Warming and elevated CO2 alter tamarack C fluxes, growth and mortality: evidence for heat stress-related C starvation in the absence of water stress.

Climate warming is increasing the frequency of climate-induced tree mortality events. While drought combined with heat is considered the primary cause of this mortality, little is known about whether moderately, high temperatures alone can induce mortality, or whether rising CO2 would prevent mortality at high growth temperatures. We grew tamarack (Larix laricina) under ambient (400 ppm) and elevated (750 ppm) CO2 concentrations combined with ambient, ambient +4°C, and ambient +8°C growth temperatures to investigate whether high growth temperatures lead to carbon limitations and mortality. Growth at +8°C led to 40% mortality in the ambient CO2 (8TAC) treatment, but no mortality in the elevated CO2 treatment. Thermal acclimation of respiration led to similar leaf carbon balances across the warming treatments, despite a lack of photosynthetic acclimation. Photosynthesis was stimulated under elevated CO2, increasing seedling growth, but not leaf carbon concentrations. However, growth and foliar carbon concentrations were lowest in the +8°C treatments, even with elevated CO2. Dying 8TAC seedlings had lower needle carbon concentrations and lower ratios of photosynthesis to respiration than healthy 8TAC seedlings, indicating that carbon limitations were likely the cause of seedling mortality under high growth temperatures.