Temperature and precipitation determine the conditions where plant species can occur. Despite their significance, to date, surprisingly few demographic field studies have considered the effects of abiotic drivers. This is problematic because anticipating the effect of global climate change on plant population viability requires understanding how weather variables affect population dynamics. One possible reason for omitting the effect of weather variables in demographic studies is the difficulty in detecting tight associations between vital rates and environmental drivers. In this paper, we applied Functional Linear Models (FLMs) to long-term demographic data of the perennial wildflower, Astragalus scaphoides. We compared models of the effect of average temperature, total precipitation, or an integrated measure of drought intensity (Standardized Precipitation Evapotranspiration Index, SPEI), on plant vital rates.
Results/Conclusions
We found that transitions to flowering and recruitment in year t were highest if winter/spring of year t was wet (positive effect of SPEI). Counterintuitively, if the preceding spring of year t-1 was wet, flowering probabilities were decreased (negative effect of SPEI). Survival of vegetative plants from t-1 to t was also negatively affected by wet weather in the spring of year t-1, and for large plants, even wet weather in the spring of t-2 had a negative effect. We assessed the integrated effect of all vital rates on life history performance by fitting FLMs to the asymptotic growth rate, log(λt). Log(λt) was highest if dry conditions in year t-1 were followed by wet conditions in the year t. Overall, the positive effects of wet years exceed their negative effects, suggesting that increasing frequency of drought conditions would reduce population viability of A. scaphoides.