Document: STE-3-18-3

Woody encroachment in grasslands and savannas: Ecosystem consequences.

ARCHER, S.R.* ¹, T.W.BOUTTON ¹ and K.A.HIBBARD ²

Texas A&M University, College Station, TX 77843 ¹
University of New Hampshire, Durham, NH 03824 ²

Abstract:
Reductions in woody biomass associated with deforestation are in contrast with changes in many semi-arid regions, where grasslands and savannas are being replaced by shrublands and woodlands. The geographic extent and causes of this phenomenon will be reviewed and impacts on biogeochemistry discussed. Studies at a subtropical savanna site with a well-documented history show the density of soil (0-20 cm) C and N in wooded landscapes (C = 2,500-5,200 g m^-2 ; N = 200-500 g m^-2) is greater than that of the grasslands being replaced (C = 2,100 g m^-2; N = 150 g m^-2), as are fluxes of CO₂, NO and non-methane hydrocarbons. Soil C and N has increased over the past 100 y (C = 12-50 g m^-2 y^-1; N = 2-5 g m^-2 y^-1), indicating net sequestration. Aboveground C and N mass in woodlands (C = 2,700 -3,100; N = 60-70 g m^-2) is 10X that of grasslands and has accumulated at rates of 42-63 (C) and 0.9-1.4 ( N) g m ^-2 y^-1 over the past 100 y. Area-weighted calculations indicate C mass has increased 12 and 800% in soils and plants, respectively with the conversion of grassland to woodland over the past century. Socioeconomic consequences of these biogeochemical changes will be explored and new approaches for monitoring changes in woody vs. herbaceous land cover in drylands will be discussed.

Keywords: land use, land cover, woody encroachment, flux, sequestration, carbon, nitrogen

This abstract is being presented at: 3:20 PM in session:
Symposium # 17: Land Use and Land Cover Change: The Last Century and Prospects for the 21st Century.