Interannual climatic variation mediates elevated CO2 and O3 effects on forest growth

We analyzed growth data from model aspen (Populus tremuloides Michx.) forest ecosystems grown in elevated atmospheric carbon dioxide (CO₂]; 518 μL L^?1) and ozone concentrations (O₃]; 1.5 × background of 30–40 nL L^?1 during daylight hours) for 7 years using free‐air CO₂ enrichment technology to determine how interannual variability in present‐day climate might affect growth responses to either gas. We also tested whether growth effects of those gasses were sustained over time. Elevated CO₂] increased tree heights, diameters, and main stem volumes by 11%, 16%, and 20%, respectively, whereas elevated ozone O₃] decreased them by 11%, 8%, and 29%, respectively. Responses similar to these were found for stand volume and basal area. There were no growth responses to the combination of elevated CO₂+O₃]. The elevated CO₂] growth stimulation was found to be decreasing, but relative growth rates varied considerably from year to year. Neither the variation in annual relative growth rates nor the apparent decline in CO₂ growth response could be explained in terms of nitrogen or water limitations. Instead, growth responses to elevated CO₂] and O₃] interacted strongly with present‐day interannual variability in climatic conditions. The amount of photosynthetically active radiation and temperature during specific times of the year coinciding with growth phenology explained 20–63% of the annual variation in growth response to elevated CO₂] and O₃]. Years with higher photosynthetic photon flux (PPF) during the month of July resulted in more positive growth responses to elevated CO₂] and more negative growth responses to elevated O₃]. Mean daily temperatures during the month of October affected growth in a similar fashion the following year. These results indicate that a several‐year trend of increasingly cloudy summers and cool autumns were responsible for the decrease in CO₂ growth response.