Functional traits are moderate predictors of above- and belowground biomass in multispecies seagrass habitats

Seagrasses are important ecosystem engineers that maintain biodiversity
and modify the abiotic and biotic environment. At present, we are lacking
a wider understanding on the functional traits that predict seagrass
biomass stock, whether trait-biomass associations vary across multispecies
seagrass habitats and which biodiversity mechanisms explain variation in
ecosystem functions in seagrass ecosystems. To explore which traits
predict biomass, we conducted a field survey along 1500 km coastline of
Western Australia, where species-rich seagrass meadows are common. We
sampled multispecies meadows at 14 sites in coastal embayments or
estuarine habitats and measured seven morphological and biochemical traits
of multiple species. Our aim was to explore the functional structure of
seagrass communities in coastal embayments and estuaries and investigate
how various components of diversity (species richness, community-weighted
mean traits (CWM), and functional dispersion (FDis)) predict above- and
belowground biomass in multispecies seagrass habitats by using piecewise
structural equation modelling.  Trait-biomass associations ranged
from strong (standardized path coefficient 0.5) to weak (< 0.2).
More traits predicted belowground than aboveground biomass, and the total
explained variance was higher when conducting separate analysis for
coastal embayments compared to including both seagrass habitats.
Site-level variation accounted for the largest part of the explained
variation in biomass stock as the overall explanatory power of traits to
biomass was low (r2 < 0.3). For individual traits, mass ratio
effects (CWM) primarily predicted biomass in both coastal embayments and
estuaries, and species were functionally similar (low FDis).  Our
study concludes that functional traits act as moderate predictors of
biomass stock across multispecies temperate seagrass habitats, but
environmental context is of more importance. Our results further
demonstrate that the main biodiversity mechanism driving biomass
allocation in multispecies seagrass communities is through dominance
rather than complementarity, and co-existing species show similarity in
their functional traits. The predictive strength of individual traits to
biomass varied between different seagrass habitats, indicating
context-dependency in trait-biomass associations. More research is needed
to understand how patterns in functional diversity are regulated by the
environment and how such patterns relate to other ecosystem properties and
services sustained by these important ecosystems.