COS 180-4 - Impacts of climate extremes on physiology and growth of red spruce, northern red oak, and paper birch seedlings in a regional forest ecotone

Abstract: Climate change is raising concerns about how forests in the northeastern United States will respond to extreme heatwaves, droughts, and their co-occurrence. Climate extremes are especially important in ecosystem transition areas, like central Maine, where species composition may change rapidly with climate change. In this study, we tested how carbon and water relations drive tree seedling growth during extreme drought, heat, and combined drought and heat. We focused on red spruce (Picea rubens), a boreal-type species, and how its response to these stressors depends on the presence of a common associate (paper birch, Betula papyrifera) and a species expected to increase abundance in this region with climate change (northern red oak, Quercus rubra). We planted 216 two- to three-year-old seedlings of red spruce, northern red oak, and paper birch in containers either alone or with another red spruce. By controlling water availability, we tested three levels of drought (irrigated, 15-day “short”, and 30-day “long”) crossed with two levels of temperature (ambient and heat; +3.5 ℃ average daily maximum temperature with passive warming chambers). We measured growth and vigor, gas exchange, and midday leaf water potential (Ψ_leaf­) across the three-month study period. We also measured stomatal responses to drought and heat, minimum epidermal conductance, and the turgor loss point (TLP, the water potential at which the plant wilts) of each species. Overall, we found dominant effects of the drought driving reductions in both physiology and growth of spruce and birch, but that oak showed considerable resistance to the drought stress. Spruce and birch both experienced declines in growth Ψ_leaf­ exceeded their TLP, but oak had a lower (more resistant) turgor loss point and was able to avoid growth declines even when it did experience some turgor loss. Despite declines in photosynthesis with higher temperatures that were even stronger when compounded by drought, the moderate heatwaves did not impact growth. Further, in this first year of the study, competition had a relatively minor effect on growth responses; however, that may change as the study continues into the recovery year. This study suggests that native species to this region like red spruce and paper birch may be quite susceptible to future climate extremes that may favor other species like northern red oak. Our work suggests that more extreme and prolonged heatwaves, particularly when combined with drought, may lead to rapid shifts in community composition at species range margins.