UC Berkeley

Global climate change is altering not only average temperatures, but also the distribution and abundance of precipitation. During winter in montane habitats, changing precipitation patterns mean changing snow fall, which will have profound ecological impacts. Snow is an effective insulator and decouples below-snow temperatures from air temperatures, providing a stable environment and determining microclimate conditions in the soil. A wide range of organisms take advantage of stable below-snow winter environments by overwinter in soil. Winter soil microclimate modulates the degree of stress, performance, and ultimately fitness for organisms that overwinter in the soil. During the coldest portions of winter, the insulating effect of snow protects organisms from potentially damaging or even lethal cold stress. However, protection from cold extremes comes at a cost: in ectothermic animals, energy consumption is dependent on body temperature, and so relatively warm below-snow conditions drain energy stores more quickly. In winter resources are limited and energy stores cannot be replenished, and complete depletion of energy stores can cause mortality. Even if organisms survive, post-winter fitness may also be impacted by energy depletion by reducing reserves available for growth and reproduction. We don’t yet understand the impacts of changing winter snow will have on overwintering organisms in montane environments. I hypothesize that winter snow cover, through regulation of winter microclimate, modulates the cause of stress and mortality (cold damage or energy depletion) for montane ectotherms overwintering in soil. By better understanding the impacts of changing winter snow cover on montane ectotherms, we may be better able to identify processes and populations that are vulnerable to climate change.

My dissertation aims to expand our understanding of the impact of snow cover on overwinter stress on insects that overwinter in the soil, by examining the impact of snow cover along elevation gradients, then extending the energy use model developed in Chapter 1 to test for impacts of plasticity. Finally I examine how overwinter conditions alter physiological processes upon emergence from overwintering. This work shows that winter snow cover has far reaching impacts on overwintering ectotherms, which are not equal across elevation, and have effects that can carry over into the growing season, expanding our understanding of changing winters and population impacts of climate change.

Chapter 1 establishes the energetic impacts and thermal challenges of changing snow cover along elevation gradients. Along elevational gradients snow cover increases but air temperature decreases, and it is unknown how these opposing gradients impact performance and fitness of organisms overwintering in the soil. I developed experimentally validated ecophysiological models of cold and energy stress over the past decade for the montane leaf beetle Chrysomela aeneicollis, along five replicated elevational transects in the Sierra Nevada mountains in California. Cold stress peaks at mid-elevations, while high elevations are buffered by persistent snow cover, even in dry years. While protective against cold, snow increases energy stress for overwintering beetles, particularly at low elevations, potentially leading to mortality or energetic trade-offs. Declining snowpack resulting in drier winters, will predominantly impact mid-elevation populations by increasing cold exposure, while high elevation habitats may provide refugia.

Chapter 2 focuses on the impacts of plasticity in metabolic-rate-temperature relationships in dormant organisms and their impacts on energy use estimates, expanding on the model developed in Chapter 1. Ecophysiological energy use models predict long-term energy use from metabolic rates, but we do not know the degree to which plasticity in metabolic intensity or thermal sensitivity impact energy use estimates. I quantified metabolic rate-temperature relationships of dormant willow beetles (Chrysomela aeneicollis) monthly from February to May under constant and variable acclimation treatments. Metabolic intensity increased through time, and acclimation altered both metabolic intensity and the thermal sensitivity. However, incorporating these two types of metabolic plasticity into energy use models did not improve energy use estimates, validated by empirical lipid measurements. Together, this indicates that while metabolic rate temperature relationships are plastic during winter, incorporating this plasticity does not improve prediction of energy use made by ecophysiological models, partly due to large individual variability in energy reserves.

Chapter 3 examines the impact that overwintering environment plays on prioritization of physiological processes upon emergence from dormancy, based on changes in gene expression. During winter, dormant organisms conserve resources through metabolic suppression and minimizing cellular processes. The transition from dormancy to active season requires a quick reversal of these processes and large-scale physiological transitions, enabling organisms to begin exploiting favorable environmental conditions. The impact of overwinter microclimate on the physiological processes involved in transitioning from dormancy to active season in insects are unknown. I conducted a field snow-cover manipulation and then profiled gene expression of willow leaf beetles Chrysomela aeneicollis during the transition out of dormancy using RNA-seq. Upon emergence from dormancy, beetles first prioritize up-regulation of transcripts associated with digestion and nutrient acquisition. The prioritization of nutrient acquisition is followed by investment into reproduction, but the timing is sex-specific with females investing sooner. Winter snow cover impacted the timing of these processes, with beetles that overwintered below snow being several days ahead of beetles that overwintered without snow. This highlights the importance of winter microclimate in regulating critical life history transitions.