PS 26-61 - The experimental manipulation of atmospheric drought disentangles the role of microclimate in biodiversity experiments

Background/Question/Methods

Drought frequency is expected to increase within the next twenty years. Drought negatively affects plants via reduced soil moisture availability below-ground and increased evaporative demand or vapor pressure deficit (VPD) above-ground. Past studies show that plant diversity can ameliorate the negative effects of drought, but these results are inconsistent between experimental and natural drought studies. Furthermore, while experimental drought studies that examine the effects of reduced soil moisture on plant growth are abundant, the effects of predicted increases in atmospheric VPD have been neglected. We propose a novel methodology as an approach to revise experimental drought studies. We manipulated atmospheric relative humidity in a biodiversity and drought experiment at California State University, Los Angeles under three atmospheric conditions (ambient, dehumidified, and humidified), two treatments of grass diversity (monoculture and eight species polyculture) and two soil drought treatments (control and drought). We assessed polyculture and individual species (Poa secunda) responses to atmospheric drought and soil drought. We examined the following hypotheses: (1) high diversity plant communities are less negatively affected by drought (due to soil drought or atmospheric drought), and (2) microclimate amelioration in high diversity plant communities will protect individual species from the negative effects of atmospheric drought (increased VPD).

Results/Conclusions

We found that soil drought only limits above-ground biomass production when atmospheric conditions are also dry. We also found that an individual species (P. secunda) was limited by increased competition in polyculture when ambient atmospheric conditions were humid but was facilitated by diversity when atmospheric conditions were dry. This study highlights that higher diversity ecosystems may be capable of protecting individual species from the negative effects of drought via facilitation. Without careful experimental manipulation of atmospheric drought, this important mechanism will be missed.