Across regions of the boreal forest, widespread growth declines and high mortality of trees have been linked to rising temperatures. In North America, paper birch (Betula papyrifera) and white spruce (Picea glauca) have suffered some of the greatest declines and evidence indicates direct heat stress and/or temperature-mediated drought stress are likely causes. Future annual temperatures will rise due to an increased frequency and duration of summer heatwaves. To date, few studies have examined the photosynthetic, photochemical and biochemical responses to reoccurring summer heatwaves in boreal trees. Due to this knowledge gap, we conducted a climate-manipulation experiment that examined the effects of heatwaves on Wisconsin-native birch and spruce seedlings. During the 2016 growing season, the seedlings were exposed to heatwaves and/or reduced water to determine if temperature-related decline in these species is influenced by direct heat stress (H), drought-induced stress (D), or an interaction of both (HD). The plants were subjected to monthly 8-day long heatwaves of 10◦C above ambient in June, July, and August. We measured photosynthetic, photochemical, and growth traits and stress related metabolites during and after each heatwave.
Preliminary data suggests that consecutive summer heatwaves affected spruce and birch seedlings differently. The HD treatment had a synergistic effect on most traits measured. Photosynthetic rates (A) decreased in spruce in H and D treatments, but HD had the strongest effect on decreasing A. In birch, A of the H plants recovered after the heatwave whereas the HD plants did not. The temperature response curves showed there was no shift in the temperature optimum in heatwave-treated spruce and only minor non-significant shifts were found in HD-treated birch. Heat and HD decreased photochemical efficiency (fv/fm) in spruce during the heatwaves and those values did not recover post heatwave. Birch showed a slight decrease in fv/fm during the heatwaves and recovered post stress.
Total biomass was reduced in by H and D treatments but most detrimentally affected were the HD plants (50-55% reduction). Leaf areas were reduced by H and HD in birch but in spruce, HD caused reductions where H increased leaf areas. Interestingly, leaf carbon and nitrogen content was not affected by any of the treatments. These results suggest the combination of HD has the strongest influence on decline than either of the stressors, heat or drought, applied alone.