Elevated CO2 and O3 Alter Productivity and Carbon Storage in Northern Temperate Forests: Results from Aspen FACE
Abstract
Three northern temperate forest communities in the north-central United States were exposed to factorial combinations of elevated carbon dioxide (CO2) and/or tropospheric ozone (O3) for 11 years, advancing from open-grown seedlings <0.25 m in height at the beginning to the experiment to closed-canopy stands that were >8 m tall. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment. In addition, we estimated cumulative net primary productivity (NPP) during the experiment and used these data to gain further insight into forest C cycling in two ways. First, we tested several canopy productivity models to understand variation in cumulative NPP. Second, we compared cumulative NPP to ecosystem C storage. Elevated CO2 enhanced ecosystem carbon (C) storage by 11%, but elevated O3 decreased ecosystem C storage by 9%. There was little variation in treatment effects on C storage across communities and no significant interactions between CO2 and O3 for any major C pools. Tree C storage increased 44% under elevated CO2 and decreased 15% under elevated O3. Neither CO2 nor O3 affected the total amount of C in the top 1 m of mineral soil. However, soil C content within the top 0.1 m of mineral soil was lower under elevated O3, whereas soil C at 0.1 to 0.2 m and 0.4 to 0.5 m in depth were lower under elevated CO2. Cumulative NPP was 42% greater under elevated CO2, 11% lower under elevated O3, and was a strong predictor of variation in C storage in plants and organic soil (n = 36, r2 = 0.96). Tree productivity comprised 95% of cumulative NPP. Stands with more cumulative canopy N (g foliar N m-2 of ground area) had greater cumulative tree productivity, but N productivity (tree productivity per canopy N) decreased as canopy N accrued. Thus, elevated CO2 increased cumulative NPP because of a 28% increase in canopy N and a 28% increase in N productivity, while elevated O3 did not impact N productivity and lowered tree productivity only through a 21% decrease in canopy N. Because N productivity decreased as canopy N accrued, the difference in canopy N between ambient and elevated O3 had a gradually smaller influence on NPP through time as the stands became more developed, eventually eliminating the O3 effect. In comparison, variation in mineral soil C content was not positively related to cumulative NPP and the effects of CO2 and O3 on this important ecosystem C pool remain poorly understood..
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B21A0461T
- Keywords:
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- 0428 BIOGEOSCIENCES Carbon cycling;
- 0476 BIOGEOSCIENCES Plant ecology;
- 0426 BIOGEOSCIENCES Biosphere/atmosphere interactions;
- 0478 BIOGEOSCIENCES Pollution: urban;
- regional and global