Understanding plant population responses to climate and land-use change is fundamental to conservation and land management. Demographic analyses accurately predict these responses. While demographic parameters are known for many tree species, they are unknown for most herbaceous plants. Here, we focus on two dominant, perennial, bunchgrasses in the ponderosa pine ecosystem of Northern Arizona: Arizona fescue (Festuca arizonica; C3) and mountain muhly (Muhlenbergia montana; C4). Mapping individual plants over time provides a precise method of determining demographic parameters, while modeling these data over time is the most realistic way to predict how species might respond to changes in climate and land-use. We used data from 68 annually remapped quadrats (2002-2018) to examine the effects of precipitation and temperature on the survival and growth of each species. We constructed life tables to examine their vital rates (survival probabilities, growth, and life expectancies) and then made population projections using the species’ state (size) and weather variables within an Integral Projection Model (IPM) framework to quantify the direct effects of seasonal precipitation and temperature on vital rates. We predicted that survival and growth of Arizona fescue, a cool-season C3 species, would increase with increased spring precipitation and temperatures, while mountain muhly, a warm-season C4 species, would increase with increased summer precipitation and temperatures.
Results/Conclusions
Vital rates varied between species, though both are characterized by type III survivorship curves (high first year mortality). First year survival probabilities were 0.17 and 0.07 and mean life expectancies were 1.19 and 1.07 years for Arizona fescue and mountain muhly, respectively. Both species had lambda values less than 1 (0.90 for Arizona fescue, and 0.88 for mountain muhly) indicating decreased population growth over time (17-year time period). Plant size was the most important predictor for survival and growth of each species and larger plant sizes had higher rates of survival and growth. Spring precipitation influenced survival and growth for Arizona fescue while summer monsoon precipitation influenced survival and growth for mountain muhly. Growth of both species increased with warmer spring temperatures; Arizona fescue responded to higher spring temperatures in March and April, while mountain muhly responded to higher temperatures in May and June. Given the dominance of these species in the understory community in the ponderosa pine ecosystem, understanding the controls on demographic parameters of these bunchgrass species is critical for predicting community response to changing environmental conditions.