Three‐dimensional soil heterogeneity modulates responses of grassland mesocosms to an experimentally imposed drought extreme

Heterogeneity is an intrinsic characteristic of soils, which regulates plant diversity and ecosystem functioning. However, whether soil heterogeneity also modulates responses of plant communities to climate change, including climate extremes, remains largely an open question. Here, we explore responses of plant communities to drought extremes across four levels of spatial soil heterogeneity, with cell sizes varying from very small to very large, i.e. 0, 12, 24 and 48 cm. These were created in mesocosms by alternating nutrient-rich and nutrient-poor substrate in three dimensions. A seed mixture of 24 grassland species was evenly sown on each mesocosm in spring. In late summer, a three-week drought was imposed with a rainout shelter. During the drought, soil water content at the mesocosm scale decreased more at intermediate (12 and 24 cm) than at small or large (0 and 48 cm) cell sizes, which was reflected in increased senescence and drought-induced heat stress. These responses could be traced to greater plant biomass coupled with higher water demand at those intermediate cell sizes, likely related to between-cell access to nutrients and effects of diversity and community composition. Our results indicate that soil heterogeneity can modulate the impact of drought extremes on plant communities, though more research is needed on the transition between intermediate and extreme cell sizes, where heterogeneity effects seem to change most. We propose that soil heterogeneity be considered more explicitly in studies of changing precipitation regimes.