Heterogeneity is an intrinsic characteristic of soils, which can significantly affect plant diversity and ecosystem functioning. Under ongoing climate change, these heterogeneity effects may change in turn. Here, we specifically focused on the interactions between soil heterogeneity and drought, as drought events are expected to increase in intensity and/or duration in many regions under climate change. We created soil heterogeneity in three dimensions in experimental mesocosms using a recently developed technique. Nutrient-rich and nutrient-poor substrates were added layer by layer, in all dimensions, in wooden boxes of 48 cm × 48 cm × 48 cm, resulting in four levels of cell size (0, 12, 24 to 48 cm). Smaller cell sizes represent higher heterogeneity. A seed mixture of 24 species that naturally occur in grasslands in Western Europe was evenly sowed on each mesocosm in May 2016. Temporary rainout shelters were installed on three of the six plots to impose a three-week long drought in August 2016. At the end of drought, irrigation was applied and recovery was studied until November 2016. We measured soil water content (SWC), canopy temperature, canopy browning and canopy greenness of each mesocosm during drought, and soil water content and canopy greenness during recovery. Shoot and root biomass were harvested after drought and shoot biomass was collected again after recovery.
Results/Conclusions
During the drought, SWC decreased more slowly at large (48 cm) than at small (0-12-24 cm) cell size, which coincided with a slower loss of canopy greenness. These responses mainly originated from nutrient-poor substrate, whereas the rate of decline of SWC and canopy greenness on nutrient-rich substrate was indifferent to cell size. The slower decrease of SWC and canopy greenness on large nutrient-poor cells matches the smaller shoot and root biomass there compared with smaller nutrient-poor cells, which in turn can be explained by less easy root access on these large nutrient-poor cells to resources in nutrient-rich cells. After the drought, plants recovered faster on nutrient-rich than on nutrient-poor substrate, probably, owing to the greater availability of resources. Overall, our results indicate that soil heterogeneity can modulate the responses of plant communities to drought events, and should thus be considered when assessing climate change impacts in terrestrial ecosystems.