COS 215-1 - Assessing the impact of microhabitat conditions on plant community responses to climate change with a seed transplant experiment

Abstract: Climate change is causing geographic range shifts in species around the globe, and understanding the factors that influence a species’ ability to expand its range is crucial for predicting future changes in biodiversity. However, our inability to accurately forecast such shifts impedes tailored policies to mitigate biodiversity loss. Underlying this problem is the outstanding ecological question of how species will perform beyond their current range. A common, yet untested, assumption in forecasting approaches is that populations will be able to survive and grow beyond current ranges in new habitats that will become (macro)climatically suitable, even though differences in microhabitat and local population dynamics could alter such responses. We aim to test whether and how such deviations will impact population response and ask: 1) How do variations in microhabitat conditions affect seed recruitment and seedling survival; 2) Do grasses, forbs, and woody species respond differently? Using the large elevational gradients of the Washington Cascade Range as a model system, we transplanted 25 tree, shrub, forb, and graminoid species across their current elevational ranges and a broad range of macroclimates, including those projected to become suitable with climate change. Over five years, we recorded seed germination, seedling survival and growth, and a suite of microhabitat parameters. Here we show that recruitment and seedling survival vary with abiotic and biotic microhabitat conditions and that these effects are highly species-dependent. We found that 84% of species successfully recruited, but only 33% percent of those responded to variations in microhabitat conditions. Interestingly, species responses are not grouped by functional traits, dormancy requirement, or cooler vs. warmer native ranges. Rather, species responses fall along moisture and temperature gradients and species recruit best in different combinations of cool-warm or moist-dry ordinated microhabitat spaces. In conclusion, our results demonstrate that microhabitat has species-specific effects on seed recruitment and seedling survival, but that many species do not respond to microhabitat variation. These variable microhabitat effects on the key life stages of plants are likely to have important implications for the ability of different plant species to expand their ranges with climate change. These findings provide valuable insights for predicting future changes in plant biodiversity and informing conservation efforts in the Washington Cascade Range and other similar ecosystems. Our work will ultimately produce the tools and knowledge base necessary to gain a deeper understanding of species response in novel environments and to construct improved range shift projections.