Forest response to elevated CO₂is conserved across a broad range of productivity

Abstract

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)...

Systematic assessment of terrestrial biogeochemistry in coupled climate–carbon models

“…The disproportionately large NEE re- sponse in CASA 0 (almost threefold larger than CN) can only be partly attributed to the higher sensitivity of NPP to CO 2 enrichment; other important factors included a higher baseline NPP and similar turnover times in pools involved with initial carbon storage. At the four model grid cells corresponding to the FACE experiments analyzed by Norby et al (2005), CASA 0 and CN had NPP increases of 17 AE 2% (b fert 5 0.43 AE 0.04) and 7 AE 3% (b fert 5 0.18 AE 0.09) during the first 5 years, respectively, compared with an observed increase of 27 AE 2% (b fert 5 0.67). Both models showed a decreasing trend in NPP response between 401N and 701N (supporting information Fig.…”

“…We made a direct comparison of temperate forest grid cell NPP increases with site level averages from Norby et al (2005) -estimating the percent increases in NPP separately for grid cells corresponding to each of the four FACE sites. The model-data differences for the four sites were used with Eqn (s2) in the supporting information to generate a scoring metric.…”

Free atmospheric CO<sub>2</sub> enrichment increased above ground biomass but did not affect symbiotic N<sub>2</sub>-fixation and soil carbon dynamics in a mixed deciduous stand in Wales

“…Just like the coarse and fine POM fractions, the micro-aggregate protected POM C and N fractions were not affected by species either (P = 0.564 and P = 0.244). Norby et al (2005) calculated an average CO 2 response of 18% based on NPP (g C m −2 y −1 ) data of four forest FACE experiments. Despite the fact that the observed above ground woody biomass data can not be compared directly to the NPP data, the response ratios (expressed as response percentage) may be related since the annual woody biomass increment (after conversion to g C m −2 ) is one of the largest increments that make up NPP.…”

“…At the 0-15 cm increment, Table 5. Additional C sinks in the forest floor -soil systems and major vegetation and soil characteristics of selected forest FACE experiments Schlesinger et al, 2006;Norby et al, 2006;Norby et al, 2005;Finzi et al, 2007;Hoosbeek and ScarasciaMugnozza, 2009;Lichter et al, 2008).…”

“…(Calfapietra et al, 2003;DeLucia et al, 1999;Gielen et al, 2005;Hamilton et al, 2002;Handa et al, 2006;Karnosky et al, 2003;Liberloo et al, 2009;Norby et al, 2002). Norby et al (2005) analyzed the response of NPP (g C m −2 y −1 ) to elevated [CO 2 ] in four forest FACE experiments and calculated the following regression NPP elev = 1.18×NPP amb + 55.4 (r 2 = 0.97). The elevated [CO 2 ] effect of 18% was significant (P < 0.001), while the positive intercept was not significantly different from zero.…”