CO2 and temperature effects on morphological and physiological traits affecting risk of drought-induced mortality

Despite a wealth of eco-physiological assessments of plant response to extreme drought, few studies have addressed the interactive effects of global change factors on traits driving mortality. To understand the interaction between hydraulic and carbon metabolic traits influencing tree mortality, which may be independently influenced by atmospheric [CO2] and temperature, we grew Eucalyptus sideroxylon A. Cunn. ex Woolls from seed in a full-factorial [CO2] (280, 400 and 640 μmol mol−1, Cp, Ca and Ce, respectively) and temperature (ambient and ambient +4 °C, Ta and Te, respectively) experiment. Prior to drought, growth across treatment combinations resulted in significant variation in physiological and morphological traits, including photosynthesis (Asat), respiration (Rd), stomatal conductance, carbohydrate storage, biomass and leaf area (LA). Ce increased Asat, LA and leaf carbohydrate concentration compared with Ca, while Cp generated the opposite response; Te reduced Rd. However, upon imposition of drought, Te hastened mortality (9 days sooner compared with Ta), while Ce significantly exacerbated drought stress when combined with Te. Across treatments, earlier time-to-mortality was mainly associated with lower (more negative) leaf water potential (Ψl) during the initial drought phase, along with higher water loss across the first 3 weeks of water limitation. Among many variables, Ψl was more important than carbon status in predicting time-to-mortality across treatments, yet leaf starch was associated with residual variation within treatments. These results highlight the need to carefully consider the integration, interaction and hierarchy of traits contributing to mortality, along with their responses to environmental drivers. Both morphological traits, which influence soil resource extraction, and physiological traits, which affect water-for-carbon exchange to the atmosphere, must be considered to adequately predict plant response to drought. Researchers have struggled with assessing the relative importance of hydraulic and carbon metabolic traits in determining mortality, yet an integrated trait, time-dependent framework provides considerable insight into the risk of death from drought for trees

Nocturnal stomatal conductance responses to rising [CO₂], temperature and drought

•The response of nocturnal stomatal conductance (gs,n) to rising atmospheric CO₂ concentration ([CO₂]) is currently unknown, and may differ from responses of daytime stomatal conductance (gs,d). Because night-time water fluxes can have a significant impact on landscape water budgets, an understanding of the effects of [CO₂] and temperature on gs,n is crucial for predicting water fluxes under future climates. •Here, we examined the effects of [CO₂] (280, 400 and 640 μmol mol⁻¹), temperature (ambient and ambient + 4°C) and drought on gs,n, and gs,d in Eucalyptus sideroxylon saplings. •gs,n was substantially higher than zero, averaging 34% of gs,d. Before the onset of drought, gs,n increased by 85% when [CO₂] increased from 280 to 640 μmol mol⁻¹, averaged across both temperature treatments. gs,n declined with drought, but an increase in [CO₂] slowed this decline. Consequently, the soil water potential at which gs,n was zero (Ψ0) was significantly more negative in elevated [CO₂] and temperature treatments. gs,d showed inconsistent responses to [CO₂] and temperature. •gs,n may be higher in future climates, potentially increasing nocturnal water loss and susceptibility to drought, but cannot be predicted easily from gs,d. Therefore, predictive models using stomatal conductance must account for both gs,n and gs,d when estimating ecosystem water fluxes.10 page(s