| dc.description.abstract | The industrial revolution began approximately 200 years ago, and since then atmospheric [CO2] has increased from 270 to 402 ppm, and global temperatures have warmed by an average of 0.85 °C. Anthropogenic climate change is expected to alter both climate mean conditions and variability, leading to the more frequent occurrence of intense climate extremes such as heat waves and drought. This increase in climatic variability is likely to profoundly affect trees, as phenological and physiological processes of these species have been shown to be more sensitive to climate extremes than changes in the mean state. Yet, investigations of physiology and phenology of trees in relation to climate extremes remains limited, especially studies that address effects of climate extremes among- and within-populations of tree species. Investigations of the effects of weather extremes on phenology and physiology among- and within-populations of trees can provide important information on future phenological and physiological patterns that may affect ecosystem-level processes such as carbon cycling. To assess phenological and physiological responses of a tree to weather extremes, I measured phenology and physiology among 43 populations of Fraxinus americana (white ash) growing in a common garden during the extreme warm year of 2012, which was also one of the driest years on record at this site, and relatively non-extreme years. Intraspecific phenological responses across extreme and non-extreme years were assessed by observing the timing of leaf emergence whereas intraspecific physiological responses across extreme and non-extreme years were assessed by measuring leaf-level carbon isotopic signatures (δ13C). Additionally, we tested the performance of a commonly used phenology model in extreme and non-extreme years. I also identified three major ploidy levels of white ash at this site (diploids, tetraploids, and hexaploids), and this allowed me to investigate the consequences of polyploidy (within-population variation) on plant water relations by measuring leaf-level mid-day water potentials, gas exchange, δ13C, xylem-specific hydraulic conductance (Ksmax), xylem fatigue, and xylem density across three populations within which all cytotypes were represented. Leaf emergence among white ash populations was accelerated by 22 d during the extreme warm year of 2012 relative to non-extreme years. Additionally, thermal requirements shifted such that greater amounts of warming were required for leaf emergence during the extreme year relative to non-extreme years and this constrained the potential for even earlier leaf emergence by 7 d. I also found rank order for leaf emergence was maintained among populations across extreme and non-extreme years, suggesting at least the relative timing of leaf emergence may be predictable during future extreme years. Most concerning was the finding that responses of white ash to an extreme warm year altered the reliability of conventional models in predicting leaf emergence. In relation to physiology, we found that the rank order in leaf-level δ13C among populations was maintained across extreme and non-extreme years. This result suggest that the origin of white ash populations may play a role in controlling physiological functioning, and that the relative physiological relationships among populations will be maintained across extreme and non-extreme years. While population had a strong effect on δ13C, we did not find that average δ13C among populations varied significantly across years. Thus, populations of white ash differ constitutively in leaf-level δ13C with little environmentally induced change. We did find a significant linear relationship between average leaf-level δ13C and growing season (March-Aug.) vapor pressure deficit (VPD); however, this relationship was significantly weakened during 2013. This suggests that the effects of extreme years on white ash physiology may persist into non-extreme years. As intense droughts are projected to become more frequent over the next century, we investigated the effect of polyploidy on white ash water relations. Different degrees of ploidy within plants can produce morphological and physiological differences in response to local environments, including larger xylem vessels and larger stomata due to increased cell size, and these traits can affect drought tolerance of individuals. I found significant effects of cytotype on mid-day leaf-level water potentials whereby water potentials increased with increasing ploidy level. I also found significant effects on leaf-level gas exchange between cytotypes where diploids showed higher rates of photosynthesis (Amax), stomatal conductance to water vapor (gs), and transpiration (E). However, I did not find any differences across populations or between cytotypes nested within populations in leaf-level δ13C, Ksmax, xylem fatigue, or xylem density. These results suggest polyploidy may affect leaf-level responses in white ash across the growing season. This research advances our understanding of phenological and physiological responses among- and within-populations of white ash to weather extremes. I report on one of the first studies to document intraspecific variation in phenology during extreme and non-extreme years, and show extreme years will fundamentally alter phenology and its cues. I also find strong population level effects on δ13C, but marginal shifts in these responses across extreme and non-extreme years, suggesting populations have similar leaf-level δ13C with little environmentally induced change. The study on the influence of polyploidy on water relations of white ash indicates genome duplication may affect physiological responses of white ash during extremely dry periods across the growing season. My documentation of phenology and physiology among- and within-populations of white ash during extreme and non-extreme years provide novel insights into the potential of white ash to respond to rapid environmental changes that are expected under future climate change scenarios with more commonly occurring weather extremes. | |
