Feedback of trees on nitrogen mineralization to restrict the advance of trees in C4 savannahs.
Remote sensing studies suggest that savannahs are transforming into more tree-dominated states; however, progressive nitrogen limitation could potentially retard this putatively CO2-driven invasion. We analysed controls on nitrogen mineralization rates in savannah by manipulating rainfall and the cover of grass and tree elements against the backdrop of the seasonal temperature and rainfall variation. We found that the seasonal pattern of nitrogen mineralization was strongly influenced by rainfall, and that manipulative increases in rainfall could boost mineralization rates. Additionally, mineralization rates were considerably higher on plots with grasses and lower on plots with trees. Our findings suggest that shifting a savannah from a grass to a tree-dominated state can substantially reduce nitrogen mineralization rates, thereby potentially creating a negative feedback on the CO2-induced invasion of savannahs by trees.
Department of Botany, University of Otago, PO Box 56, Dunedin 9016, New Zealand steven.higgins@otago.ac.nz.
Steven I
SI
Higgins
Department of Biological Sciences, University of Cape Town, Private Bag, Rondebosch 7701, South Africa.
Moagi
M
Keretetse
Department of Biological Sciences, University of Cape Town, Private Bag, Rondebosch 7701, South Africa.
Edmund C
EC
February
D017753:Ecosystem; D005246:Feedback; D065948:Grassland; D009584:Nitrogen / Q000737:chemistry*; D058458:Nitrogen Cycle; D006109:Poaceae / Q000502:physiology*; D011891:Rain; D012621:Seasons; D012987:Soil / Q000737:chemistry; D013019:South Africa; D014197:Trees / Q000502:physiology*
D012987:Soil; D009584:Nitrogen
bush encroachment; elevated CO2; negative feedback; progressive nitrogen limitation; savannah