Seedling establishment and growth is a prerequisite for local population persistence and adaptation as well as for range expansion. While adult trees can often persist through extensive climatic variation, the conditions allowing seedlings to survive and grow are often more restrictive. From 1999 to 2009, USGS researchers monitored seedlings in more than a dozen long-term plots in Sequoia-King’s Canyon and Yosemite National Parks. We modeled seedling and sapling survival and transition to larger size classes as a function of species, size, and various climatic variables, within a Bayesian framework. Predictive loss was used to compare fit across models. We also examined the first year (spring 2017-2018) performance of ponderosa, Jeffrey, and western white pine seedlings from source populations across the state planted at 4 sites across an elevation gradient in Sequoia NP. This experiment is intended to test the degree of local adaptation and within-species variation in climate responses.
Results/Conclusions
We found that higher July maximum temperature, either directly or as a deviation from the long-term plot mean, tended to be associated with lower survival and growth. Precipitation and snow effects were weaker and more variable. Higher precipitation/snow values tended to have a positive effect on survival while higher-than-average levels tended to have a negative effect on growth - perhaps because seedlings were buried in snow for longer periods in high precipitation years. Of the planted seedlings, those from warmer or drier home climates tended to exhibit higher survival. Transfer effects were asymmetric, with seedlings transferred to sites hotter or drier than their home climate exhibiting a bigger decrease in survival than those transferred to cooler/moister sites. This suggests that if, as predicted by many climate models, the Sierra Nevada continues to become warmer and drier the establishment of most tree species will be negatively impacted. However, intraspecific variation in responses suggests that assisted migration/gene flow could be an option to counteract these negative effects.