UC Santa Cruz

Warming due to climate change will be felt throughout California. Increased climatic variability is also expected, and will impact physiological processes beyond what is predicted from changes in mean conditions. Knowledge of how these changes will affect dominant forest species in the Sierra Nevada is critical for anticipating secondary effects on water resources, wildlife, and other ecosystem services. In order to better understand the effect of weather variability on tree growth, I compared the sensitivity of four regionally dominant tree species to variable temperature and precipitation across a 500 m elevation gradient at annual and seasonal temporal scales.

My first chapter takes advantage of the end of a historic multi-year drought and included an unusually wet winter, to understand how very dry and very wet conditions constrain photosynthesis and growth. All species demonstrated phenotypic plasticity in response to temporal differences in precipitation on both inter-annual and seasonal timescales. Net photosynthesis in Pinus contorta decreased from an early season 2016 average of 12.4 to 6.89 μmol CO2 m-2 s-1 later in the summer, but increased 14.1% between seasons in the wet year. By contrast, elevation had almost no effect on instantaneous photosynthetic gas exchange, CO2 response curve parameters, or stem water potential in any of the years for any of the species. My results show these species demonstrated considerable ability to tolerate and recover from an extreme drought event.

In my second chapter, I examined sensitivity of these species at longer time scales, by comparing tree age structure and variability in annual ring growth at these same elevations. I also assessed sensitivity of annual ring-width to variation in April 1 snow water content, a proxy for total water year precipitation over an 88-year period starting in 1929. I found higher snow water content had a positive effect on RWI at the lower elevations, but this effect was reversed at the highest site. Snow water content of the previous year was also important for some sites and species, supporting the idea that conditions over multiple years are involved in controlling growth. The age and size structure of P. contorta were not consistent with upslope migration, though a distributional shift for Abies magnifica is possible at the highest elevation.

In addition, I sought to understand community belief systems about climate change in the vicinity of two National Forests near my field sites. This aspect of my research may help forest managers frame management actions and policies to better communicate with these citizenries. My third chapter quantified the underlying belief systems and attitudes towards climate change expressed by local community members near the Inyo and Sierra National Forests. I used a combination of two previously developed survey instruments: 12 statements assessing a respondent’s adherence to one of four dominant cultural types posited by Cultural Theory, and the Global Warming’s Six Americas survey developed by the Yale Program on Climate Change Communication. Despite the demographic similarities between the two National Forests, their populations differed in both their acceptance of and attitudes toward climate change and in their expressed agreement with the four cultural types.

My results provide reasons for optimism in the face of climate change in the eastern Sierra Nevada. With regard to the social context, even though acceptance of climate change was not universal, it was expressed by the majority of participants in both Forests. While significant ecological impacts are evident, tree at my sites demonstrated the capacity to respond plastically to dramatic meteorological variation in their photosynthetic rates, water status, and growth. One of the study species even appears to be responding distributionally. Increasing climate variability will likely subject these trees to new extreme events in the future, but thus far, they have shown an unexpected ability to tolerate and recover from extreme drought.