Climate change predictions derived from coupled carbon-climate models are highly dependent on assumptions about feedbacks between the biosphere and atmosphere. One critical feedback occurs if C uptake by the biosphere increases in response to the fossil-fuel driven increase in atmospheric [CO 2] (''CO2 fertilization''), thereby slowing the rate of increase in atmospheric [CO 2]. Carbon exchanges between the terrestrial biosphere and atmosphere are often first represented in models as net primary productivity (NPP). However, the contribution of CO 2 fertilization to the future global C cycle has been uncertain, especially in forest ecosystems that dominate global NPP, and models that include a feedback between terrestrial biosphere metabolism and atmospheric [CO 2] are poorly constrained by experimental evidence. We analyzed the response of NPP to elevated CO 2 (Ϸ550 ppm) in four free-air CO 2 enrichment experiments in forest stands. We show that the response of forest NPP to elevated [CO 2] is highly conserved across a broad range of productivity, with a stimulation at the median of 23 ؎ 2%. At low leaf area indices, a large portion of the response was attributable to increased light absorption, but as leaf area indices increased, the response to elevated [CO 2] was wholly caused by increased light-use efficiency. The surprising consistency of response across diverse sites provides a benchmark to evaluate predictions of ecosystem and global models and allows us now to focus on unresolved questions about carbon partitioning and retention, and spatial variation in NPP response caused by availability of other growth limiting resources.CO2 fertilization ͉ global change ͉ leaf area index ͉ net primary productivity A nalysis and prediction of the effects of human activities, particularly the combustion of fossil fuels, on climate and the biological, physical, and social responses to changing climate require an integrated view of the complex interactions between the biosphere and atmosphere. Carbon cycle models are now being coupled to atmosphere-ocean general circulation climate models to achieve a dynamic analysis of the relationships between C emissions, atmospheric chemistry, biosphere activity, and climatic change (1-3).Exchanges between the terrestrial biosphere and atmosphere are represented in models using empirical and theoretical expressions of net primary productivity (NPP), the net fixation of C by green plants into organic matter, or the difference between photosynthesis and plant respiration. Because the photosynthetic uptake of carbon that drives NPP is not saturated at current atmospheric concentrations (4), NPP should increase as fossilfuel combustion adds to the atmospheric [CO 2 ]. Increased C uptake into the biosphere in response to rising [CO 2 ] (''CO 2 fertilization'') can create a negative feedback that slows the rate of increase in atmospheric [CO 2 ] (3, 5). Hence, assumptions regarding CO 2 fertilization of the terrestrial biosphere greatly affect predictions of future atmospheric [CO 2 ] (3)...

The northern hemisphere temperate and boreal forests currently provide an important carbon sink; however, current tropospheric ozone concentrations ([O 3 ]) and [O 3 ] projected for later this century are damaging to trees and have the potential to reduce the carbon sink strength of these forests. This meta-analysis estimated the magnitude of the impacts of current [O 3 ] and future [O 3 ] on the biomass, growth, physiology and biochemistry of trees representative of northern hemisphere forests. Current ambient [O 3 ] (40 ppb on average) significantly reduced the total biomass of trees by 7% compared with trees grown in charcoal-filtered (CF) controls, which approximate preindustrial [O 3 ]. Above-and belowground productivity were equally affected by ambient [O 3 ] in these studies. Elevated [O 3 ] of 64 ppb reduced total biomass by 11% compared with trees grown at ambient [O 3 ] while elevated [O 3 ] of 97 ppb reduced total biomass of trees by 17% compared with CF controls. The root-to-shoot ratio was significantly reduced by elevated [O 3 ] indicating greater sensitivity of root biomass to [O 3 ]. At elevated [O 3 ], trees had significant reductions in leaf area, Rubisco content and chlorophyll content which may underlie significant reductions in photosynthetic capacity. Trees also had lower transpiration rates, and were shorter in height and had reduced diameter when grown at elevated [O 3 ]. Further, at elevated [O 3 ], gymnosperms were significantly less sensitive than angiosperms. There were too few observations of the interaction of [O 3 ] with elevated [CO 2 ] and drought to conclusively project how these climate change factors will alter tree responses to [O 3 ]. Taken together, these results demonstrate that the carbonsink strength of northern hemisphere forests is likely reduced by current [O 3 ] and will be further reduced in future if [O 3 ] rises. This implies that a key carbon sink currently offsetting a significant portion of global fossil fuel CO 2 emissions could be diminished or lost in the future.

Summary1. The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34 -36 nL L − 1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3. After 3 years of exposure to 560 µ mol mol − 1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.

Summary• Concentrations of atmospheric CO 2 and tropospheric ozone (O 3 ) are rising concurrently in the atmosphere, with potentially antagonistic effects on forest net primary production (NPP) and implications for terrestrial carbon sequestration.• Using free-air CO 2 enrichment (FACE) technology, we exposed north-temperate forest communities to concentrations of CO 2 and O 3 predicted for the year 2050 for the first 7 yr of stand development. Site-specific allometric equations were applied to annual nondestructive growth measurements to estimate above-and below-ground biomass and NPP for each year of the experiment.• Relative to the control, elevated CO 2 increased total biomass 25, 45 and 60% in the aspen, aspen-birch and aspen-maple communities, respectively. Tropospheric O 3 caused 23, 13 and 14% reductions in total biomass relative to the control in the respective communities. Combined fumigation resulted in total biomass response of − 7.8, +8.4 and +24.3% relative to the control in the aspen, aspen-birch and aspensugar maple communities, respectively.• These results indicate that exposure to even moderate levels of O 3 significantly reduce the capacity of NPP to respond to elevated CO 2 in some forests.