Paper Reviewed
Russell, J.M. and Ward, D. 2014. Remote sensing provides a progressive record of vegetation change in northern KwaZulu-Natal, South Africa, from 1944 to 2005. International Journal of Remote Sensing 35: 904-926.

Focusing on the open savannah and grasslands of northern KwaZulu-Natal, South Africa, Russell and Ward (2014) employed "multi-temporal sets of aerial photographs, taken between 1944 and 2005, as they describe it, "to observe the phenomenon of woody plant encroachment in four neighboring study sites," while also analyzing the sites' rainfall and temperature trends; and they go on to report that this effort revealed that these and other local environmental factors - such as fire, herbivory, and land use - did not have any significant effects on woody-plant encroachment or increases in vegetative cover, playing only modifying roles in this major landscape-altering phenomenon.

Furthermore, the two South African researchers report that even when average regional rainfall dropped below the long-term mean, the woody cover of the region continued to increase. Neither could they attribute this vegetative cover increase to an observed increase in average annual temperature, because, as they report, "the timing of the response does not synchronize with this increase." And they thus concluded that there must "be an additional driver" of the observed increase in the region's woody cover. And because of the fact that "rising atmospheric CO2 concentrations have been shown to have a significant impact on the vigor and survival of savannah trees and shrubs," as described by Kgope et al. (2009), they ultimately concluded that "rising CO2 concentrations may well be the overriding driver, with local drivers tempering the effects of CO2 to a greater or lesser extent at the various study sites."

Reference
Kgope, B.A., Bond, W.J. and Midgley, G.F. 2009. Growth responses of African savanna trees implicate atmospheric [CO2] as a driver of past and current changes in savanna tree cover. Austral Ecology 35: 451-463.