Effects of climate extremes on tree species’ occurrence by life stage
Climate change is a threat to life on Earth, and efforts to understand how widespread temperate forest species handle erratic climate and increasing temperature are imperative for management and policy. We examine the effect of climate means versus climate extremes to determine tree species’ probability of occurrence by life stage across the Pacific coastal United States containing carbon-rich, temperate conifer forests. We study climate means and extremes that have direct bearing on plant fitness, growth, and mortality including growing season length, seasonal vapor pressure deficit, minimum temperature by season, degree of freeze-thaw events occurring in the spring and fall, and seasonality of precipitation. These are derived from gridded climate data at a monthly time step across the period from 1900 to 2007. We summarize climate extremes for empirical niche modeling using a new approach. We first test for differences among different potential groupings of years in terms of magnitudes of anomalies using Multivariate Response Permutation Procedure or MRPP. We examine different potential combinations of climatic oscillation indices as potential grouping variables. We choose the best grouping scheme per climate variable to summarize climatic anomalies as predictors for niche modeling of trees. We calculate average measures of climate deviation per time interval defined by combinations of climatic oscillations, which we call oscillation schemes. We build niche models for presence/absence data of seedlings and adults across 22 tree species using data from USDA Forest Inventory and Analysis (FIA). To build a model for a species and climate variable, we supply the long-term climate mean and related extremes (summarized by oscillation scheme) to the model building process using Non-Parametric Multiplicative Regression (NPMR). This process is repeated for 22 species, two life stages, and seven climate variables.
Results indicate that 27% of the models had a climate extreme explain more variability than a climate mean. Extremes associated with freeze-thaw events, seasonality of precipitation, and winter minimum temperature were most frequently represented. Extremes were less often chosen for models of seedling niches compared to adults (p=0.054). The reason for this is likely that the adult niche has longer exposure to climatic events and thus a higher likelihood of being shaped by climate extremes. Additionally, we argue that facilitation of seedling establishment by microclimatic refuge may be an important and overlooked process that determines seedling realized niches as broader than theoretically considered.