Quantifying how fitness changes across environmental gradients can identify key environmental drivers of distribution shifts. I focused on California forests because of the vast coverage of historic and contemporary datasets and because of widespread interest in how populations will cope with changes in water availability. I explored three questions: Do species’ demographic sensitivity to climatic water deficit (CWD) explain observed distribution shifts? Are species distributed where they perform (grow) well? And how will migration, or lack thereof affect the mismatch between distribution and performance? I constructed generalized linear occurrence models from the Wieslander (historic) and FIA (contemporary) data sets and calculated the extremes and the optimum observed CWD and elevation values for each species. I then constructed linear growth models to predict diameter growth at each of these key CWD values. Sensitivity was calculated as the slope of the predicted growth function from the maximum to the minimum over the observed CWD range. I used these values as predictors for shifts in elevation determined from the occurrence models. Mismatch between distribution and performance was calculated as the difference between CWD at optimum growth and CWD at maximum probability of occurrence.
Results/Conclusions
Preliminary data analysis suggests there is no relationship between growth sensitivity to CWD and observed elevation shifts. This could be true for many reasons: other vital rates may be important, other covariates may be important or there may be other ecological processes limiting these species’ distributions. Although, further data analysis is necessary to ensure congruency of the predictor variables in historic and contemporary datasets. Some species have significant mismatches between the CWD at optimum growth and maximum probability of occurrence, as many of the species’ 95% credible intervals of the mismatches do not overlap 0 . If growth is truly an important component of fitness, these mismatches show that populations have some pressure to keep up with their climatic niche. Repeating these analyses with full population models to address this are future directions.