Effects of elevated atmospheric CO2 and tropospheric O3 on tree branch growth and implications for hydrologic budgeting.

The forest hydrologic budget may be impacted by increasing CO(2) and tropospheric O(3). Efficient means to quantify such effects are beneficial. We hypothesized that changes in the balance of canopy interception, stem flow, and through-fall in the presence of elevated CO(2) and O(3) could be discerned using image analysis of leafless branches. We compared annual stem flow to the results of a computerized analysis of all branches from the 2002, 2004, and 2006 annual growth whorls of 97 ten-year-old trees from the Aspen Free-Air CO(2) and O(3) Enrichment (Aspen FACE) experiment in Rhinelander, WI. We found significant effects of elevated CO(2) and O(3) on some branch metrics, and that the branch metrics were useful for predicting stem flow from birch, but not aspen. The results of this study should contribute to development of techniques for efficient characterization of effects on the forest hydrologic budget of increasing CO(2) and tropospheric O(3).