The fate of carbon in a mature forest under carbon dioxide enrichment

Abstract

Atmospheric carbon dioxide enrichment (eCO 2) can enhance plant carbon uptake and growth 1,2,3,4,5 , thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO 2 concentration 6. While evidence gathered from young aggrading forests has generally indicated a strong CO 2 fertilization effect on biomass growth 3,4,5 , it is unclear whether mature forests respond to eCO 2 in a similar way. In mature trees and forest stands 7,8,9,10 , photosynthetic uptake has been found to increase under eCO 2 without any apparent accompanying growth response, leaving an open question about the fate of additional carbon fixed under eCO 2 4,5,7,8,9,10,11. Here, using data from the first ecosystemscale Free-Air CO 2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responds to four years of eCO 2 exposure. We show that, although the eCO 2 treatment of ambient +150 ppm (+38%) induced a 12% (+247 g C m-2 yr-1) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for ~50% of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO 2 , and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO 2 fertilization as a driver of increased carbon sinks in global forests. Main text Globally, forests act as a large carbon sink, absorbing a significant portion of the anthropogenic CO 2 emissions 1,12 , an ecosystem service that has tremendous social and

Understanding the uncertainty in global forest carbon turnover

“…Reduced rates of mortality due to elevated atmospheric CO 2 concentration (H6b) are conceptually included in five of the TBMs through the growth efficiency concept (Table 3) and are evident in the overall response for two of them (Table 5). Increased plant production under elevated CO 2 follows wellestablished leaf-level responses of photosynthesis and wateruse efficiency to atmospheric CO 2 concentration and is supported by detailed stand-level modelling (Liu et al, 2017), but is hard to verify with observations in mature trees (Jiang et al, 2020;Walker et al, 2019). If trees expend their extra NPP on growing proportionally larger, thereby increasing their respiration demands, then the positive effect of enhanced NPP could be offset.…”

“…Recently, however, a number of studies have found τ to have comparable or even larger importance than NPP when assessing the response of C veg to environmental change using terrestrial biosphere models (TBMs; Ahlström et al, 2015a;Friend et al, 2014;Galbraith et al, 2013;Johnson et al, 2016;Thurner et al, 2017), with large divergence in TBM projections of τ over the 21st century depending on forcing (Ahlström et al, 2015a) or the choice of TBM (Friend et al, 2014). The divergence that can be traced to TBM structure and parameterisation (Nishina et al, 2015) has not been closely analysed in terms of the contributions of specific underlying processes, interactions and driver dependencies, or their basis in knowledge from real-world ecosystems.…”

Mature forest shows little increase in carbon uptake in a CO2-enriched atmosphere

“…However, the tools for scrutinizing such changes remain imprecise. On page 252, Ouyang et al 2 report the development of a computational platform that uses an artificial-intelligence (AI) approach to assess cardiac ultrasound video and to provide continuous, beat-by-beat measurement of cardiac pump function.…”

“…A small number of enrichment experiments have been conducted in young forests, but there is a paucity of knowledge about the CO 2 -fertilization effect in mature forests. On page 227, Jiang et al 2 present results of the Free-Air CO 2 Enrichment (FACE) experiment in a mature forest in Australia. Their estimate of the CO 2 -fertilization effect is among the lowest yet reported.…”

“…Carbon dioxide enrichment in an Australian eucalyptus forest. Jiang et al2 increased the atmospheric CO 2 concentration in circular plots to observe the effects on the main carbon pools and fluxes in the forest. The cranes were used to take researchers to the top of the forest canopy, and the ring structures transport and pump CO 2 into the plots.…”