Ambient conditions influence drought effects in climate change manipulation experiments: Effect sizes, rain-out shelter artifacts, and ways forward
Extreme drought events pose challenges to ecosystem functioning. Ecological response to drought is studied worldwide in a growing number of field experiments by the use of rain-out shelters. Yet, the few emerging meta-analyses face severe challenges in the comparability of the studies. This is potentially because drought stress on plant created by rain-out shelters manipulations are modified by ambient weather conditions outside the shelters. Rain-out shelters are further criticized for creating confounding micro-climatological artifacts e.g. alter radiation and temperature, which may influence plant responses. However, a detailed quantification of the build-up of drought effects within rain-out shelters under opposing outside weather conditions and of micro-climatological artifacts is missing.
Here, we examined the physiological and growth responses of plants in drought manipulation experiments under rain-out shelter, rain-out shelter artifact control, and ambient control during opposing outside micro-climatological conditions. Furthermore, we tested rain-out shelter artifacts on community productivity and whether the effect of drought manipulation on community productivity in long term field experiment can be explained by ambient conditions.
Plants below the rain-out shelters showed lower stomatal conductance, lower effective quantum yield, lower leaf water potential and lower biomass production during warm outside ambient weather conditions with high evaporative demand than during cold conditions with low evaporative demand. Plant responses were correlated with both single and multiple effects of ambient outside micro-climate conditions. Rain-out shelter artifacts on plant responses were non-significant.
We conclude that drought manipulations using the rain-out shelter technique are strongly affected by ambient weather conditions. This finding has implications for the comparability among studies and even between years within single studies. Relating drought responses to continually measured micro-climatological parameters such as air temperature, vapor pressure deficit and soil moisture can facilitate meaningful comparisons and meta-analyses among the numerous existing and upcoming climate change manipulation experiments.