Dispersal of a forest liana into grasslands and post-establishment stand expansion

Abstract

Encroachment into the moist coastal grasslands within eThekwini Municipality by Dalbergia obovata, an indigenous forest liana that forms dense monospecific stands, poses a potential biodiversity threat. Identification of the mechanisms of D. obovata diaspore dispersal from neighbouring adjacent forests, and subsequent patterns of propagation within the grasslands, was required to understand how D. obovata encroached into the grasslands. A simple wind dispersal model for D. obovata diaspores was developed to determine potential dispersal distances under different wind conditions. Field surveys were undertaken to map the distribution of plants within stands of D. obovata in the grasslands of two urban nature reserves and these were compared against historic orthophoto records to determine when establishment occurred. Stands were excavated to investigate the primary method of propagation by D. obovata once it had become established. The results of the wind dispersal modelling and field surveys showed that D. obovata dispersal was a product of the height of diaspore release and wind velocity. For successful dispersal to occur, diaspores would have to occupy positions close to the forest margin. Successful establishment of D. obovata was infrequent, and likely associated with extreme weather events, but once established, clonal propagation via roots and lateral branches resulted in expansion of the stands. The progressive expansion of the stands in the grasslands represents a threat to the grasslands.