Abstract
Warming and drought will occur with increased frequency and intensity at high latitudes in the future. How heat and water stress can influence tree mortality is incompletely understood. The aim of this study was to evaluate how carbon resources, stem hydraulics, and wood anatomy and density determine the ability of black spruce saplings to survive daytime or night-time warming (+ 6 °C in comparison with control) in combination with a drought period. Plant water relations, the dynamics of non-structural carbohydrates and starch, mortality rate, and wood anatomy and density of saplings were monitored. Warming, in conjunction with 25 d of water deficit, increased sapling mortality (10% and 20% in night-time and daytime warming, respectively) compared with the control conditions (0.8%). Drought substantially decreased gas exchange, and also pre-dawn and mid-day leaf water potential to values close to -3MPa which probably induced xylem embolism (xylem air entry point, P₁₂, being on average around -3MPa for this species). In addition, the recovery of gas exchange never reached the initial pre-stress levels, suggesting a possible loss of xylem hydraulic conductivity associated with cavitation. Consequently, mortality may be due to xylem hydraulic failure. Warmer temperatures limited the replenishment of starch reserves after their seasonal minimum. Lighter wood was formed during the drought period, reflecting a lower carbon allocation to cell wall formation, preventing the adaptation of the hydraulic system to drought. Saplings of black spruce experienced difficulty in adapting under climate change conditions, which might compromise their survival in the future.
Highlights
In boreal ecosystems, an increase in temperature of ~2–4 °C could have important consequences on tree growth and suris expected by 2060 due to global warming
During maximum xylem growth, when saplings are more susceptible to dry conditions (Rossi et al, 2006), two irrigation regimes were applied: (i) control, consisting of maintaining the soil water content at ~80% of field capacity; and (ii) water deficit, in which irrigation was withheld for 25 d in June [from day of the year (DOY) 158 to 182] in 184 saplings per thermal condition
During the water deficit period, the temperature in T0 varied between 14 °C and 22 °C
Summary
An increase in temperature of ~2–4 °C could have important consequences on tree growth and suris expected by 2060 due to global warming In the last decades, Peng et al (2011) found that regional drought increased tree mortality in mature stands of the Canadian boreal forest. Drought conditions affect growth and survival of stand regeneration (Payette and Filion, 1985; Hogg and Schwarz, 1997), because young trees are more vulnerable to root embolism and stomatal closure (Domec et al, 2004; Mueller et al, 2005). The increases in temperature occurring during drought lead to a more rapid dehydration of young trees because of a higher evapotranspiration demand (Angert et al, 2005)
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