Future climate change scenarios for precipitation predict greater frequency of extreme events, namely drought and heavy rainfall. The common currency for understanding ecosystem functioning among these studies is primary production, and results have been mixed as to whether drought reduces biomass or not. Moreover, climate change experiments in grassland systems have predominantly focused on drought events, with less attention paid to predicted scenarios of heavy rainfall. Combining both ends of precipitation extremes with multiple ecosystem responses may be necessary to adequately understand grassland systems regulation by, and response to, future precipitation regimes. Here, we report on the effects of heavy rainfall relative to previously reported drought effects, both from a six-year experiment beginning in 2005 in a Central European grassland system manipulating rainfall. Grassland communities were planted at multiple taxonomic and functional diversity levels, and drought and heavy rainfall treatments were imposed by simulating 100-year events based on the region’s climate records. We focused holistically on the ecosystem and report on measured response parameters representing five ecosystem function categories: Primary production, water regulation, gas exchange, nutrient cycling, and community response. Data were analyzed with linear mixed-effects modeling accounting for the repeated measures design, and significance evaluated at p<0.05.
Results/Conclusions
Heavy rainfall induced significant responses in 24 of the 30 measured parameters in at least one year. Most striking were the observed increases to all five primary production parameters, due to the fact that drought was previously shown to have no significant effect on any of them despite significant reductions in soil moisture. Additionally, we observed increased soil microbial activity in our experiment under heavy rainfall scenarios. Decomposition rates, mycorrhization rates and increased soil microbial biomass were observed alongside increases to available plant soil nitrogen in the form of nitrate and ammonium. This in turn was accompanied by changes in plant physiology and increased community invasibility, competitive effects, and plant consumer abundance. Collectively, this indicates that mesic temperate grasslands are either water limited, or increased water availability has more immediate impacts on ecosystem function. In our grassland system, response dynamics for heavy rainfall were rapid and promoted increased abundance of organisms across multiple levels of organization. This is in contrast to drought effects, which initiated ecosystem-regulating functions while maintaining levels of plant biomass. This suggests that focusing on drought effects in climate change research misses the potentially more important effect of periods of heavy rainfall in future precipitation regimes.