UC Berkeley

In a world of changing climate, changes in the magnitude and timing of fluctuations in environmental conditions are expected to alter plant functioning. Plants are already responding to anthropogenic climate change and in the next hundred years they will face an increase in the severity, duration, and frequency of extreme climatic events, such as heatwaves. To understand the impacts of climatic extremes on plants we need to study the exposure and vulnerability of natural systems in the context of natural climate variability. Few studies have examined plant responses to heatwaves - short-term, intense increases in temperature. Here, the ecophysiology of the California native shrub Heteromeles arbutifolia is explored, with the goal of determining the extent of within species variation in ecophysiological responses to heatwaves.

Chapter 1 examines if the ability to tolerate heatwaves varies among distant populations of H. arbutifolia. Vulnerability to heatwaves may depend on genetic variation in phenology, growth, reproduction and physiological tolerances, and these factors may vary within a species among different populations. This study showed that two populations of a widespread California native shrub responded differently to an artificial heatwave. In the greenhouse, regardless of treatment, plants from the northern site had higher stomatal conductance and plants from the southern site had higher nighttime respiration. An artificial spring heatwave treatment, when water availability was high but air temperature suddenly increased 5°C, resulted in increased transpiration by plants from both sites but the magnitude of the increase was greater in plants from the northern site. However, our results also show that an artificial spring heatwave did not result in lower photosynthetic rates or higher respiration rates, indicating that high water availability may have allowed plants from both sites to cope with extreme temperatures.

Chapter 2 examines the responses of two growth forms of H. arbutifolia to naturally occurring heatwaves in the field. . H. arbutifolia is a resprouter, with plants regenerating after a fire from shoots produced by belowground meristems fuelled by non-structural carbohydrate reserves. There is strong evidence that resprouting individuals have higher photosynthetic and transpiration rates than mature non-sprouting individuals. Therefore, vulnerability to heatwaves may depend on plant growth form. This study shows that when seasonal water availability was lower during a fall heatwave, plants reduced stomatal conductance, but when water availability was higher during a spring heatwave stomatal conductance was unaffected by the heatwave. Resprouts in this study did have higher electron transport rates and higher transpiration rates, yet these differences did not result in mature shrubs and resprouts responding differently to either a spring or fall heatwave. Detecting a strong heatwave response in the field for drought tolerant shrub species such as H. arbutifolia may require heatwaves of larger magnitude or heatwaves coupled with severe drought. These extreme heatwaves or combinations of extreme heatwaves and drought are rare and therefore difficult to capture in the field; however, it is just such events that are likely to be the most important to predict plant vulnerabilities as extreme events become more common in the future.

Chapter 3 examines the seasonal patterns of water and carbon use in the two growth forms of H. arbutifolia. Most of the research comparing resprouting and mature individuals of a species happens immediately following a disturbance. To assess the relative risks to an obligate resprouting species it is important to understand the long-term performance of both forms of the plant. This study focuses on differences 4-years and 5-years post-disturbance. We found that many aspects of the physiology of two types of had converged but that resprouts had higher photosynthetic rates at the end of the dry season. If differences between mature and resprouting shrubs are more pronounced when leaves are experiencing water limited conditions, this could have important implications during drought years or under projected climate change if the precipitation regime changes.

This research is some of the first to examine the influence of ecotypic variation in response to heatwaves. Overall, these studies show that H. arbutifolia is likely to be robust to heatwaves of the magnitude studied. H. arbutifolia exhibits within species variation despite growing in a wide range of environments throughout its range. Understanding the magnitude of within species variation between different populations and different forms of the plant (i.e. resprouting and mature individuals) will be important to determine the vulnerability of the species to future climate change.