Current and future impacts of drought and ozone stress on Northern Hemisphere forests

Rising ozone (O3) concentrations, coupled with an increase in drought frequencydue to climate change, pose a threat to plant growth and productivity which couldnegatively affect carbon sequestration capacity of Northern Hemisphere (NH)forests. Using long-term observations of O3 mixing ratios and soil water content(SWC), we implemented empirical drought and O3 s tress p arameterizations i n acoupled stomatal conductance-photosynthesis model to assess their impacts onplant gas exchange at three FLUXNET sites: Castelporziano, Blodgett and Hyytiälä.Model performance was evaluated by comparing model estimates of gross primaryproductivity (GPP) and latent heat fluxes (LE) against present-day observations.CMIP5 GCM model output data were then used to investigate the potential impactof the two stressors on forests by the middle (2041-2050) and end (2091-2100)of the 21st century. We found drought stress was the more significant as it reducedmodel overestimation of GPP and LE by ~11%-25% compared to 1%-11%from O3 stress. However, the best model fit to observations at all the study siteswas obtained with O3 and drought stress combined, such that the two stressorscounteract the impact of each other. With the inclusion of drought and O3 stress,GPP at CPZ, BLO and HYY is projected to increase by 7%, 5% and 8%, respectively,by mid-century and by 14%, 11% and 14% by 2091-2100 as atmospheric CO2 increases.Estimates were up to 21% and 4% higher when drought and O3 stresswere neglected respectively. Drought stress will have a substantial impact on plantgas exchange and productivity, off-setting and possibly negating CO2 fertilizationgains in future, suggesting projected increases in the frequency and severity ofdroughts in the NH will play a significant role in forest productivity and carbonbudgets in future.