The concept of the fundamental niche, or the suite of environmental conditions that a species needs to persist, is the foundation of how we predict species distributions and their responses to changing climate. However, these predictions rest on the assumption that the conditions where a species is most abundant or most likely to be found are also the optimal conditions for species’ demographic performance (growth, survival, and reproduction). This assumption has rarely been tested because individual-level demographic data is not often collected on the scale of species distributions. The Forest Inventory and Analysis (FIA) program, which collects tree-level demography data in plots spanning the US, provides a unique opportunity to quantify the relationship between species’ performance and distribution. For 62 tree species in CA, OR, and WA, we used hierarchical Bayesian models to quantify the magnitude of mismatch (i.e. the distance along a climate gradient between a species’ optimal demographic performance and peak abundance), and the demographic cost of the mismatch. We used these measures to test the overall hypothesis that species are most abundant where they perform well.
Results/Conclusions
We modeled species’ performance (growth, survival, recruitment) as a function of local stand characteristics and soil water balance (SWB), a key climatic variable in water-limited forests in Western North America. We found substantial mismatch between the SWB value where each species is most abundant and the SWB value at the maximum demographic performance (diameter growth, survival, and seedling recruitment). Interestingly, the magnitude and direction of this mismatch varied among species, with some distributions being displaced to wetter than conditions than demographically optimal and others to drier conditions. We found a significant (p<0.05) positive relationship between the magnitude of the mismatch and the demographic cost to the species, but again with substantial variation among species depending on their demographic sensitivity to SWB. The high degree of variability among species in this mismatch-cost relationship suggests different fundamental constraints shaping species abundances along environmental gradients, and also underlines key challenges for predicting species distributions based on current occurrence patterns.