California has experienced several strong droughts during recent decades. Other droughts and extreme climate events are expected with ongoing climate change, and will likely impact the montane forests of the Sierra Nevada. Different species of oaks, pines or firs adapted to different elevations will likely have diverse reactions to climate change, making changes in forest dynamics challenging to predict. Annual tree growth can be used to study these reactions, and understand the relation between climate variability and tree dynamics.
We used data collected by the USGS from 26 long-term plots in Sequoia-Kings Canyon National parks and Yosemite National park to parametrize a climate-dependent annual growth model for the twelve species occurring within the plots. Trees were measured every 3-5 years, so we used a hierarchical model to estimate annual tree growth. Models also included annual climate parameters to account for variability in temperature, water availability, or both.
Most of the species (except red fir, ponderosa pine, and oaks) reduced their growth during warm years. The results for precipitation and water availability are more diverse. For instance, red and white firs have opposite growth responses to most climate variables, showing that species from the same genus - often considered as functionally similar- can be quite different.
Some growth variation can be explained by the distribution of the species across altitude. Species found at higher altitudes, where short growing seasons can limit growth, tend to growth faster during warmer and drier years, while species found at lower altitudes tend to growth faster during cooler and wetter years.
Our results suggest that species present in the Sierra will not be impacted the same way by climate change. Differences in growth can lead to changes in forest composition and dynamics, and have to be taken into account to better model, predict, and manage the impacts of climate change on forests.