Abstract
Forests play a pivotal role in mitigating global warming as an important carbon sink. Recent global greening trends reflect a positive influence of elevated atmospheric CO2 on terrestrial carbon uptake. However, increasingly frequent and intense drought events endanger the carbon sequestration function of forests. This review integrates previous studies across scales to identify potential global trends in forest responses to drought and elevated CO2 as well as to identify data needs in this important research field. The inconsistent responses of ecosystem respiration to drought contributes to the change of forest net CO2 exchange, which depends on the balance of opposite effects of warming and water stress on respiration. Whether CO2 fertilization can offset the effects of drought remains controversial, however, we found a potential overestimation of global CO2 fertilization effects because of increasing water stress and other limitations such as light and nutrients (N, P) as well as the possibility of photosynthetic acclimation.
Highlights
Forest ecosystems play a key role in the land-atmosphere system and provide wide ecological services and socio-economic benefits, including forest products, water regulation, biodiversity, soil protection, spiritual and inspirational needs, recreational activities and aesthetic value [1]
Similar results were found in Lindroth et al [37], they suggest that most forests in their study experienced a decline of net ecosystem productivity in the 2018 summer drought across Northern Europe, and the forest carbon balance change was largely explained by decreased heterotrophic respiration and lack of precipitation
Piao et al.’s [67] modelling studies based on ORCHIDEE suggest that 49% of the increase of leaf area index (LAI) in growing season in the northern hemisphere is contributed by atmospheric CO2
Summary
Forest ecosystems play a key role in the land-atmosphere system and provide wide ecological services and socio-economic benefits, including forest products, water regulation, biodiversity, soil protection, spiritual and inspirational needs, recreational activities and aesthetic value [1]. Forests capture atmospheric CO2 and sequester it into their biomass and soil, and release oxygen back to the atmosphere, which is the buffer against global warming. Higher WUE under elevated atmospheric CO2 makes plants more tolerant to water stress [6], frequent occurrence of extreme droughts and heat stress lead to the decline of forest productivity and biomass accrual by widespread tree mortality and wildfires [7]. The extent to which climate change affects forest carbon uptake and the interaction of drought and elevated CO2 on plant stress resistance is still unclear. Since the global warming and severity of extreme events will continue to increase if greenhouse gas emissions are unmitigated, understanding the implications of the capacity of forests to sequester carbon under climate scenarios remains a pressing and crucial need
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