Dioecious plants often display sexual segregation in habitat preference and trait expression due to contrasts in reproductive costs. Females may be maladapted to environments with limited available resources, or habitats where resources are diminishing due to climate change. Reduced fitness in female individuals compared to males could lead to skewed sex ratios and reduce population fitness of dioecious species, including one of the most widely distributed dioecious tree species in North America, Acer negundo. The goal of this study was to evaluate how climate warming and drought may enhance sexual segregation in productivity and physiological stress in A. negundo. To address this goal, I measured radial growth and carbon isotope ratios (δ13C) in tree-ring cellulose of 22-year male and female A. negundo trees growing in a common garden in Salt Lake City, UT. The trees were originally transplanted as one-year old cuttings from a nearby site that was 6.5 °C cooler that the common garden. I hypothesized that 1) δ13C would be lower (more negative) in late growth that is formed during the hottest months of the growing season in males than in females, and during years with no supplemental watering, indicating lower stress from heat and drought in males than in females. And 2) radial growth would be greater in males under warm, well-watered conditions and the addition of drought will exacerbate the difference between males and females. To test these hypotheses, cores were extracted from the main stem of nine male and nine female trees with an increment borer. Annual growth was measured on each core and cellulose was extracted to measure annual δ13C ratios. Males had a 0.63‰ lower mean δ13C than females in years after supplemental water had ceased (p = 0.03) and a 4.12 mm wider radial growth compared to females while irrigated (p = 0.02). Although these data did not support my hypotheses per se, results nevertheless indicate that females are more likely to be maladapted to climate warming and drought to a greater extent than males. If so, a combination of drought and heat stress may have deleterious impacts on the population fitness of Acer negundo and other similar dioecious tree species.

Climate change is making the arid southwestern U.S. (“Southwest”) warmer and drier. Decreases in water availability coupled with increases in episodic heat waves can pose extraordinary challenges for native riparian tree species to persist in their current ranges. However, the morpho-physiological mechanisms that these species deploy to cope with extreme temperature events are not well understood. Specifically, how do these species maintain leaf temperatures within a safe operational threshold in the extreme conditions found across the region? Morpho-physiological mechanisms influencing intraspecific local adaptation to thermal stress were assessed in Populus fremontii using two experimental common gardens. In a common garden located near the mid-point of this species’ thermal distribution, I studied coordinated traits that reflect selection for leaf thermal regulation through the measurement of 28 traits encompassing four different trait spectra: phenology, whole-tree architecture, and the leaf and wood economic spectrum. Also, I assessed how these syndromes resulted in more acquisitive and riskier water-use strategies that explained how warm-adapted populations exhibited lower leaves temperatures than cool-adapted populations. Then, I investigated if different water-use strategies are detectable at inter-annual temporal scales by comparing tree-ring growth, carbon, and oxygen isotopic measurements of cool- versus warm-adapted populations in a common garden located at the extreme hottest edge of P. fremontii’s thermal distribution. I found that P. fremontii’s adaptation to the extreme temperatures is explained by a highly intraspecific specialized trait coordination across multiple trait scales. Furthermore, I found that warmer-adapted populations displayed 39% smaller leaves, 38% higher midday stomatal conductance, reflecting 3.8 °C cooler mean leaf temperature than cool-adapted populations, but with the tradeoff of having 14% lower minimum leaf water potentials. In addition, warm-adapted genotypes at the hot edge of P. fremontii’s distribution had 20% higher radial growth rates, although no differences were detected in either carbon or oxygen isotope ratios indicating that differences in growth may not have reflected seasonal differences in photosynthetic gas exchange. These studies describe the potential effect that extreme climate might have on P. fremontii’s survival, its intraspecific responses to those events, and which traits will be advantageous to cope with those extreme environmental conditions.

Soil organic carbon (SOC) is a critical component of the global carbon (C) cycle, accounting for more C than the biotic and atmospheric pools combined. Microbes play an important role in soil C cycling, with abiotic conditions such as soil moisture and temperature governing microbial activity and subsequent soil C processes. Predictions for future climate include warmer temperatures and altered precipitation regimes, suggesting impacts on future soil C cycling. However, it is uncertain how soil microbial communities and subsequent soil organic carbon pools will respond to these changes, particularly in dryland ecosystems. A knowledge gap exists in soil microbial community responses to short- versus long-term precipitation alteration in dryland systems. Assessing soil C cycle processes and microbial community responses under current and altered precipitation patterns will aid in understanding how C pools and cycling might be altered by climate change. This study investigates how soil microbial communities are influenced by established climate regimes and extreme changes in short-term precipitation patterns across a 1000 m elevation gradient in northern Arizona, where precipitation increases with elevation. Precipitation was manipulated (50% addition and 50% exclusion of ambient rainfall) for two summer rainy seasons at five sites across the elevation gradient. In situ and ex situ soil CO2 flux, microbial biomass C, extracellular enzyme activity, and SOC were measured in precipitation treatments in all sites. Soil CO2 flux, microbial biomass C, extracellular enzyme activity, and SOC were highest at the three highest elevation sites compared to the two lowest elevation sites. Within sites, precipitation treatments did not change microbial biomass C, extracellular enzyme activity, and SOC. Soil CO2 flux was greater under precipitation addition treatments than exclusion treatments at both the highest elevation site and second lowest elevation site. Ex situ respiration differed among the precipitation treatments only at the lowest elevation site, where respiration was enhanced in the precipitation addition plots. These results suggest soil C cycling will respond to long-term changes in precipitation, but pools and fluxes of carbon will likely show site-specific sensitivities to short-term precipitation patterns that are also expected with climate change.