A hot and dry future: warming effects on boreal tree drought tolerance.

By 2100, climate change is predicted to warm high-latitude summers by 3–8 °C (ACIA 2005, Christensen et al. 2007). High temperatures will become more common in the boreal region and, in the absence of significantly increased precipitation, drought frequency and intensity will increase. Much of the North American boreal consists of forests dominated by black spruce (Picea mariana (Mill.) B.S.P.), a major timber crop that dominates frequently flooded soils, but has shallow roots (<20 cm deep) and therefore experiences drought stress in the absence of frequent rain (Viereck and Johnston 1990). Black spruce growth is negatively impacted by heat, due in part to rapid soil drying from greater evapotranspiration (ACIA 2005, Angert et al. 2005). Increased temperatures and drought potentially threaten the dominance of black spruce in the boreal forest, and have been identified as major causes for the sharp increases in mortality rate of boreal tree species, including black spruce, that have occurred in North America since the 1960s (Peng et al. 2011). Experiments investigating the response of black spruce to warming and drought provide insight into the mechanisms that underlie the reduced growth and increased mortality seen in this species in the field. Growth at elevated temperatures reduces the carbon balance of black spruce seedlings, which in turn limits growth, as well as reducing the root to shoot ratio, which may make trees more prone to drought stress (Way and Sage 2008a, 2008b, Way et al. 2011). Stomatal closure appears to be the main mechanism by which black spruce avoids lethal water stress (Blake and Li 2003), with conductance declining at relatively high leaf water potentials (−0.25 MPa) and complete closure observed below −1.5 MPa (Grossnickle and Blake 1986, Eastman and Camm 1995, Dang et al. 1997). However, by reducing stomatal conductance to prevent water loss, seedlings substantially reduce their ability to assimilate carbon, which may further exacerbate heatrelated growth declines. In this issue of Tree Physiology, Balducci et al. (2013) look at how water stress affects xylem formation, gas exchange and survival in black spruce seedlings grown at elevated temperatures of +2 °C and +5 °C above the ambient. While a 32-day drought reduced stomatal conductance and photosynthesis, there was little evidence for an effect of temperature on these leaf-level parameters, which rapidly recovered fully after rewatering. But warming did alter the impact of drought on seedling survival: over 12% of the drought-stressed spruce died in the warmest treatment, compared with 2% in the ambient temperature environment. The drought also affected xylem formation over the season. When water was resupplied to the drought-stressed, ambient temperature-grown spruce had similar cambial activity to the ambient temperature, watered seedlings after 2 weeks of recovery. But growth at elevated temperatures delayed recovery of cambial activity for an extra 2 weeks, and reduced wood density. Overall, the effect of drought was much stronger than that of increased growth temperatures on the parameters examined, although the delayed recovery of growth and increased mortality from water stress at elevated temperatures implies that seedlings that develop in a warmer climate may be more vulnerable to drought. While the cause of increased drought-related mortality in warm-grown spruce in Balducci et al. (2013) is unknown, it echoes recent trends seen in forest stands (Peng et al. 2011) and lends support to the concept that combined increases in heat and drought are responsible for rising mortality in boreal Commentary