Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests

Abstract

Three young northern temperate forest communities in the north-central United States were exposed to factorial combinations of elevated carbon dioxide (CO2 ) and tropospheric ozone (O3 ) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO2 enhanced ecosystem C content by 11%, whereas elevated O3 decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO2 and O3 . Treatment effects on ecosystem C content resulted primarily from changes in the near-surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r(2) = 0.96). Elevated CO2 enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m(-2) ) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O3 lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (∆NPP/∆N) decreased through time with further canopy development, the O3 effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O3 and less soil C from 0.1 to 0.2 m in depth under elevated CO2 . Overall, these results suggest that elevated CO2 may create a sustained increase in NPP, whereas the long-term effect of elevated O3 on NPP will be smaller than expected. However, changes in soil C are not well-understood and limit our ability to predict changes in ecosystem C content.

Keywords: air pollution; carbon sequestration; carbon storage; elevated carbon dioxide (CO2); free-air CO2 enrichment (FACE); net primary productivity (NPP); nitrogen; soil carbon.

Fig 1

Ecosystem carbon content after 11 years of fumigation at the Aspen FACE experiment. Data are averaged across the three forest community types and include soil to 1 m in depth. The height of each bar segment represents mean size of each pool and the total bar height represents ecosystem C content for each treatment. For simplicity, soil C below 0.5 m in depth is grouped into a single pool because there were no significant treatment effects. Significant (P ≤ 0.05) effects of the treatment gases and the size of these effects (%) are shown to the right of the figure. Pools without significant treatment effects are denoted with ‘–’. With the exception of two small pools (foliage, groundcover plants), there were no significant treatment × community interactions. More detailed results can be found in Tables S1 and S2.

Fig 2

(a) NPP, (b) actual NPP effect sizes and effects modeled from canopy N differences, (c) canopy N, and (d) marginal N productivity [(NPP_{tree(elevated)} − NPP_{tree(ambient)})/(Canopy N_(elevated) − Canopy N_(ambient))]. In (b), black lines show actual NPP effect sizes (elevated/ambient, 1 = no effect) and red lines show effect sizes modeled from canopy N differences; black symbols shown in (b) only when actual NPP effects are significant (P < 0.05). In (b), groundcover plants are assumed to be unresponsive to modeled changes in NPP_tree. Bars are ±1SE.

Fig 3

The relationship between cumulative NPP (through 2008) and C stored within the plant, dead wood and roots, and soil organic horizon pools at the conclusion of the experiment (2009). The overall regression fit is r² = 0.96, although the amount of ecosystem C relative to NPP is smaller under elevated O₃ in the aspen community (O₃ × Community: P = 0.003).

Fig 4

Cumulative canopy N in relation to (a) cumulative tree productivity and (b) N productivity, with lines displayed representing mixed model estimates of these relationships (community effects not shown for simplicity). Slopes in (b) do not differ, but intercepts differ by community (P = 0.031) and between ambient CO₂ and elevated CO₂ (P < 0.001). Ozone effects on the slopes and intercepts were not significant (P > 0.25). The simplified model in (a) has a fit of r² = 0.87. Community effects in the full model (Table S5; r² = 0.93) shift the lines vertically.