Effects of Water Availability and Drought on Tree Growth, Individual- and Population-Level Growth Synchrony, and Mismatches Between Species’ Performance and Their Distribution

The ecological impacts of global change extend through multiple scales affecting individuals, local populations and communities and species distributions. Species-specific tolerances to interacting climate variables can give rise to species’ differing abilities to track their suitable habitat. Differing tracking abilities can in turn lead to novel assemblages of species further altering community dynamics. All of these questions can be better resolved by exploring population and community processes at broader regional scales or across environmental gradients. Forests of the western United States are an excellent study system as government agencies have collected vast amounts of individual tree data spread across the country and in addition to high spatial resolution, temporal patterns are recorded in the wood as annual growth rings. Forests are also ecologically as well as economically important and are vulnerable to multiple global change factors including changes in fire regimes, pest outbreaks, and drought. Here I address three research topics regarding the response of Western forests to climate variability at multiple scales. First, I leveraged the spatial coverage of the Forest Inventory and Analysis (FIA) database to quantify how growth, survival, and reproduction responses to soil water availability are related to distributions of 59 tree species in California, Oregon, and Washington. This revealed extensive mismatches between demographic performance and species distributions that result in dramatic decreases in performance, and depend on traits related to dispersal rates and competition. Second, using tree ring data, I analyze drought resilience among three conifers and explore how factors at local scales like tree size and regional scales affect resilience. Despite being drought-adapted, populations from xeric environments still experienced detrimental effects of drought and across the range, large trees suffered from droughts through poor recovery. Last, I explore patterns in synchrony of tree ring growth across space and time to shed light on community dynamics. Synchrony among species reflects a mixture of niche partitioning and neutral dynamics at play as well as environmental stress. Over the past century, synchrony among tree growth has increased overall which has the potential to alter community dynamics and ecosystem function