Drivers of riparian forest change in Mapungubwe National Park, South Africa

Background/Question/Methods

Narrow bands of riparian forests that occur along perennial rivers in semi-arid savannas have greater productivity and biodiversity than the surrounding terrestrial savanna. In Mapungubwe National Park, in northern South Africa, riparian forests have contracted, and extensive stands of closed-canopy forest once dominated by Acacia xanthophloea, Faidherbia albida, Ficus sycomorus have been replaced by savanna. Water availability for these forests has been altered, with groundwater diversion to support nearly mining, as well as reduced flow in the Limpopo River. To determine the potential importance of altered water availability as a driver of riparian forest change, we measured differences in source water-use using the xylem-water isotopic signature to identify differences by: (1) species, (2) growth form, and (3) distance from the river.

Results/Conclusions

Resampling of trees originally marked in 1990 revealed average annual mortality rates of 4.9%, 2.4% and 1.6% for A. xanthophloea, F. albida, and F. sycomorus, respectively, while the sub-canopy species Croton megalobotrys is expanding. Of the tree species sampled, F. albida and F. sycomorus show similar reliance on water from the river channel. A. xanthophloea, Philenoptera violacea and Xanthocercis zambesiaca had greater reliance on groundwater. The water isotopic signature of C.megalobotrys was distinct from all other species measured and suggests a unique source of water used.  While declining river flows and extreme floods are the most likely causes of the decline of two of the dominant forest species, factors not related to water availability, such as browsing by elephants, must be involved for A. xanthophloea. Thus, multiple and interacting factors are having species-specific impacts in the Mapungubwe riparian forests, with consequences for habitat availability, hydrology, and biodiversity in this system.