Current estimates of the amount of carbon stored in terrestrial vegetation range from 450 to 650 Pg C, with forests comprising 80% of this carbon sink. Plants utilize CO2 during photosynthesis, but also produce it during respiration. The net effect is an uptake of CO2 from the atmosphere equivalent to ~60 Pg C/year. As such, terrestrial vegetation plays a key role in the global carbon cycle. Hymus and Valentini (Chapter 2, this volume) examine the role plants play in controlling atmospheric CO2 concentrations and the potential changes in this large sink for CO2 in the face of changing land use and climate.
They discuss the CO2 fertilization effect, whereby plants respond to elevated CO2 concentrations in the atmosphere by increasing CO2 uptake, and the uncertainties that revolve around this potentially very important negative feedback to anthropogenic climate change.
The determinants of terrestrial primary production, past and present, are also discussed, as is the potential for increased CO2 sequestration in the 21st century through both natural and managed changes in the carbon sink strength of terrestrial vegetation. Our primary impact on the terrestrial vegetation CO2 sink has been through our alterations in land use. A switch from forested land to agricultural crops means that carbon incorporated into plant tissues is taken out of the atmosphere for a much shorter time, and so the effectiveness of the plants as CO2 sinks is much reduced.
Globally, through a combination of afforestation and reforestation, it has been estimated that between 0.2 and 0.6 Pg C/year could be sequestered in above- and below-
ground biomass by 2010, with improved management practices and land-use change providing an additional 0.57 and 0.44 Pg C/year, respectively.
Hymus and Valentini conclude that, though terrestrial vegetation has played a vital role in buffering anthropogenic CO2 emissions in the past and has the potential to do so in the future, its continuation as a leading sink for our own CO2 emissions cannot be taken for granted given the continuing uncertainty in its drivers.
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