Drylands play a dominant role in global carbon cycling and are particularly vulnerable to increasing temperatures that enhance evapotranspiration and accelerate drying of terrestrial surfaces. Most warming experiments to date capture only community-level data and fail to track individual plants, potentially leading to divergent patterns that can arise in relation to population dynamics versus predicting a species persistence and survival. In a 13-year ecosystem warming experiment in a southwestern dryland, we investigated the consequences of rising temperature on the widespread, keystone grass species Achnatherum hymenoides (Indian ricegrass). In an effort to make inferences about the likelihood of this species surviving and how dryland ecosystem function may change in the future, we tested for shifts in aboveground biomass, phenology, photosynthesis (i.e., net photosynthesis and photosynthetic acclimation), and for signs of water stress after 13 years of continuous warming. We also evaluated population cover and recruitment in an effort to reveal how changes in population dynamics and individual growth patterns interact.
Results/Conclusions
Warming dramatically affected plant growth through large enhancements in aboveground production, with individual plants producing twice as much aboveground biomass and doubling the number of culms (i.e., photosynthetic surfaces) they produced. However, there were fewer individuals in warmed plots due to lower survival and recruitment over the course of the experiment, reducing overall plant cover. As a result, while measured plants are much larger in the warmed plots, they also represent more of the younger individuals who germinated in our experiment’s warmer world conditions. Plants also responded to warming through large changes in phenological cycles, advancing spring green-up by 8.5 ± 2.9 days, date of first flower by 10.8 ± 1.3 days, and senescence by 2.2 ± 1.1 days, leading to a longer growing season (+6.3 days). No treatment effects were found on photosynthetic optima or temperature optima, but net photosynthesis in the warmed plots was reduced by 30% when soil moisture was not limiting. This suggests individual plants did not acclimate to changes in temperature, likely due to low photosynthetic plasticity. Overall, our study demonstrates a plastic response of A. hymenoides to tolerate 13 years of warming by shifting growth and aboveground allocation strategies and downregulating CO2 fixation to prevent plant damage. Together, these results suggest A. hymenoides may be capable of facing increased temperatures but that the species may not be as abundant nor will it represent as much of the system’s total plant cover.