Community-wide trait means and variations affect biomass in a biodiversity experiment with tree seedlings

The structure of a plant community in terms of functional traits can
strongly affect community productivity. Two components may contribute to
this, community-wide trait means (mass-ratio hypothesis) or community-wide
trait variations (diversity hypothesis) across species and individuals. We
compared the explanatory power of the two hypotheses for explaining
biomass variation among individuals and communities as measures of
productivity. We set up experimental communities of tree seedlings that
ranged in species richness from 1‒8. We measured above- and belowground
biomass, leaf area, specific leaf area and height of all individuals
separately after 16 months of growth and calculated species-level values
by averaging across all individuals of each species grown in monoculture.
Using species- and individual-level trait data, we calculated community-
(CWM) and neighbourhood-weighted mean (NWM) values to assess the
mass-ratio hypothesis and community functional dissimilarity (FD) and
average trait dissimilarity of target individuals to neighbours (ATD),
i.e. trait variations, to assess the diversity hypothesis. CWMs explained
much more variation in community biomass than did species richness or FD,
supporting the mass-ratio hypothesis. The explanatory power of CWMs
increased when calculated with individual-level data, demonstrating
positive effects of trait adjustments of individuals in response to
species richness, i.e. plasticity in the broad sense. FD calculated with
individual-level data also increased community biomass, lending some
support to the diversity hypothesis. The effects of trait means and
variations on community biomass were related to effects of trait means and
variations on the biomass of individuals. Overall, our results suggest
that trait means rather than trait variations may drive community biomass
productivity at the earliest stage of forest development.