Relationships between aboveground woody biomass and soil organic carbon in a semi-desert grassland.
Throop, Heather*,1, Archer, Steven1, McMurtry, Chad1, McClaran, Mitchel1, 1 University of Arizona, Tucson, AZ
ABSTRACT- The proliferation of woody plants into grasslands and savannas has been a prominent global land cover change over the past century. Recent estimates suggest that increased woody plant cover accounts for a significant fraction of the Northern Hemisphere carbon (C) sink. Considerable uncertainty exists, however, in the magnitude and spatial distribution of woody encroachment C sinks. Elevated under-canopy soil organic carbon (SOC) pools have been widely reported, but little is known about the spatial distribution of SOC relative to tree canopies. Understanding this distribution is critical, however, to developing accurate landscape-scale assessments of woody proliferation impacts on ecosystem C pools. We compared SOC and aboveground C pools for encroaching mesquite (Prosopis velutina) in an Arizona semi-desert grassland. We developed predictive equations of SOC distribution relative to tree size and bole-to-canopy position based on spatially-intensive sampling around 17 trees. We used these equations to predict SOC distribution around 31 trees with known aboveground C pools. SOC was significantly elevated close to the bole under large trees (>4m canopy radius). Aboveground C mass strongly predicted SOC pools (y=0.50x+3192.7, R2=0.84). SOC accounted for 56% of combined SOC + aboveground C pools on average, although the relative size of SOC pools declined with tree size (y=-11.98x+82.22, R2=0.56). Under-canopy SOC pools were elevated moderately relative to inter-canopy values (mean 19% increase). The relationships between above- and belowground C pools will facilitate landscape-level assessments of woody encroachment C sinks. Canopy radius can be used to successfully predict both aboveground and SOC pools.
Key words: desert grassland, woody encroachment, carbon cycling
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