title: Determinants of woody encroachment and cover in African savannas.
abstract: Savanna ecosystems are an integral part of the African landscape and sustain the livelihoods of millions of people. Woody encroachment in savannas is a widespread phenomenon but its causes are widely debated. We review the extensive literature on woody encroachment to help improve understanding of the possible causes and to highlight where and how future scientific efforts to fully understand these causes should be focused. Rainfall is the most important determinant of maximum woody cover across Africa, but fire and herbivory interact to reduce woody cover below the maximum at many locations. We postulate that woody encroachment is most likely driven by CO2enrichment and propose a two-system conceptual framework, whereby mechanisms of woody encroachment differ depending on whether the savanna is a wet or dry system. In dry savannas, the increased water-use efficiency in plants relaxes precipitation-driven constraints and increases woody growth. In wet savannas, the increase of carbon allocation to tree roots results in faster recovery rates after disturbance and a greater likelihood of reaching sexual maturity. Our proposed framework can be tested using a mixture of experimental and earth observational techniques. At a local level, changes in precipitation, burning regimes or herbivory could be driving woody encroachment, but are unlikely to be the explanation of this continent-wide phenomenon.
authors.affiliation: Department of Biosciences, Wallace Building, Swansea University, Singleton Park, Swansea, SA2 8PP, UK. a.p.devine@swansea.ac.uk.|Environment and Sustainability Institute, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK. a.p.devine@swansea.ac.uk.
authors.forename: Aisling P
authors.initials: AP
authors.lastname: Devine
authors.affiliation: Environment and Sustainability Institute, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK.
authors.forename: Robbie A
authors.initials: RA
authors.lastname: McDonald
authors.affiliation: Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB, UK.
authors.forename: Tristan
authors.initials: T
authors.lastname: Quaife
authors.affiliation: Environment and Sustainability Institute, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK.
authors.forename: Ilya M D
authors.initials: IM
authors.lastname: Maclean
mesh_terms: D003247:Conservation of Natural Resources; D017753:Ecosystem; D065948:Grassland; D014197:Trees; D014934:Wood
keywords: Carbon; Grassland; Rangeland; Savannah; Shrub invasion