Negative impacts of summer heat on Sierra Nevada tree seedlings

Abstract

Understanding the response of forests to climate change is important for predicting changes in biodiversity and ecosystem services, including carbon storage. Seedlings represent a key demographic stage in these responses, because seedling establishment is necessary for population persistence and spread, and because the conditions allowing seedlings to survive and grow are often more restrictive than those adults can tolerate. However, detailed long-term data on the responses of individual seedlings to climatic variation are rare. Here, we analyze a decade of seedling growth and survival data collected in 25 long-term plots located in Sequoia and Yosemite National Parks. The forest types represented range from lower-elevation mixed conifer, to red fir-dominated, to high-elevation western white pine stands. Twelve species, including two oaks and ten conifers, are found across these plots. Survival and transition to larger size classes are modeled within a Bayesian framework as a function of species, size, fire, and various climate variables. We initially expected that precipitation-related variables would have a strong effect on seedling survival and growth in this Mediterranean-climate region and that some species might respond positively to higher temperatures if they are adapted to warmer low-elevation areas or to higher-than-average temperatures if they are high-elevation species whose growing seasons are limited by cold. However, we found that higher July maximum temperature or temperature anomalies were associated with lower survival and growth for most species, while precipitation and snow effects were weaker and more variable. Higher precipitation had a positive effect on survival of most species, but snow effects and precipitation effects on growth could be negative. Given the estimated climate responses, under potential future scenarios of higher summer temperatures and altered precipitation, most species are predicted to have lower survival and reduced growth. This is likely to favor species that have relatively high survival and growth and less negative responses to climatic shift (such as several of the pines) and perhaps lead to a more open forest structure-or potentially transition to non-forest vegetation types where seedling recruitment does not keep pace with adult mortality.

Rethinking fire‐adapted species in an altered fire regime

“…For pines, which are less sensitive to warming and drying than other species (Moran et al. ), shrub effects under a novel fire regime may be more important drivers of decreasing regeneration in our study area than changes in climate, though more research is needed on climate and shrub effects on regenerating pines.…”

Modeling the forest dynamics of the Sierra Nevada under climate change using SORTIE-ND

“…Growth of all age classes and seedling recruitment are affected by climate too. Our recent analysis found that hotter summer temperatures reduced the survival and growth of most species of Sierra Nevada tree seedlings (Moran et al 2019). Shifts in forest composition have already been observed.…”

“…For adult growth, the two best climate predictors were January minimum temperature and precipitation (Aubry-Kientz and Moran 2017) and for adult mortality July maximum temperature and precipitation (Appendix 2). For the survival of seedlings >10 cm in height, the best-fit model included July maximum temperature and total precipitation anomalies averaged over the current and past year, while for growth the current year July maximum temperature and total snow performed better (Moran et al 2019). However, projections for snow are highly uncertain because they combine already uncertain precipitation projections with calculations of how much falls as snow and the melt rate.…”

“…where θ i,t is the probability of surviving. Class 0 seedlings were initially given the size effect estimated for seedlings <10 cm (Moran et al 2019), but this resulted in an unrealistic increase in tree density (A4) because the established seedlings ( >10 cm) used to fit the model tend to have much higher survival than younger seedlings. Therefore, the class 0 and 1 size effects were "hand-tuned" (see Appendix 4 for an explanation).…”

“…Species that are found in hotter/drier environments or that show more positive growth or survival response to higher temperatures will tend to increase in abundance and/or basal area in the climate change scenarios relative to the control scenario. (Moran et al 2019) have been published. Plot characteristics and analyses of fecundity and adult mortality are described in Appendix 2 and Appendix 3.…”