Belowground traits and biomass data from California grassland in 2015

Understanding precipitation controls on functional diversity is important
in predicting how change in rainfall patterns will alter plant
productivity in the future. Trait-based approaches can provide predictive
knowledge about how certain species will behave and interact with the
community. However, how functional diversity relates to above- and
belowground biomass production in variable rainfall conditions remains
unclear. Here, we tested the role of mass ratio and niche complementarity
hypotheses in shaping above- and belowground biomass-functional diversity
relationships in seasonal drought. We implemented a fully crossed
experiment that manipulated drought timing (fall dry, spring dry,
consistent dry, and ambient rainfall) and community composition
(grass-dominated, forb-dominated, and mixed grass-forb) in a California
annual grassland. Plant communities with mixed functional groups showed
higher above- and belowground biomass than either the grass- or
forb-dominant communities. We found divergent functional diversity-biomass
relationships for above- and belowground biomass. Aboveground biomass
decreased with community-weighted means (CWMs) of SLA and height,
supporting the mass ratio hypothesis, which posits that dominant species
with specific traits drive biomass production of the community.
Belowground biomass showed no evidence of either mass ratio hypothesis or
niche complementarity. While biomass was largely unaffected by the timing
of drought in one season, we found community-wide functional trait shifts
in response to rainfall treatments. Aboveground traits shifted to higher
SLA in consistent dry compared to ambient. Belowground traits shifted to
longer, finer and denser roots in fall and consistently dry, and shorter
and coarser roots in spring dry. Functional diversity buffered biomass
production by enabling shifts in above- and belowground functional traits
across variable rainfall conditions.