1.Trait-based models of ecological communities typically assume intraspecific variation in functional traits is not important, though such variation can change species trait rankings along gradients in resources and environmental conditions, and thus influence community structure and function. 2. We examined the degree of intraspecific relative to interspecific variation, and reaction norms of 11 functional traits for 57 forest understory plant species, including: intrinsic water-use efficiency (iWUE), Δ15N, 5 leaf traits, 2 stem traits and 2 root traits along gradients in light, nitrogen, moisture and understory cover. 3. Our results indicate that interspecific trait variation exceeded intraspecific variation by at least 50% for most, but not all traits. Intraspecific variation in Δ15N, iWUE, leaf nitrogen content and root traits was high (47-70%) compared with most leaf traits and stem traits (13-38%). 4. Δ15N varied primarily along gradients in abiotic conditions, while light and understory cover were relatively less important. iWUE was related primarily to light transmission, reflecting increases in photosynthesis relative to stomatal conductance. Leaf traits varied mainly as a function of light availability, with some reaction norms depending on understory cover. Plant height increased with understory cover, while stem specific density was related primarily to light. Resources, environmental conditions and understory cover did not contribute strongly to the observed variation in root traits. 5. Gradients in resources, environmental conditions and competition all appear to control intraspecific variability in most traits to some extent. However, our results suggest that species cross-over (i.e., trait rank reversals) along the gradients measured here are generally not a concern. 6. Intraspecific variability in understory plant species traits can be considerable. However, trait data collected under a narrow range of environmental conditions appears sufficient to establish species rankings and scale between community and ecosystem levels using trait-based models. Investigators may therefore focus on obtaining a sufficient sample size within a single set of conditions rather than characterizing trait variation across entire gradients in order to optimize sampling efforts.

Burton_etal_TraitDataWhole plant, leaf, stem and root trait data for 57 forest understory species from western Oregon, USA. Growth forms are: F = fern, G = graminoid (grasses, sedges and rushes), H = herbaceous forbs, LS = low clonal shrubs, TS = tall shrubs. GLI = gap light index (%). Total_pct_cover = total cover of understory vegetation surrounding specimen; soil__n = total soil nitrogen (%); CMD = climatic moisture deficit (mm); DELTA15N = Δ15N = δ15Nleaf - δ15Nsoil; iWUE = intrinsic water use efficiency (A/g); SLA = specific leaf area (mm2/mg); LNC = leaf nitrogen content (mg/g); Narea = leaf nitrogen per area (g/m2); LDMC = leaf dry matter content; leaf size (cm2); SSD = stem specific density (mg/mm3); SRL = specific root length (m/g).