Dennis Ojima, Bill Parton, Becky McKeown, Dave Schimel, Melannie Hartman, and VEMAP members.  2001.   IHDP/WCRP/IGBP Global Change Open Science Conference, Amsterdam, the Netherlands 10-13 July 2001.

Abstract

        Transient calculations for North America under historical transient climate (1895-1993) and under two different climate change scenarios (Hadley and Canadian Climate Centre) with 1% increase in GHG forcing and aerosol feedbacks were completed.  These simulations were made with CO2 and without CO2 implemented in the Century model over the potential vegetation and current vegetation including agricultural land cover masks.   These results have been used in the National Assessment of Climate Impacts and the Great Plains Regional Assessment.  Coordinated efforts between the Century group and collaborators at Oregon State University and the USFS continued work with MAPSS-Century (MC) coupling to develop a transient version of the MC model. These developments provides a more dynamic way to represent changes in vegetation structure and a more mechanistic fire simulator.  The MC simulations still represents dynamics of natural vegetation and relies on simulations of contemporary land use for North American from the Century static vegetation analysis.

        The VEMAP models agreed substantially on the size and spatial distribution of the CO2-driven sink, and so we conclude that processes not modeled, such as abandonment of agricultural land and recovery from forest harvest, result in larger sinks than does CO2 fertilization itself.  Agriculture does not appear to result in a large sink today, but remains a modest sink throughout the future scenarios, even without simulating intentional measures to sequester carbon in croplands.  Cropland sinks can be enhanced through adoption of certain management practices which reduces removal of crop residue, erosion soil carbon losses, and decreased tillage.

        The VEMAP results have two implications for this debate.  First, if 50-70% of today’s sinks are due to agricultural abandonment and forest regrowth, this process reflects decisions made up to a century ago and may soon saturate.  On the other hand, the VEMAP dynamic vegetation models suggest that climate change could induce broad shifts to woodier, higher carbon biomes, although this tendency is weakened if there are large-scale decreases in water. A sink due to forest expansion could persist because of the long response times of forest expansion, even after physiological mechanisms are saturated.  Despite these shifts, the vegetation models suggest smaller carbon sinks overall because of higher fire frequencies. These results imply potential opportunities, by facilitating the expansion of woody vegetation together with a significantly enhanced risk due to higher disturbance frequency.  The balance will depend upon both ecosystem management and the details of the climate change that actually occurs.

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