A Meta-analysis of Elevated CO Effects on Woody Plant Mass, Form, and Physiology

Quantitative integration of the literature on the effect of elevated CO on woody plants is important to aid our understanding of forest health in coming decades and to better predict terrestrial feedbacks on the global carbon cycle. We used meta-analytic methods to summarize and interpret more than 500 reports of effects of elevated CO on woody plant biomass accumulation and partitioning, gas exchange, and leaf nitrogen and starch content. The CO effect size metric we used was the log-transformed ratio of elevated compared to ambient response means weighted by the inverse of the variance of the log ratio. Variation in effect size among studies was partitioned according to the presence of interacting stress factors, length of CO exposure, functional group status, pot size, and type of CO exposure facility. Both total biomass (W ) and net CO assimilation (A) increased significantly at about twice ambient CO, regardless of growth conditions. Low soil nutrient availability reduced the CO stimulation of W by half, from +31% under optimal conditions to +16%, while low light increased the response to +52%. We found no significant shifts in biomass allocation under high CO. Interacting stress factors had no effect on the magnitude of responses of A to CO, although plants grown in growth chambers had significantly lower responses (+19%) than those grown in greenhouses or in open-top chambers (+54%). We found no consistent evidence for photosynthetic acclimation to CO enrichment except in trees grown in pots <0.5 l (-36%) and no significant CO effect on stomatal conductance. Both leaf dark respiration and leaf nitrogen were significantly reduced under elevated CO (-18% and -16% respectively, data expressed on a leaf mass basis), while leaf starch content increased significantly except in low nutrient grown gymnosperms. Our results provide robust, statistically defensible estimates of elevated CO effect sizes against which new results may be compared or for use in forest and climate model parameterization.