed grassland ecosystems to assess net impacts on GWP. Soil C and N stocks either both increased or both decreased for most studies. Soil C and N sequestration were tightly linked, resulting in little change in C: N ratios with changes in management. Within grazing treatments N2O made a minor contribution to GWP (0.1-4%), but increases in N2O fluxes offset significant portions of C sequestration gains due to fertilization (10-125%) and conversion (average = 27%). Results from this work demonstrate that even when improved management practices result in considerable rates of C and N sequestration, changes in N2O fluxes can offset a substantial portion of gains by C sequestration. Even for cases in which C sequestration rates are not entirely offset by increases in N2O fluxes, small increases in N2O fluxes can substantially reduce C sequestration benefits. Conversely, reduction of N2O fluxes in grassland soils brought about by changes in management represents an opportunity to reduce the contribution of grasslands to net greenhouse gas forcing