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)...

Forest ecosystems are important sinks for rising concentrations of atmospheric CO 2. In previous research, we showed that net primary production (NPP) increased by 23 ؎ 2% when four experimental forests were grown under atmospheric concentrations of CO 2 predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary to sustain the high rates of forest NPP under free-air CO 2 enrichment (FACE). In this study, experimental evidence demonstrates that the uptake of N increased under elevated CO 2 at the Rhinelander, Duke, and Oak Ridge National Laboratory FACE sites, yet fertilization studies at the Duke and Oak Ridge National Laboratory FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, nitrogen-use efficiency increased under elevated CO 2 at the POP-EUROFACE site, where fertilization studies showed that N was not limiting to tree growth. Some combination of increasing fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO 2. Regardless of the specific mechanism, this analysis shows that the larger quantities of C entering the below-ground system under elevated CO 2 result in greater N uptake, even in N-limited ecosystems. Biogeochemical models must be reformulated to allow C transfers below ground that result in additional N uptake under elevated CO 2 .global change ͉ net primary production

Fine-root production and turnover are important regulators of the biogeochemical cycles of ecosystems and key components of their response to global change. We present a nearly continuous 6-year record of fine-root production and mortality from minirhizotron analysis of a closed-canopy, deciduous sweetgum forest in a free-air CO 2 enrichment experiment. Annual production of fine roots was more than doubled in plots with 550 ppm CO 2 compared with plots in ambient air. This response was the primary component of the sustained 22% increase in net primary productivity. Annual fine-root mortality matched annual production, and the mean residence time of roots was not altered by elevated CO 2, but peak fine-root standing crop in midsummer was significantly higher in CO 2-enriched plots, especially deeper in the soil profile. The preferential allocation of additional carbon to fine roots, which have a fast turnover rate in this species, rather than to stemwood reduces the possibility of long-term enhancement by elevated CO 2 of carbon sequestration in biomass. However, sequestration of some of the fine-root carbon in soil pools is not precluded, and there may be other benefits to the tree from a seasonally larger and deeper fine-root system. Root-system dynamics can explain differences among ecosystems in their response to elevated atmospheric CO 2; hence, accurate assessments of carbon flux and storage in forests in a globally changing atmosphere must account for this unseen and difficult-to-measure component.R esearchers who attempt to quantify C pools and fluxes in terrestrial ecosystems often find it expedient to measure only the aboveground components despite the wide recognition that a substantial fraction of net primary production occurs belowground (1). Fine roots (ephemeral, nonwoody roots generally of diameters Ͻ1 mm) are important regulators of biogeochemical cycling (2), and their turnover is a key component of C sequestration in soil (3). Root production often is enhanced when the plants are grown in a CO 2 -enriched atmosphere (4, 5). However, the difficulty in measuring these roots, which cannot generally be seen, are not easily sampled, and can grow and die rapidly and unpredictably, means that they are left out of both retrospective (6) and prospective (7) studies of the potential for terrestrial ecosystems to sequester additional C in response to a changing atmosphere and climate. Fine-root production and distribution in forests exhibit high spatial and temporal variation, making their characterization especially difficult, and most of the experimental evidence on tree root responses to elevated CO 2 comes from static or short-term observations that are strongly confounded with plant developmental trajectories (5). Free-air CO 2 enrichment (FACE) experiments (8) provide an opportunity for longer-term observations under realistic conditions in fully developed forests stands.We used minirhizotron observation tubes and video imaging (9, 10) to quantify fine-root production and mortality in a FACE experim...