Heat Waves Alter Carbon Allocation and Increase Mortality of Aleppo Pine Under Dry Conditions

Abstract

Climate extremes are likely to occur more frequently in the future, including a combination of heat waves and drought. However, the responses of trees to combined stress and their post-stress recovery are not fully understood yet. Therefore, this study investigated the responses of semi-arid Pinus halepensis seedlings to moderate drought, heat and combined heat-drought stress, as well as post-stress recovery. The seedlings were grown under controlled conditions and exposed to two 4-days-long heat periods, reaching air temperature maxima of 42 • C and vapor pressure deficit (VPD) of 7 kPa. Day-and nighttime canopy gas exchange was measured and differences in shoot and root allocation of non-structural carbohydrate (NSC) compounds (soluble sugars, starch, cyclitols, and carboxylic acids) assessed. Fluorescence parameters, nitrate levels, proline content and shoot water potential (ψ) provided additional indicators for stress severity and recovery performance. During the heat periods, net photosynthesis and stomatal conductance decreased immediately. This decline was modest under well-watered conditions, with transpiration and dark respiration rates remaining high and despite reductions in root NSC content, trees recovered following heat release. This was not the case in the heat-drought treatment, where stress resulted in high mortality rates and the few surviving seedlings showed reduced gas exchange rates and low root NSC content, while leaf nitrate and proline remained elevated even 3 weeks after heat release. Shoot ψ indicated that hydraulic failure was not the reason for mortality in the heat-drought seedlings. Instead, we argue that low transpiration rates, which resulted in needle temperatures >47 • C during heat stress (6 • C above air temperature) have caused irreversible damage. In summary, it could be demonstrated that heat waves in combination with moderate drought can either result in increased mortality or, if the seedlings survive, in delayed recovery. This highlights the potential of an increase in heat wave temperatures to trigger forest decline in semi-arid regions.

Hot drought reduces the effects of elevated CO2 on tree water‐use efficiency and carbon metabolism

“…Gauthier et al , 2014). However, an acclimation of leaf respiration to elevated growth temperatures has been reported in many instances (Reich et al , 2016; Drake et al , 2019), and R Shoot has been shown to decline during consecutive heatwaves (Birami et al , 2018). Likely explanations for the early downregulation of R Shoot and R Root that we have found in response to heat stress are: first, reduced respiratory demand due to downregulation of growth and maintenance respiration; second, adenylate restriction caused by ATP turnover decline; and/or third, reduced C availability (O'Leary et al , 2019).…”

“…3a). We refrained from temperatures above 40°C as tree mortality has been found to strongly increase in Aleppo pine seedlings above this threshold, particularly in combination with drought (Birami et al , 2018). As during a typical heatwave in the Yatir Forest (Tatarinov et al , 2016), vapor pressure deficit (VPD) increased alongside increasing temperature, and this increase was slightly greater in the drought‐treated saplings due to low E (Fig.…”

“…Heatwaves during extended drought periods can be a main cause of forest decline (Anderegg et al , 2013). Hot droughts are particularly stressful because evaporative demand is high, while H 2 O availability is low and trees need to tightly regulate H 2 O loss (Ameye et al , 2012; Ruehr et al , 2016; Birami et al , 2018). This typically induces stomatal closure to maintain the integrity of the H 2 O transporting system (Tyree & Zimmermann, 2002).…”

Joint forcing by heat waves and mowing poses a threat to grassland ecosystems: Evidence from a manipulative experiment

“…As several prior laboratory-based studies showed, the HW effect was first noticed at the cellular level, which resulted in a decrease in photosynthetic enzyme activity and low stomatal conductance and had effects on other eco-physiological characteristics Zinta et al, 2014). Recent findings have confirmed that HWs significantly alter plant photosynthesis and respiration (Qu, Chen, Bunce, Zhu, & Sicher, 2018;Rashid et al, 2018), reduce CO 2 uptake (Tatarinov et al, 2016), and lead to the redistribution of carbon and nitrogen in a plant or community (Birami et al, 2018;Giri, Heckathorn, Mishra, & Krause, 2017). Here, mowing had a similar effect on the community but directly removed plant parts (photosynthetic product) rather than reducing the accumulation rate.…”

Environmental Controls on the Seasonal Variation in Gas Exchange and Water Balance in a Near-Coastal Mediterranean Pinus halepensis Forest

“…Improving our understanding of the driving factors that control Aleppo pine responses to climatic conditions is important for managing the species and forecasting its responses to climate variability, extremes, and change. Relationships between various physiological traits and the abiotic environment have been reported in the literature for Aleppo pine [10][11][12]. Studies have demonstrated a strong stomatal regulation in the species and coordination between foliage water potential and stomatal conductance to balance water loss [13].…”