Previous studies on how climate affects plant phenology have mostly focused on either natural climate gradients or climate manipulation experiments, and focused on just temperature rather than both temperature and soil moisture. To address this, we embedded four climate treatments (control, warming, drought, and warming+precip) at three prairies across a 520 km latitudinal Mediterranean climate gradient within the Pacific Northwest (PNW) to examine the responses of plant phenology at both the population and community levels. We ask: will the phenology of individual species, as well as entire communities of prairie plants, respond similarly across sites to future climatic conditions? Will changes to temperature or soil moisture be more predictive of phenological responses?
We increased canopy temperatures in the warming treatments by 2.5°C, decreased precipitation by 40% using rainout shelters in the drought treatment, and irrigated the warming+precip treatment with additional rainfall to ameliorate a warming-induced drying effect. At the population-level, we monitored first and peak flowering dates of eight PNW-native, range-restricted focal species. At the community-scale, we used normalized difference vegetation index to measure plot greenness and estimate seasonal biomass growth, decline, and quantity through time.
Results/Conclusions:
Temperature was a stronger control than moisture on phenology at both the population and community levels. The effects of drought never differed from control while warming+precip never differed from warming, and temperature variables were greater at predicting phenological responses. For individual species, warming advanced flowering regardless of whether a species was within or beyond its current geographic range. Site also affected flowering times, but somewhat idiosyncratically. At the community level, biomass was affected by warming in the following ways: suppression at the southern site during the spring, enhancement at the central site during the winter-spring followed by suppression during the summer, and enhancement at the northern site during the winter-spring and fall. These circumstances caused a warming-induced reduction in growing season length at the southern site, a shift in the growing season at the central, and a lengthening of the growing season at the northern site. Peak biomass and decline generally occurred earlier moving north to south and under warming. Lastly, the rate of senescence was greater moving north to south. Our results emphasize that even in a moisture-limiting Mediterranean climate, temperature is a key control over phenological shifts.