OOS 18-5 - Experimental warming and drought may accelerate and extend leafing phenology while delaying and shortening some plants' reproductive phases in tallgrass prairies

Background/Question/Methods
Climate change increasingly threatens the functioning of grassland ecosystems, likely leading to higher average temperatures and more extreme drought events in the Midwestern U.S. over the next century. Plant phenology is an important contributor to ecosystem functioning, since the timing of leafing can alter soil chemistry while the timing of flowering and fruiting may affect foraging by animals and alter plants’ reproductive life cycles. Previous work indicates that warming may extend the growing season and facilitate earlier flowering while drought may constrain or delay growth. To date, few experiments have explored the simultaneous effects of warming and drought on grassland plant phenology. Using a field experiment in Minnesota, U.S., we investigated the phenological responses of 8 native grassland plant species (separated into grasses and forbs) to fully-crossed warming and drought treatments over 8 growing seasons. Data included weekly phenophase observations and overhead camera images. Warming was simulated using infrared heaters set to +2.5°C above ambient while drought was simulated by installing movable rainout shelters that excluded ~40% of summer precipitation events. We used linear mixed effects modeling to test whether warming, drought, and warming + drought treatments drove earlier phenology or changed the length of leafing, flowering, and fruiting phases.

Results/Conclusions
After accounting for species effects, warming (W) and drought (D) accelerated leaf emergence (W: p<0.01, D: p<0.001) and extended leaf phase duration (W: p<0.001, D: p<0.05). Both treatments also drove changes in flowering (W: p<0.01, D: p<0.001), which was accelerated in most cases but was surprisingly delayed in forbs under warming. Drought had a significant effect on flower duration (p<0.01), which was shortened in forbs and extended in grasses. Warming and drought also altered ripe fruit emergence (W: p<0.001, D: p<0.001). Both treatments delayed fruiting in grasses while warming alone accelerated fruiting in forbs. Fruiting duration was significantly shortened by drought (p<0.001). Other than a significant interaction between warming and drought on fruit emergence by species (p<0.05), no other significant treatment interactions were observed. Year was also a significant predictor of phenophase emergence and length, indicating that treatment effects may differ between warmer/drier or cooler/wetter years and warranting further study. In summary, climate change may alter community-level phenology in prairies by accelerating phase emergence, extending leafing, and shortening reproductive phases. This research has important implications for conservation work that considers how future climate change could alter the ability of prairies to support livestock or pollinators and otherwise maintain ecosystem functioning.