Woody encroachment over 70 years in South African savannas: overgrazing, global change or extinction aftershock?

Woody encroachment in “open” biomes like grasslands and savannas is occurring globally.Both local and global drivers, including elevated CO2, have been implicated in these increases. The relative importance of different processes is unresolved as there are few multisite, multi land-use, evaluations of woody plant encroachment. We measured 70 years of woody cover changes over a 1020km2 area covering four land uses (commercial ranching, conservation with elephants, conservation without elephants and communal rangelands) across a rainfall gradient in South African savannas. Different directions of woody cover change would be expected for each different landuse, unless a global factor is causing the increases. Woody cover change was measured between 1940 and 2010 using the aerial photo record. Detection of woody cover from each aerial photograph was automated using eCognitions’ Object based image analysis (OBIA). Woody cover doubled in all land uses across the rainfall gradient, except in c...