Authors Beatriz Aguirre and Alexandra (Sasha) Wright discuss the findings of their new article – The experimental manipulation of atmospheric drought: Teasing out the role of microclimate in biodiversity experiments.

This article is also part of our upcoming Special Feature on Facilitative Mechanisms!

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Drought occurrence is increasing due to anthropogenic climate change. Drought can negatively affect ecological plant communities via reduced water belowground (soil drought) and increased evaporative demand or vapor pressure deficit (VPD) aboveground (atmospheric drought). Past studies have shown that plant diversity can ameliorate the negative effects of drought in plant communities, however the results to date are inconsistent between experimental and natural drought studies. While studies on the negative effects of reduced soil moisture on plant growth are abundant, the effects of predicted increases in atmospheric VPD have been neglected.

In this study, we figured out how to manipulate atmospheric humidity in an outdoor field experiment for the first time. We directly manipulated atmospheric relative humidity under three atmospheric conditions (ambient, dehumidified, and humidified), (Figure 1) two treatments of native perennial grass diversity (monoculture and 8 species polyculture), and two soil drought treatments (control and drought) (Figure 2).

We found that plant communities produced less biomass when soil moisture was limiting (in the soil drought treatment), and atmospheric conditions were dry (relative humidity was 12.1% lower in the ambient atmospheric drought treatment) (Figure 3). However, when atmospheric humidity was high (as is common under rainout shelters in many drought experiments), the soil moisture manipulation had no effect on community biomass production.

This demonstrates the importance of studying the two components of drought simultaneously. Past drought experiments that only manipulated soil moisture may have been underestimating the negative effects of drought by as much as 50%.

Finally, past focus on water limitation during drought may have also undermined our ability to tease out a new mechanism underlying biodiversity-productivity patterns. Evidence from the stress gradient hypothesis points us towards the importance of aboveground facilitative interactions as an alternative mechanism for the biodiversity-stability hypothesis during drought. Higher diversity communities may ameliorate microclimate conditions (VPD, relative humidity and temperature) more comprehensively than lower diversity communities and this may protect even sensitive species from the most negative effects of drought. We found that our model species, Poa secunda, performed qualitatively better in polyculture (facilitation) when atmospheric conditions were dry (ambient treatment) (Figure 4). However, Poa secunda performed worse in polyculture than in monoculture (competition) when atmospheric conditions were humid. Drought may have overall negative effects on ecosystems, and drive both increased competition and increased facilitation. However, aboveground microclimate amelioration effects (and facilitation) would be missed, in experimental manipulations of drought that only manipulate soil moisture.

Our results showed that in our Mediterranean study system: (1) community biomass production is only limited when soil drought and atmospheric drought (high VPD) occur simultaneously (Figure 3); and (2) individual species are limited by increased competition in polyculture when ambient atmospheric conditions are humid but facilitated by diversity when atmospheric conditions are dry (Figure 4). Overall, our results make an important addition towards our understanding of the role of atmospheric conditions in drought experiments and the mechanisms conferring resistance to high diversity plant communities.

Beatriz A. Aguirre California State University Los Angeles, USA and Cornell University, USA
Alexandra (Sasha) Wright California State University Los Angeles, USA

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You can read the full article by Aguirre, Hsieh, Watson & Wright on Journal of Ecology: The experimental manipulation of atmospheric drought: Teasing out the role of microclimate in biodiversity experiments