Effects of growing conditions and source habitat on plant traits and functional group definition

1. Plant functional groups are used to describe patterns of community organ ization. However, they are defined either by suites of correlated traits or by species groupings, and the responses of these two definitions to changi ng environmental conditions are unknown. 2, We assessed 14 growth and morphological traits under low- and high-resou rce conditions of 42 annual plant species from two source communities in Is rael that differed in resource availability. As current theory predicts, pl ants growing in the high-resource treatment were larger, had twofold greate r relative growth rate (RGR) and thinner leaves, and allocated less biomass to roots than plants grown in the low-resource treatment. Differences in t hese traits were less consistent between the two source communities. Instea d, taxonomic groups (grasses, legumes and a group of other forbs), regardle ss of source, differed in most characteristics. 3, Three general groups of species (functional groups) were identified in b oth resource treatments using cluster analysis on ail 14 traits. In both re source treatments monocots were almost completely separated into one distin ct cluster, regardless of source habitat, while the two other, mainly dicot , clusters were partially separated by habitat. However, the species compos ition and trait characterization of the dicot clusters differed strongly be tween treatments. Under low-resource conditions the two dicot clusters were separated by size traits and seed mass, but under high-resource conditions , they were separated by above-ground size, morphology and RGR. 4. Principal components analysis demonstrated inconsistency in relationship s among traits and species groupings between treatments. The first two prin cipal components emphasized different aspects of growth depending on the tr eatment; the third axis was defined by growth rates. As with the cluster an alysis, plots of species scores revealed relatively little separation of sp ecies by habitat. 5, The response of each species varies for different traits and with growin g conditions. Variation may differ among species within a functional group, producing different definitions of functional groups under different exper imental conditions. Because most functional group analyses are performed on data collected without manipulation of growing conditions, conclusions con cerning the response of species or communities to changes in environmental conditions may be problematic.