Do abandoned farmlands promote spread of invasive alien plants? Change detection analysis of black wattle in montane grasslands of the Eastern Cape.
Invasive alien plants (IAPs) pose a threat to ecosystem integrity and generally provide more disservices than services. Acacia mearnsii (black wattle) was introduced to South Africa for its provisioning and economic potential. Since then, it has colonized large areas and it is a high priority taxon by the Working for Water (WfW) programme. This study establishes the nature of the spread of A. mearnsii in grasslands in the Eastern Cape Province of South Africa, with the aim of providing feedback to the WfW programme on IAP expansion and focal areas of concern. Land cover change detection analysis using historical aerial photographs showed that A. mearnsii has spread more extensively into disturbed (abandoned cultivation and overgrazed) grasslands compared to undisturbed natural grasslands from the initial plantings prior to 1958. Using the point-centred quarter method to determine density, and an allometric equation to determine above-ground biomass, a growth model for A. mearnsii was prepared. The incremental annual increase in the standing biomass of A. mearnsii using the model, validated against MODIS Net Primary Productivity data, was estimated to be ∼10.5 ton DM ha-1 yr-1. The spread rate of wattle maybe up to 10% and several alternative control strategies need to be considered.