Combined heat-drought stress imposes lagged effects on tree carbon and water relations
Heat and drought waves co-occur naturally, but so far we have only a limited understanding on how plants respond to multiple stress and their ability to recover. In this study we investigate the response of carbon and water relations to heat and heat-drought waves followed by recovery periods in Douglas-fir and black locust saplings grown under controlled conditions.
Tree carbon and water fluxes responded strongly to the treatments, with largest water deficits occurring under coupled heat-drought stress. Day- and nighttime transpiration increased with high temperatures and evaporative demand as long as water supply was sufficient. Assimilation in both tree species declined strongly under heat and heat-drought conditions while dark respiration increased with stress intensity in Douglas-fir but not in black locust. We found lagged effects on gas-exchange during the recovery periods, likely resulting from reduced stomatal conductance. Black locust did not compensate for stress-induced growth and biomass losses during recovery, in contrast to Douglas-fir where stem growth continued longer in previously stressed trees while it already ceased in the control treatment. In summary, high temperature events, specifically in combination with drought, strongly affected tree carbon and water cycling and caused lasting effects beyond the actual stress period. These legacy effects need to be included in vegetation models to allow accurate predictions of stress impacts.