Periods of low precipitation and increasing atmospheric temperature are having adverse effects on tree and forest growth and survival in part via limitations upon photosynthesis. Understanding how different species and plant functional types will physiologically respond to the combination of drought and heat is therefore critical for modeling and predicting the fate of trees and forests under future climate conditions. In addition, climate is changing at a faster pace than the slow natural adjustment rates of most species to their rapidly shifting habitat conditions, especially for long-lived tree species, which poses a major challenge for management, conservation, and preservation strategies and policies. Therefore the current traditional conservation/preservation methods, such as providing migration corridors and increasing suitable habitat at range margins will need to be supplemented with a more direct, innovative, and adaptive approach. One such approach is that of managed migration, which is the intentional translocation of species outside their current habitat in order to reduce anticipated loss of biodiversity caused by climate change related stress. This dissertation addresses these problems through the experimental examination of heat, drought, and the combination of heat and drought on two conifer species that have differing drought tolerances, which can serve as global models for a range of physiological mechanisms to deal with water and heat stress, as well as a social science investigation into the preservation of an iconic conifer tree, the coast redwood into Oregon via managed migration.

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.

Bromus tectorum L. (Poaceae, ‘cheatgrass’) is a flammable, non-native, annual grass from Eurasia that has spread throughout the western United States, exerting serious impacts on ecosystems by fueling frequent fires and displacing native vegetation. Over the last decade, B. tectorum has begun expanding into higher elevations, where it currently coexists with native vegetation and has not yet altered fire regimes. In this dissertation, I examine the impacts of climate change and management activities on high-elevation B. tectorum populations in the eastern Sierra Nevada, California. In Chapter 1, I explore the impacts of changing patterns of snow and rain on B. tectorum as compared to native species. I found that B. tectorum responses to precipitation change (snow and rain) were of greater magnitude than the responses of native species at the University of California Valentine Eastern Sierra Reserve. Bromus tectorum density followed the declining trend in winter snowpack over three years, and within-year snowpack reductions advanced the timing of B. tectorum phenology. Species-specific responses to spring rainfall showed some variation in different years of the study, highlighting the importance of both winter and spring precipitation and suggesting that inter-annual variability will generate different impacts on different species depending on specific conditions. Chapter 2 presents a greenhouse experiment to study the impacts and interactions of temperature, competition, and facilitation on native species and B. tectorum. Higher temperatures in the greenhouse dramatically decreased B. tectorum reproductive allocation and increased vegetative biomass. Bromus tectorum tended to have negative impacts on neighboring species by reducing their biomass, reproductive allocation, stomatal conductance, and photosynthetic rates (suggesting possible competition), while a native nitrogen-fixing forb tended to have positive impacts on these variables for B. tectorum and other native species (suggesting possible facilitation). Chapter 3 is an applied research project conducted at Devils Postpile National Monument to assess the impacts of extreme disturbance events and fuels management activities on fuel loads and B. tectorum invasion. An extreme windstorm substantially elevated fuel loads, which were reduced in areas that were treated for hazardous fuel reduction. Anthropogenic disturbance from fuel reduction projects was not creating establishment sites for B. tectorum, but areas that were subjected to a high-severity wildfire burn about twenty years ago provided loci of B. tectorum invasion. Prioritizing monitoring efforts in high-severity burned areas, especially when they coincide with high levels of disturbance from recreational activities, such as along hiking trails and near pack stations, would strengthen active management of B. tectorum infestations at Devils Postpile National Monument. Overall, this dissertation shows that various climatic changes and anthropogenic impacts could influence mechanisms for future high-elevation B. tectorum expansion in different ways.

Bromus tectorum L. is an annual grass native to Eurasia that was introduced into the U.S.A. in the late 1800s and has become dominant in large parts of the Intermountain West. At high density, it can alter the fire regime and subsequently act to displace native shrub and bunchgrass communities. In the eastern Sierra Nevada, California, B. tectorum exists in low-density populations and its impacts have been minimal. However, agents of global change, such as changing climate and increased anthropogenic nitrogen (N) deposition, will likely affect plant species distributions and may facilitate the spread of B. tectorum. In this dissertation, I examine: (1) how climate change and increased N deposition might affect B. tectorum spread, (2) how increased N deposition might affect species diversity and B. tectorum dominance, (3) the ecological effectiveness of B. tectorum control techniques, and (4) the feasibility of B. tectorum control in the eastern Sierra Nevada, CA, considering social logistical, and regulatory factors.

Through a series of in-situ experiments, I found that a shift from snow to rain could increase B. tectorum dominance in the region, depending on timing, frequency, and magnitude of rain events. Effects of increased N on B. tectorum were dependent on rainfall. Through a four-year study monitoring species composition, I found that B. tectorum dominance has increased significantly at the site at the expense of native forb species richness. In disturbed areas, B. tectorum cover is approaching the threshold for increased fire risk, which could result in more significant impacts for the region. Several methods of control were effective in reducing B. tectorum and increasing the dominance of native species, including hand pulling, soil solarization, and mulching. However, through interviews with local land managers, I identified a number of regulatory, social, and logistical obstacles to active B. tectorum control in the region. Future work should be focused on overcoming these barriers to prevent increased impacts of B. tectorum at its high elevation range margin.