Extreme climate events, such as drought and heat waves, shape the vegetated landscape with lasting effects on the composition, dynamics and carbon balance of vegetation. While the death of mature trees during drought is the most prominent visual feature of the landscape, it is the loss of highly-susceptible seedlings and saplings which generates the most significant long-term ecological impact. Species from seasonally water-limited systems have evolved in response to water stress, but the concomitant effect of extended drought, warmer average air temperatures, and heat waves may extend beyond the threshold or persistence niche for many species. In addition, there are conflicting opinions about the impact of elevated CO2 on seedling survival during drought and heat waves. In our research, we address whether growth in elevated CO2 and warmer average temperatures ameliorates or exacerbates tree seedling sensitivity to heat waves and drought. Our objectives are to use environmentally controlled glasshouse and field facilities to (1) determine the impact of elevated CO2 and temperature on plant traits (e.g. physiology, leaf area, leaf and stem anatomy) under well-watered conditions prior to initiation of drought and heat waves; (2) assess the role of these CO2 and temperature treatment driven traits in the degree of tree susceptibility to drought and heat waves; (3) address the universality of tree response through better characterisation of plant strategies (rooting depth, stomatal physiology, leaf area display, C storage, etc) during extreme events; and (4) parameterise a whole-plant physiological model to greatly improve predictions of plant mortality under current and future climate scenarios.
Results/Conclusions
In eucalypts grown under elevated CO2 and temperature conditions for nine months in well-watered conditions, we observed development of distinct plant traits that affected sapling response to the subsequent prolonged two-month drought. Elevated CO2 increased photosynthesis, leaf area and C storage, while elevated temperature increased stomatal conductance and reduced C storage. Trees in elevated temperature died first, due to massive water loss, while elevated CO2 did not ameliorate the drought. Overall, trees in elevated CO2 and temperature were most susceptible to drought due to large leaf area (CO2 effect) and high stomatal conductance (Temp effect) leading to rapid and significant transpirational water loss. Overall, we concluded that there was greater potential for mortality in a future, warmer, elevated CO2 world with more frequent dry periods. We are continuing this research effort with more woody species and including heat waves to gain further understanding of this complex issue.