title: Determinants of woody cover in African savannas.
abstract: Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.
authors.affiliation: Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523, USA. mahesh@nrel.colostate.edu
authors.forename: Mahesh
authors.initials: M
authors.lastname: Sankaran
authors.forename: Niall P
authors.initials: NP
authors.lastname: Hanan
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Scholes
authors.forename: Jayashree
authors.initials: J
authors.lastname: Ratnam
authors.forename: David J
authors.initials: DJ
authors.lastname: Augustine
authors.forename: Brian S
authors.initials: BS
authors.lastname: Cade
authors.forename: Jacques
authors.initials: J
authors.lastname: Gignoux
authors.forename: Steven I
authors.initials: SI
authors.lastname: Higgins
authors.forename: Xavier
authors.initials: X
authors.lastname: Le Roux
authors.forename: Fulco
authors.initials: F
authors.lastname: Ludwig
authors.forename: Jonas
authors.initials: J
authors.lastname: Ardo
authors.forename: Feetham
authors.initials: F
authors.lastname: Banyikwa
authors.forename: Andries
authors.initials: A
authors.lastname: Bronn
authors.forename: Gabriela
authors.initials: G
authors.lastname: Bucini
authors.forename: Kelly K
authors.initials: KK
authors.lastname: Caylor
authors.forename: Michael B
authors.initials: MB
authors.lastname: Coughenour
authors.forename: Alioune
authors.initials: A
authors.lastname: Diouf
authors.forename: Wellington
authors.initials: W
authors.lastname: Ekaya
authors.forename: Christie J
authors.initials: CJ
authors.lastname: Feral
authors.forename: Edmund C
authors.initials: EC
authors.lastname: February
authors.forename: Peter G H
authors.initials: PG
authors.lastname: Frost
authors.forename: Pierre
authors.initials: P
authors.lastname: Hiernaux
authors.forename: Halszka
authors.initials: H
authors.lastname: Hrabar
authors.forename: Kristine L
authors.initials: KL
authors.lastname: Metzger
authors.forename: Herbert H T
authors.initials: HH
authors.lastname: Prins
authors.forename: Susan
authors.initials: S
authors.lastname: Ringrose
authors.forename: William
authors.initials: W
authors.lastname: Sea
authors.forename: Jörg
authors.initials: J
authors.lastname: Tews
authors.forename: Jeff
authors.initials: J
authors.lastname: Worden
authors.forename: Nick
authors.initials: N
authors.lastname: Zambatis
mesh_terms: D000349:Africa; D000818:Animals; D018533:Biomass; D003889:Desert Climate; D017753:Ecosystem; D006109:Poaceae / Q000502:physiology; D011891:Rain; D012987:Soil / Q000032:analysis; D014197:Trees / Q000502:physiology*; D014934:Wood
chemical_list: D012987:Soil