Tropospheric O₃ compromises net primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO₂

Abstract

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.

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

“…In addition to controlling spatial variability in response, N availability may also be a factor in determining whether the responses observed here can be sustained for decades (38,39). The data in our analysis all come from fast-growing, early successional stands, and there has been no evidence to date for a negative feedback on NPP through N availability in these stands (20,40,41).…”

“…Where soils are poor or prolonged water limitation occurs, represented only through within-site variation in our dataset, forests may have limited capacity to support any response to CO 2 enrichment (37). Concurrent increases in tropospheric ozone could negate the productivity increases from elevated [CO 2 ] (17,20). In addition to controlling spatial variability in response, N availability may also be a factor in determining whether the responses observed here can be sustained for decades (38,39).…”

“…The annual increments of stem wood (I wood ) and coarse woody root (I coarse root ) were estimated by applying site-specific allometric equations to periodic measurements of the diameter of trees in current and elevated CO 2 plots. Harvest data from the POP-EUROFACE (19) and AspenFACE experiments (20); and the tree height-diameter relationship for trees at DukeFACE (21) indicate that exposure to elevated [CO 2 ] did not affect the allometric relationships between plant parts. The allometric equation used at ORNL-FACE incorporated basal area, height, taper, and wood density to reduce the possibility of the allometry being altered by elevated [CO 2 ] (15).…”

“…At ORNL-FACE (28) and PopFACE (29), fine-root production was measured directly by using minirhizotrons and in-growth cores. The DukeFACE experiment used a compartment flow model for estimating fine-root production (30); AspenFACE calculated fine-root turnover from published rates of aspen fine-root production and mortality (31) that were then applied to allometrically determined peak standing fine-root biomass (20). Other components of NPP (e.g., losses to herbivory and root exudation) were not included in our analysis because data were not available.…”

New exposure-based metric approach for evaluating O3 risk to North American aspen forests

“…We expect that, based upon earlier measurements (Karnosky et al, 1999(Karnosky et al, , 2003a, our new metric-based exposureeresponse models will be validated against growth change observed within the range of uncertainty inherent in the models. Along with Hogsett et al (1997), we of course recognize that modeled estimates of growth change due to O 3 derived from aspen monocultures at Aspen FACE and our field plantations may be modified to a degree across the forest landscape as more O 3 -sensitive aspen clones may not compete as well when growing with other more O 3 -tolerant species like B. papyrifera (McDonald et al, 2002;King et al, 2005).…”