Effects of climate change and grassland management on plant population dynamics are mediated by changes in reproductive success

Climate change is one of the largest threats to grassland plant species, which can be modified by land management. Although climate change and land management can separately and interactively influence plant demography this has been rarely considered within one experimental set-up. We used a large-scale experiment to quantify the effects of grassland management, climate change and their joint effect on the demography and population growth rate of 11 native plant species. We parameterized integral projection models with four years of demographic data to project the population growth rate. We hypothesized, plants would perform better in ambient than in the future climate treatment that creates hotter and drier summer conditions and that plant performance in grazing vs. mowing would vary across species and depend on their traits. Due to extreme drought events, over half of our study species went quasi extinct, which highlights how extreme climate events can influence long term experimental results. Of the persistent species, only one supported our expectations, and the rest either had higher population growth rates in the future climate treatment or showed no significant difference in population growth between climate treatments. Species with shorter flowering durations performed better in the mowing treatment while those with longer flowering durations performed better in the grazing treatment. The population growth rates of these species were sensitive to changes in vital rates related to reproduction and recruitment. Depending on the species we found effects of land management and climate change on population growth rates but we did not find strong support for interactive effects among both factors. Experiments combined with measuring plant demographic responses provide a way to isolate the effects of different drivers on the long-term persistence of species, and to identify the demographic vital rates that are critical to manage in the future. Our study suggests that it will become increasingly difficult to maintain species with preferences for moister soil conditions, that traits such as flowering duration might predict responses to management, and that vital rates such as reproduction and recruitment are disproportionately important.