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 (r2 = 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 (ΔNPP/ΔN) 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.

Large but decreasing effect of ozone on the European carbon sink

“…However, some studies have found that O 3 can decrease soil carbon content. Talhelm et al (2014), for example, found O 3 reduced carbon content in near surface mineral soil of forest soils exposed to 11 years of O 3 fumigation. Hofmockel et al (2011) found elevated O 3 reduced the carbon content in more stable soil organic matter pools, and Loya et al (2003) showed that the fraction of soil carbon formed in forest soils over a 4-year experimental period when fumigated with both CO 2 and O 3 was reduced by 51 % compared to the soil fumigated with CO 2 alone.…”

“…In the Feng et al (2008) meta-analysis of wheat, O 3 reduced aboveground biomass (−18 % at a mean O 3 concentration of 70 ppb) photosynthetic rate (−20 % at a mean O 3 concentration of 73 ppb) and g s (−22 % at a mean O 3 concentration of 79 ppb). One of few long-term field-based O 3 exposure studies (AspenFACE) showed that after 11 years of exposing mature trees to O 3 (mean O 3 concentration of 46 ppb), O 3 decreased ecosystem carbon content (−9 %) and decreased NPP (−10 %), although the O 3 effect decreased through time (Talhelm et al, 2014). Zak et al (2011) showed this was partly due to a shift in community structure as O 3 -tolerant species, competitively inferior in low-O 3 environments, outcompeted O 3 -sensitive species.…”

Satellite based estimates underestimate the effect of CO2 fertilization on net primary productivity

“…the range for the FACE experiments, which typically increase CO 2 from ~370 to ~550 ppm) ( Figure 1). Furthermore, S15's synthesis of FACE data is incomplete as it omits several years of published data 13,14 , and incorrectly estimates an overall effect size by taking the median across experiments, species and years, rather than calculating a more appropriate response ratio 15 .…”

Current and future ozone risks to global terrestrial biodiversity and ecosystem processes

“…In experimental forests, O 3 reduced the C content in woody tissues and in the near‐surface mineral soil (Talhelm et al., 2014), and in more stable SOM pools (Hofmockel, Zak, Moran, & Jastrow, 2011), but data from a high‐elevation grassland experiment indicated that soil C remains unchanged, possibly because a low C input was compensated by reduced turnover (Volk et al., 2011), as discussed above. Similarly, in a modeling study, the replacement of sensitive by more tolerant plant species or genotypes (as also discussed above) in a temperate deciduous forest led to unchanged biomass C stocks in the long term (>100 years) (Wang, Shugart, Shuman, & Lerdau, 2016).…”