COS 165-3 - Negative demographic responses to climate in two dominant grasses over the last 60 years

Abstract: Background/Questions/Methods



The southwestern United States is becoming hotter and drier, with longer intervals between precipitation events. This directional change may drive shifts in dominant vegetation, particularly in arid ecosystems where high variability can cause rapid change in plant composition. Drought tolerant perennial grasses are the dominant lifeform in the understory of the ponderosa pine forests of northern Arizona, but decades of prolonged drought coupled with increased temperatures may cause declines in the growth, survival, and reproduction of these species. Utilizing permanent 1-m² quadrats put in place over a century ago, detailed plant census data collected at the individual level over the last 20 years, and annual weather data and climate normal, we applied integral projection models (IPMs) to model demographic responses of Muhlenbergia montana (MUMO; C4) and Festuca arizonica (FEAR; C3), two dominant perennial bunchgrasses in this system to climate data from 1970 until present. We asked 1) How have MUMO and FEAR responded to average climate between 1970 and present? 2) Do MUMO and FEAR respond similarly to climate drivers? And 3) Do MUMO and FEAR display similar demographic trends in response to climate extremes?



Results/Conclusions



For both species, populations were declining both in response to climate normal between 1970-2010 and in response to average climate in the last 20 years (l= 0.92 for FEAR, and 0.91 for MUMO). While survival in MUMO was shaped by climate variables, specifically growing season temperature and vapor pressure deficit (VPD), survival of FEAR was not influence by climate. Growth of both MUMO and FEAR was shaped by climate, with MUMO growth responding positively to cool growing season temperatures and high growing-season precipitation, and FEAR responding to positively to cool growing season temperatures, low spring VPD, and high winter precipitation. Both FEAR and MUMO responded similarly to climate extremes, with severely decreased l in response to hot, dry conditions (l= 0.56 for MUMO, and 0.68 for FEAR) and positive population growth in cool, wet conditions (l = 1.08 for MUMO, and 1.07 for FEAR). These results suggest that under projected warmer, drier climate conditions, populations of both MUMO and FEAR are likely to shrink in response to both average and extreme conditions.