Growth responses of aspen clones to elevated carbon dioxide and ozone

The International Panel of Climate Change (IPCC) has concluded that the greenhouse gases, carbon dioxide (CO2) and tropospheric ozone (O3), are increasing concomitantly globally. Little is known about the effect of these interacting gases on growth, survival, and productivity of forest ecosystems. In this study we assess the effects of three successive years of exposure to combinations of elevated CO2 and O3 on growth responses in a 5 trembling aspen (Populus tremuloides) clonal mixture in a regenerating stand. The experiment is located in Rhinelander, Wisconsin, USA (45° N 89° W) and employs free air carbon dioxide and ozone enrichment (FACE) technology. The aspen stand was exposed to a factorial combination of 4 treatments consisting of elevated CO2 (560 ppm), elevated O3 (episodic exposure-90 μl l-1 h-1), a combination of elevated CO2 and O3, and ambient control in 30 m treatment rings with 3 replications. Our overall results showed that our 3 growth parameters including height, diameter and volume were increased by elevated CO2, decreased by elevated O3, and were not significantly different from the ambient control under elevated CO2 + O3. However, there were significant clonal differences in the responses; all 5 clones exhibited increased growth with elevated CO2, one clone showed an increase with elevated O3, and two clones showed an increase over the control with elevated CO2 + O3, two clones showed a decrease, and one was not significantly different from the control. Notably, there was a significant increase in current terminal shoot dieback with elevated CO2 during the 1999-2000 dormant seasons. Dieback was especially prominent in two of the five clones, and was attributed to those clones growing longer into the autumnal season where they were subject to frost. In summary, our results show that elevated O3 negates expected positive growth effects of elevated CO2 in Populus tremuloides in the field. Our results suggest that future climate model predictions should take into account the offsetting effects of elevated O3 on CO2 enrichment when estimating future growth of trembling aspen stands. © 2003 Elsevier Ltd. All rights reserved