Drought responses of two gymnosperm species with contrasting stomatal regulation strategies under elevated [CO₂] and temperature

Abstract

Future climate regimes characterized by rising [CO2], rising temperatures and associated droughts may differentially affect tree growth and physiology. However, the interactive effects of these three factors are complex because elevated [CO2] and elevated temperature may generate differential physiological responses during drought. To date, the interactive effects of elevated [CO2] and elevated temperature on drought-induced tree mortality remain poorly understood in gymnosperm species that differ in stomatal regulation strategies. Water relations and carbon dynamics were examined in two species with contrasting stomatal regulation strategies: Pinus radiata D. Don (relatively isohydric gymnosperm; regulating stomata to maintain leaf water potential above critical thresholds) and Callitris rhomboidea R. Br (relatively anisohydric gymnosperm; allowing leaf water potential to decline as the soil dries), to assess response to drought as a function of [CO2] and temperature. Both species were grown in two [CO2] (C(a) (ambient, 400 μl l(-1)) and C(e) (elevated, 640 μl l(-1))) and two temperature (T(a) (ambient) and T(e) (ambient +4 °C)) treatments in a sun-lit glasshouse under well-watered conditions. Drought plants were then exposed to a progressive drought until mortality. Prior to mortality, extensive xylem cavitation occurred in both species, but significant depletion of non-structural carbohydrates was not observed in either species. Te resulted in faster mortality in P. radiata, but it did not modify the time-to-mortality in C. rhomboidea. C(e) did not delay the time-to-mortality in either species under drought or T(e) treatments. In summary, elevated temperature (+4 °C) had greater influence than elevated [CO2] (+240 μl l(-1)) on drought responses of the two studied gymnosperm species, while stomatal regulation strategies did not generally affect the relative contributions of hydraulic failure and carbohydrate depletion to mortality under severe drought.

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

“…Most studies of CO 2 fertilization are not focused on tree mortality and have not been concurrent with drought (Jentsch et al 2007), limiting their value for understanding interactions between [CO 2 ] and drought . In two particularly relevant [CO 2 ]-by-drought experiments, elevated [CO 2 ] did not ameliorate drought stress nor delay tree seedling mortality during combined heat and drought treatments in Eucalyptus (Duan et al 2014), nor did it delay mortality in two gymnosperms (Pinus radiata and Callitris rhomboidea; Duan et al 2015). There also is substantial evidence that CO 2 fertilization effects are limited by nutrient availability, especially nitrogen and phosphorus (Norby et al 2010, Fernández-Martínez et al 2014, Zhang et al 2014a.…”

Anisohydric behavior linked to persistent hydraulic damage and delayed drought recovery across seven North American tree species

“…However, multiple factors complicate a universal application of the isohydry-anisohydry framework. For example, some anisohydric trees have been found to embolize readily during severe water stress (Duan et al, 2015;Mencuccini et al, 2015;Pellizzari et al, 2016). This suggests that, rather than embolism resistance traits determining a species' degree of anisohydry, maintaining C assimilation vs avoiding hydraulic damage is the core tradeoff that determines a species' hydraulic strategy.…”

Temperature response surfaces for mortality risk of tree species with future drought

“…We hypothesized that (1) increased temperatures would result in earlier seedling mortality from drought for both species, and (2) species differences in drought tolerance would lead to earlier mortality for P. ponderosa than P. edulis. We based our first hypothesis on the findings of several drought-induced mortality experiments with singlestep temperature treatments that earlier mortality was observed under higher temperatures for small trees of P. edulis (Adams et al 2009), and seedlings of Eucalyptus radiata (Duan et al 2014), Pinus radiata (Duan et al 2015), and ten tree species from the south-central US . We based our second hypothesis on past findings that P. edulis is less vulnerable to xylem embolism from drought (Martínez-Vilalta et al 2004), has a lower sensitivity of radial growth to drought (Adams and Kolb 2005), occurs at drier sites, and experienced less drought-induced mortality at sites where the species co-occur, than P. ponderosa (Koepke et al 2010).…”