Forests are an integral part of the global carbon cycle; consequently, any change in their capacity to assimilate carbon through photosynthesis has significant implications for global climate change. The concentrations of two major greenhouse gases, carbon dioxide ([CO2]) and tropospheric ozone ([O3]) have this potential through direct alteration of photosynthesis. The impact of rising [CO2] on gross photosynthetic carbon assimilation, i.e. gross primary productivity (GPP), for trees grown under current and future elevated [CO2] using Free-Air Concentration Enrichment (FACE) technology was examined using a physiologically based model of plant productivity. Results show that although elevated [CO2] enhances GPP, the magnitude of the stimulation decreases with closure of the light environment within the tree canopy. To determine the magnitude of [O3] impacts, the extensive, yet disparate evidence compiled in the peer-reviewed literature over more than 50 years was examined using meta-analytic techniques providing the first quantitative summary of the entirety of the O3 literature to date. The meta-analysis summarized 8589 independent measurements of tree biomass, growth, physiology and biochemistry. Results show that current [O3] is depressing photosynthesis by 11% relative to the [O3] present in the atmosphere prior to the Industrial Revolution. This indicates that carbon uptake by forests is already diminished by [O3]. By 2050, the reductions could reach 18% with further reductions by 2100. It is also shown that there is a significant parallel decrease in forest biomass under current [O3] (7%) and if [O3] continues to rise, the reductions will become progressively more severe reaching 11% by 2050 and 17% by 2100. This implies that northern hemisphere forests, which are currently acting as carbon sink, are unlikely to keep pace with the rise in [CO2] and could diminish as a result of rising [O3]. It is, however, largely unknown what the impact of rising [CO2] and [O3] will be in combination. Progressing scientific understanding of the impacts of human-induced changes on the global carbon cycle and climate change will only be possible through an integrated modeling assessment of these concurrent changes.
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