Water availability is the decisive factor for the growth of two tree species in the occurrence of consecutive heat waves

The frequency and intensity of heat waves are predicted to increase as a consequence of climate warming, yet no experimental evidence exists for the potential magnitude of these effects. Therefore, we investigated whether consecutive heat waves of different severities would have the same impact on tree growth as a constant increase in temperature with equivalent heat sum and considered (CO2) and available soil moisture as potential interacting factors. Seedlings of Quercus rubra and Pinus taeda were grown under ambient or elevated (CO2) (380/700 μmol CO2 mol−1) and three temperature treatments: ambient +3 °C, moderate heat wave every second week (+6 °C) or severe heat wave every fourth week (+12 °C). All temperature treatments had the same average temperature (+3 °C above ambient) across the five-month experimental growth period. Half the seedlings were watered to field capacity (50% (v/v) on average), the other half to 30%. Stem height and diameter were measured monthly. Biomass accumulation and partitioning were measured after five months. In ambient (CO2), seedlings of both species had significantly less biomass (up to 39%) when exposed to +12 °C heat waves compared to a constant +3 °C increase. These decreases were mitigated by elevated (CO2) in Quercus but not in Pinus. Despite having the lowest biomass, Quercus seedlings in the +12 °C heat wave treatment grew taller (up to 17%) than all other Quercus seedlings. In contrast, Pinus seedlings in the +12 °C heat wave treatment had consistently lower height and stem diameter compared with all other treatments within each soil water treatment. Soil water treatment had a greater effect than (CO2) on stem height, diameter and total biomass, with reduced growth under low soil water content compared with high soil water content (up to 55% smaller and up to 63% less biomass). We conclude that (a) heat waves produced more stress than the same amount of heat applied uniformly; (b) elevated (CO2) at least partially mitigated negative effects of heat waves; and (c) drought was a more severe stressor than heat waves.