How do drought and warming influence survival and wood traits of Picea mariana saplings?

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.

Keywords: Carbon balance; climate change; drought; survival; warming; wood anatomy; wood density..

Fig. 1.

Daily temperatures experienced by black spruce saplings in the three thermal conditions (T0, control temperature; T+Day, temperature increase during the day; T+Night, temperature increase during the night) during the greenhouse experiment from April to October. Grey background corresponds to the water deficit period during June.

Fig. 2.

Volumetric water content (VWC) of soil in irrigated saplings (filled circles) and non-irrigated saplings (open circles) before, during, and after the water deficit period (grey background) under the three thermal conditions (T0, control temperature; T+Day, temperature increase during the day; T+Night, temperature increase during the night) during the greenhouse experiment in 2011. Vertical bars represent the standard deviation.

Fig. 3.

Pre-dawn leaf water potential (Ψ_pd; MPa), midday leaf water potential (Ψ_md; MPa), CO₂ assimilation (maximum photosynthesis rate, A _max; μmol m^–2 s^–1), and gas exchange (stomatal conductance, g _s; mol m^–2 s^–1) of black spruce saplings before, during, and after the water deficit period (WDp) under the three thermal conditions during the greenhouse experiment in 2011.

Fig. 4.

Mean percentage loss of hydraulic conductance (PLC%) versus xylem pressure (MPa) for black spruce saplings The vulnerability curve was obtained with the cavitron technique.

Fig. 5.

Cell features and wood density of the tracheids produced by irrigated (black curves) and non-irrigated (grey curves) black spruce saplings under three thermal conditions (T0, control temperature; T+Day, temperature increase during the day; T+Night, temperature increase during the night) along portions of an annual tree ring.

Fig. 6.

Non-structural carbohydrates (NSCs) in mg g_dw ^–1 in cambium and in xylem, and starch concentration in xylem (mg g_dw ^–1) in black spruce saplings before, during, and after the water deficit period (grey background) under three thermal conditions (T0, control temperature; T+Day, 6 °C higher daytime temperature; T+Night, 6 °C higher night-time temperature). Filled and open white circles indicate the two irrigation regimes. Asterisks indicate statistically significant differences between the two irrigation regimes (Wilcoxon test; P≤0.05).