Abstract It is now reasonably well understood that the human impact on the environment since industrialization has led to significant changes in climate. Here we attempt to develop a predictive understanding of the effects that future changes in climate may have on vegetation structure and species diversity. We do this through a determination of the relationship between radial growth and water source for Widdringtonia cedarbergensis Marsh. Our results show that there was no significant relationship between monthly radial growth, as determined using dendrometer bands, and rainfall. There is, however, a significant relationship between the δ18O composition of the water extracted from the trees and the rain δ18O values. We speculate that W. cedarbergensis exploits water derived from rain that flows off the rocky substrate of the study area into sumps between the bedding planes of the rocks on which they grow. This runoff occurs rapidly during rain events resulting in δ18O values for the trees sourcing this water not to be significantly different from that of the rain. Rainfall therefore has to be sufficient to refill these sumps on which the trees are dependent. The dendrometer bands reflect a slow but steady growth of the trees at the study site. While this growth is not dependent on rainfall, it is dependent on reliable access to available water. If climate change predictions for the region are realized and rainfall is reduced then this species will be affected. W. cedarbergensis is endemic to only a very small area within the Cedarberg Mountains in South Africa and is also one of the few trees growing in the fynbos. The extinction of this species in the wild will fundamentally affect both the vegetation structure and species composition of the region.