An investigation of the prerequisite conditions which enable invasion of moist grasslands by Dalbergia obovata.

Abstract

Encroachment by Dalbergia obovata into moist coastal grasslands within eThekwini Municipality was identified as a potential threat to the conservation of this threatened habitat type. An investigation was undertaken to confirmation whether or not D. obovata was in fact encroaching into the city's grasslands and to identify the probable underlying drivers and local factors that contributed to encroachment. The investigation comprised four areas of focus. The first was an assessment of the management practices and records of D. obovata encroachment extracted from the available reserve management records of seventeen nature reserves and conservation areas. The summarised data from the records was then processed using a Chi-square test and Principal Components Analysis. The second phase focussed on the application of field surveys to record the distribution patterns of D. obovata within four selected management areas. The results were then captured in a GIS map against which the results of the Chi-square and PCA tests were compared. In phase three a simple theoretical ballistics model was developed from experimental data to predict the anomochoric distribution of D. obovata diaspores from the forest into adjacent grasslands and the critical positioning of parent plants in the host trees in relation to the edge of the canopy. The final focal area involved the mapping of individual plants within stands of D. obovata and comparison against sixteen years of historical orthophoto records in order to determine the historic patterns and rates at which D. obovata became established and propagated within the grasslands. During this part of the investigation the vegetative propagation of D. obovata by clonal propagation from lateral branches and roots was identified and which provided an explanation of how D. obovata was able to form large dense stands with no evidence of canopy contact mortality. The final conclusions were that D. obovata parent plants had to be positioned on the edge of the forest canopy for diaspores to successfully escape the canopy, that successful establishment of D. obovata in grasslands from any particular parent plant was infrequent and that D. obovata did not become established in the presence of large mixed feeder herbivores. However once established in the grasslands, the ability to spread via clonal propagation enabled D. obovata to both outcompete grass species for available resources and be resilient to conventional bush encroachment management practices which relied on controlled burning programs.