Ability of seedlings to survive heat and drought portends future demographic challenges for five southwestern US conifers.

Climate change and disturbance are altering forests and the rates and locations of tree regeneration. In semi-arid forests of the southwestern United States, limitations imposed by hot and dry conditions are likely to influence seedling survival. We examined how the survival of one-year seedlings of five southwestern United States conifer species whose southwestern distributions range from warmer and drier woodlands and forests (Pinus edulis Engelm., P. ponderosa Dougl. Ex Laws.) to cooler and wetter subalpine forests (Pseudotsuga menziesii (Mirb.) Franco, Abies concolor (Gord. & Glend.) Lindl. Ex Hildebr., and Picea engelmannii Parry ex Engelm.) changed in response to low moisture availability, high temperatures, and high vapor pressure deficit in incubators. We used a Bayesian framework to construct discrete-time proportional hazard models that explained 55% to 75% of the species-specific survival variability. We applied these to the recent climate (1980-2019) of the southwestern US as well as 1980 to 2099 CMIP5 climate projections with the RCP8.5 emissions pathway. We found that the more mesic 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 are not projected to experience the conditions we tested as early in the 21st century as the more xeric P. edulis and P. ponderosa, leading to lower percentages of their existing ranges predicted to experience seedling-killing conditions. By late-century, extensive areas of each species southwestern range could experience climate conditions that increase the likelihood of seedling mortality. These results demonstrate that empirically derived physiological limitations can be used to inform where species composition or vegetation type change are likely to occur in the southwest US.