Climate change will increase global surface temperatures by 1.5°C during the 21st century. As a consequence, Regional Climate Models Atmospheric-Ocean Global Climate Models predict an increase in drought susceptibility across North America in the next 20 years; this is due to reduced precipitation and increased vapor pressure deficit (VPD). Increased drought severity has the capacity to negatively affect plant growth and carbon storage in many different types of ecosystems worldwide. Importantly, modern outdoor drought experiments are largely focused on rainfall reductions, with very few outdoor manipulations of evapotranspiration, humidity, or VPD. Here we describe an outdoor humidity manipulation that was conducted in a mesocosm experiment at California State University Los Angeles. We manipulated native perennial grass diversity at two levels (Poa secunda in monoculture and Poa secunda growing in an 8-species mixture). We increased atmospheric humidity of mesocosms by ~11% using commercial grade humidifiers running for 12 hours per day. We reduced atmospheric humidity using silica desiccant. We also manipulated soil moisture at levels similar to the 50-year average and ~10% of the 50-year average.
We ran this experiment for four years and found that aboveground biomass production was identical in dry soils compared with ambient soils, but only when the atmosphere was humid. When the atmosphere was dry, biomass production was ~50% lower during drought. We found that species interactions (competition and facilitation) also shifted in response to atmospheric drying (but not soil drying). We found that Poa traits shifted towards deeper roots and reduced leaf area, but only when soil drought and atmospheric drought were combined. Overall community composition of our higher diversity mesocosms shifted towards Bromus carinatus during atmospheric drought, but Poa secunda during soil drought. Finally, we found that soil nitrate concentrations were lower in humidified pots, even when no plants were present. This suggests that humidity is sufficient to initiate microbial activity in these dry-adapted ecosystems. The conditions under rainout shelters are not meant to manipulate atmospheric conditions. This may mean that we are missing the strength, shape, or sign of expected responses to drought in the future.