The experimental manipulation of atmospheric drought: Teasing out the role of microclimate in biodiversity experiments

Drought occurrence is increasing due to anthropogenic climate change.
Drought can negatively affect plants via reduced water belowground and
increased evaporative demand or vapor pressure deficit (VPD) aboveground.
Past work has shown that plant diversity can ameliorate the negative
effects of drought in plant communities, but these results are
inconsistent between experimental and natural drought studies. Further,
while studies on the negative effects of reduced soil moisture on plant
growth in drought experiments are abundant, the effects of predicted
increases in atmospheric VPD have been neglected. We directly manipulated
atmospheric relative humidity in a biodiversity and drought experiment at
the California State University, Los Angeles (CA, USA) under three
atmospheric conditions (ambient, dehumidified, and humidified), two
treatments of native perennial grass diversity (monoculture and 8 species
polyculture), and two soil drought treatments (control and drought). We
assessed both polyculture plant community and individual species (Poa
secunda) responses to atmospheric drought and soil drought. We found that
soil drought only limits aboveground biomass production when atmospheric
conditions are also dry. We also found that Poa secunda was limited by
increased competition in polyculture when ambient atmospheric conditions
were humid but was facilitated by diversity when atmospheric conditions
were dry.