Recurrent Mild Drought Events Increase Resistance Toward Extreme Drought Stress

The frequency and magnitude of extreme weather events such as drought are expected to increase in the future. At present, plant responses to recurrent extreme events have been sparsely examined and the role of stress history on subsequent stress response has been widely neglected. In a long-term field experiment, we investigated the response of grassland and heath communities to a very severe drought event, which exceeded the duration of projected drought scenarios. During the preceding 6 years, the plant communities experienced scenarios of varying water supply, including annually recurring drought, heavy rain, regular watering, and natural drought periods. Single species and plant communities that were regularly watered in the preceding years revealed the highest tissue die-back under a very severe drought when compared to plants that experienced mild or severe drought stress before. Contrary to expectations, the root to shoot ratio did not increase due to previous recurrent drought occurrences. Furthermore, pre-exposure effects on Vaccinium myrtillus and Plantago lanceolata tissue die-back and reproductive biomass (P. lanceolata) were altered by community composition. Recurrent mild drought stress seems to improve drought resistance of plant communities and species. Potential reasons could be epigenetic changes or soil biotic legacies. Morphological legacies such as altered root to shoot ratio did not play a role in our study. Imprinting events which trigger this ecological stress memory do not have to be extreme themselves. Thresholds, longevity of effects, and the role of biodiversity shown by the importance of community composition require further attention.