University of New Mexico Albuquerque, New Mexico, United States
Abstract: Climate change and high-severity wildfires are altering forests and the rates and locations of tree regeneration. While seed dispersal limitations can be overcome by planting in large severely burned patches, limitations imposed by hot and dry conditions are likely to reduce seedling survival in post-fire environments. We examined how the survival of one-year seedlings of five southwestern United States (US) conifer species whose southwestern distributions range from warmer and drier woodlands and forests (Pinus edulis, P. ponderosa) to cooler and wetter subalpine forests (Pseudotsuga menziesii, Abies concolor, and Picea engelmanii) changed in response to low moisture availability, high temperatures, and high vapor pressure deficit in incubators. We used a Bayesian framework to construct species-specific piecewise exponential regression models that explained 60% to 80% of the species-specific survival variability. We then applied these models to each species’ current range in the southwestern US using recent climate (1980-2019) and projected climate (1980-2099). We expected that species vulnerability to high temperature and drought would vary as a function of the climate that typifies their ranges such that species found in hotter and drier areas (i.e. P. edulis) would have longer survival times than those found in subalpine forests (i.e. P. engelmannii). Further, we expected that the area at risk of reduced survival within a species’ range would increase for each species and the overall survival of each species would decrease with projected warming and drying, specifically at landscape positions that experience greater solar insolation or warmer temperatures (i.e. southern aspects and lower elevations). We found that lower elevations within a given species’ range had low survival under projected climate and that range contraction was greatest for species that currently occupy warm-dry conditions. Higher elevation species (i.e. P. menziesii, A. concolor, and P. engelmannii) were more susceptible to the effects of hot and dry periods. However, their existing ranges were not projected to experience the conditions we tested as early in the 21st century as species at lower elevations. This resulted in smaller proportions of their existing ranges experiencing high mortality conditions. These results demonstrate that empirically derived physiological limitations can be used to identify where species composition or vegetation type change are likely to occur in the southwest US.