Extreme heat events have increased in number, duration, and intensity in recent decades and have caused widespread decline among forest trees. The combined effect of increased evaporative demand with decreased photosynthetic activity during heatwaves has been implicated as a driver of forest mortality. Species from boreal forests appear to be more sensitive to increasing temperatures compared to other regions, which may be due to an inability to photosynthetically acclimate to warmer temperatures. Paper birch (Betula papyrifera) and white spruce (Picea glauca) are two such boreal species in decline. Water availability has been shown to limit growth in paper birch and has been suggested to limit growth in white spruce, but few studies have examined their capacity for thermal photosynthetic acclimation. We subjected greenhouse grown paper birch and white spruce seedlings to two consecutive summers of heatwave stress to determine if thermal acclimation of photosynthesis and respiration occur. Each summer, the seedlings were exposed to three 8-day long heatwaves at 10◦C above ambient. We measured photosynthetic temperature responses, dark respiration (Rd), gas exchange, and basal area growth before and after heatwave stress.
Results/Conclusions
The data suggest that both paper birch and white spruce seedlings have a limited ability to thermally acclimate photosynthesis and Rd after two summers of consecutive heatwaves. We found no change in the temperature optimum of photosynthesis (Topt), the photosynthetic rate at Topt (Aopt), or in the shape of the curve (b) in the heatwave-treated birch. The heatwave-treated spruce marginally increased Topt but not Aopt, and slightly widened b. Measurements of Rd showed that at warmer nighttime temperatures, there was some evidence of acclimation in the heat-treated birch but not in the spruce. Additionally, carbon assimilation was severely reduced by heat and water stress during as well as not during heatwave events in both species. Low basal area growth was also observed in the heat and water stressed plants; the largest reduction in growth was shown by the combined effects of reduced water and heat. The data suggests that the lower growth observed in the heatwave-treated birch and spruce may be attributed to reduced carbon assimilation and to their inability to thermally acclimate both photosynthesis and Rd to elevated temperatures. These two factors may prevent these species from remaining functional during future heatwave events.