Effects of extreme rainfall on biotic interactions in grassland ecosystems: The importance of waterlogging timing and frequency

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Extreme rainfall events and subsequent waterlogging are going to increase in frequency and duration under climate change in many parts of the world, including Central Europe. This will not only directly affect single species or individuals, but will also lead to shifts in biotic interactions and will thus feedback on plant productivity and fitness. However, knowledge about the effects of climate change on biotic interactions and specifically of waterlogging on biotic interactions is largely missing. Further, after-effects of extreme weather events on communities and interactions, as well as the role of timing of events for community response are neglected. Changes found as an immediate response can change over time, as preconditions for biotic interactions are also altered by extreme weather events. Direct effects on one particular interaction, e.g. loss of mycorrhiza may subsequently affect other biotic interactions after a time lag, e.g. herbivory or pollination. More frequent, repeated waterlogging or stress events may thus lead to totally different responses when compared to effects of a single disturbance, as they act upon already shifted biotic interactions. The timing of waterlogging events will also play a large, yet still unstudied role for ecosystem response. In a controlled mesocosm experiment incorporating a mesic grassland diversity gradient (1, 6, 9 species) and manipulating waterlogging events both in timing (spring vs. autumn) and in frequency/repetition (single vs. repeated waterlogging) I aim to i) investigate the effects of waterlogging on plants and plant communities and on biotic interactions (plant-decomposer, plant-pollinator, plant-mycorrhiza, plant-herbivore, plant-pathogen and plant-plant interactions)ii) disentangle direct effects of waterlogging on single actor from effects mediated by shifts in plant leaf qualityiii) assess the longevity of shifts in biotic interactions and possible feedback on primary production, herbivory, decomposition and community composition and diversityiv) study the role of timing of waterlogging for system response and compare the response towards single vs. repeated waterlogging eventsIn the proposed project, numerous biotic interactions will be assessed simultaneously and over several growing seasons, comparing effects of spring vs. autumn waterlogging and comparing effects of single vs. repeated waterlogging. Mesic grassland is chosen as study system, because of its great agricultural importance in Central Europe and because it might be prone to more intense waterlogging in the future, but is currently not adapted to extreme waterlogging. The overall aim is to identify drivers of ecosystem response towards stress in biotic interactions (e.g. which interactions are resilient and which not, or which changes in interactions amplify or buffer effects over a longer time-period) and to investigate cascading of effects over interaction levels and over time.