Effect of repeated heat waves, elevated [CO2] and low water availability on growth of Quercus rubra and Pinus taeda seedlings

Background/Question/Methods: The frequency and intensity of heat waves are predicted to increase. We investigated whether repeated heat waves of different severities would have the same impact as an equivalent constant increase in temperature and considered atmospheric [CO2] and soil moisture as potential interacting factors. We measured growth, biomass accumulation and partitioning of Quercus rubra and Pinus taeda seedlings grown under ambient or elevated [CO2] (380/700 mmol CO2 mol-1) and three heat treatments: ambient +3°C, a moderate heat wave every second week (+6°C) or a severe heat wave every fourth week (+12°C). All treatments had the same average temperature across the five month growing season. Half the seedlings were watered to a soil water content near field capacity, half to 50% of that amount. Height and diameter were measured monthly. Biomass accumulation was measured after 5 months. Leaf gas exchange was also measured before, during and after a mid-summer heat wave event. Results/Conclusions: In ambient [CO2], seedlings had significantly less total biomass when exposed to +12oC heat waves compared to a constant +3oC increase. These differences were eliminated under elevated [CO2] in Q. rubra but not in P. taeda. Total biomass accumulation in seedlings subjected to +6°C heat waves was not significantly different than in the constant +3oC control treatment. The +12oC heat waves also affected stem, root and leaf biomass partitioning, but to a different degree in the two species. Generally, the Q. rubra seedlings were more sensitive to the heat wave treatments than the P. taeda seedlings. All measures of growth, including height, diameter and leaf, stem, root and total biomass accumulation were significantly reduced in the low water treatment. Water availability had a much greater effect on growth than either [CO2] or temperature. Reduced net photosynthesis during a heat wave event and in the low water treatment was correlated with reduced growth. We conclude that a) heat waves can produce more stress than the same amount of heat applied uniformly; b) elevated [CO2] can partially mitigate negative heat wave effects and c) drought was a more severe stress than repeated heat waves.