Limited effect of ozone reductions on the 20‐year photosynthesis trend at Harvard forest

Abstract

Ozone (O ) damage to leaves can reduce plant photosynthesis, which suggests that declines in ambient O concentrations ([O ]) in the United States may have helped increase gross primary production (GPP) in recent decades. Here, we assess the effect of long-term changes in ambient [O ] using 20 years of observations at Harvard forest. Using artificial neural networks, we found that the effect of the inclusion of [O ] as a predictor was slight, and independent of O concentrations, which suggests limited high-frequency O inhibition of GPP at this site. Simulations with a terrestrial biosphere model, however, suggest an average long-term O inhibition of 10.4% for 1992-2011. A decline of [O ] over the measurement period resulted in moderate predicted GPP trends of 0.02-0.04 μmol C m s yr , which is negligible relative to the total observed GPP trend of 0.41 μmol C m s yr . A similar conclusion is achieved with the widely used AOT40 metric. Combined, our results suggest that ozone reductions at Harvard forest are unlikely to have had a large impact on the photosynthesis trend over the past 20 years. Such limited effects are mainly related to the slow responses of photosynthesis to changes in [O ]. Furthermore, we estimate that 40% of photosynthesis happens in the shade, where stomatal conductance and thus [O ] deposition is lower than for sunlit leaves. This portion of GPP remains unaffected by [O ], thus helping to buffer the changes of total photosynthesis due to varied [O ]. Our analyses suggest that current ozone reductions, although significant, cannot substantially alleviate the damages to forest ecosystems.

Recent pause in the growth rate of atmospheric CO2 due to enhanced terrestrial carbon uptake

“…Diffuse light changes have also been reported and are estimated to have increased uptake between 1960 and 1980 by 0.44 PgC per year globally39. The effect of ozone on the terrestrial carbon cycle is uncertain, due to unknowns regarding plant-specific sensitivities to ozone40 and the effect of canopy structure44. Ultimately, there are a myriad of factors that influence the carbon cycle, particularly at regional scales.…”

Interannual variability in ozone removal by a temperate deciduous forest

“…During other periods (e.g., 2001–2014), stomatal uptake may contribute substantially to IAV in at Harvard Forest, as suggested by a factor of 2 increase in IAV in g s estimated from H 2 O EC during 2001–2014 (when O 3 EC are unavailable) versus 1992–2000. Constraining stomatal O 3 uptake is necessary to quantify the impacts of O 3 ‐induced damage on long‐term trends and variability in GPP [e.g., Sitch et al ., ; Fares et al ., ; Yue et al ., ] and water use efficiency [e.g., Keenan et al ., , ; Holmes , ; Hoshika et al ., ; Lombardozzi et al ., ].…”

“…However, source attribution and top-down approaches using observations (e.g., satellite data) to infer precursor emissions and their IAV rely on accurate estimates of sinks such as O 3 dry deposition. Our analysis using 11 years (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) Sitch et al, 2007;Fares et al, 2013;Yue et al, 2016] and water use efficiency [e.g., Keenan et al, 2013Keenan et al, , 2014Holmes, 2014;Hoshika et al, 2015;Lombardozzi et al, 2015].…”

Future inhibition of ecosystem productivity by increasing wildfire pollution over boreal North America

“…With the Sitch et al (2007) scheme, the YIBs model simulates reasonable GPP responses to [O 3 ] in North America (Yue and Unger, 2014;Yue et al, 2016). Generally, damage to GPP increases with the enhancement of ambient [O 3 ], but with varied sensitivities for different plant species (see Fig.…”

“…Future projections of wildfire emissions from are applied as input to ModelE2-YIBs model to project fire-induced O 3 and aerosol concentrations in the 2010s and 2050s. The impacts of the boreal fire O 3 on forest photosynthesis are predicted using the flux-based damage algorithm proposed by Sitch et al (2007), which has been fully evaluated against available O 3 damage sensitivity measurements globally and over North America (Yue and Unger, 2014;Yue et al, 2016Yue et al, , 2017. Fire aerosols induce perturbations to radiation, meteorology, and hydrology, leading to multiple influences on the land carbon uptake.…”