Global S&T Development Trend Analysis Platform of Resources and Environment
Carbon dioxide enrichment of a mature forest resulted in the emission of the excess carbon back into the atmosphere via enhanced ecosystem respiration, suggesting that mature forests may be limited in their capacity to mitigate climate change.
Atmospheric carbon dioxide enrichment (eCO(2)) can enhance plant carbon uptake and growth(1-5), thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2 concentration(6). Although evidence gathered from young aggrading forests has generally indicated a strong CO2 fertilization effect on biomass growth(3-5), it is unclear whether mature forests respond to eCO(2) in a similar way. In mature trees and forest stands(7-10), photosynthetic uptake has been found to increase under eCO(2) without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO(2) unclear(4,5,7-11). Here using data from the first ecosystem-scale Free-Air CO2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responded to four years of eCO(2) exposure. We show that, although the eCO(2) treatment of +150 parts per million (+38 per cent) above ambient levels induced a 12 per cent (+247 grams of carbon per square metre per year) 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 half 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 CO2 fertilization as a driver of increased carbon sinks in global forests.
Outdoor air pollution adversely affects human health and is estimated to be responsible for five to ten per cent of the total annual premature mortality in the contiguous United States(1-3). Combustion emissions from a variety of sources, such as power generation or road traffic, make a large contribution to harmful air pollutants such as ozone and fine particulate matter (PM2.5)(4). Efforts to mitigate air pollution have focused mainly on the relationship between local emission sources and local air quality(2). Air quality can also be affected by distant emission sources, however, including emissions from neighbouring federal states(5,6). This cross-state exchange of pollution poses additional regulatory challenges. Here we quantify the exchange of air pollution among the contiguous United States, and assess its impact on premature mortality that is linked to increased human exposure to PM2.5 and ozone from seven emission sectors for 2005 to 2018. On average, we find that 41 to 53 per cent of air-quality-related premature mortality resulting from a state'
