Vegetation responses to invasive alien plant clearing along the Sabie River in and adjacent to the Kruger National Park
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Date
2009-03-25T07:32:55Z
Authors
Morris, Taryn Lee
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Abstract
Alien plant invasions are a global problem and have widespread ecological and economic
impacts. River ecosystems are highly prone to invasion by alien plants due to their
dynamic hydrology and the ability of water to efficiently transport alien plant
propagules. This, together with continuous access to water and nutrient resources,
facilitates invasions. The recognized impact of woody alien species in reducing scarce
water resources in South Africa inspired the inception of the Working for Water (WfW)
programme. This programme aims to increase future water yields through the large scale
clearing of invasive alien plants (IAPs), while simultaneously addressing poverty
alleviation. Since its inception, the programme has been operating under the assumption
that ecosystems would “self repair” after the removal of IAPs, but little monitoring has
occurred to determine if this is in fact the case.
In the Kruger National Park (KNP) invasive alien species have been declared one
of the greatest threats to ecosystem biodiversity. Riparian zones are the most invaded
systems due to the continuous influx of alien propagules from the poorly managed upper
reaches of the major river catchments, combined with the capacity of river systems to
facilitate invasions. WfW have executed the majority of the active clearing operations on
an ongoing basis in the KNP for several years. However, little monitoring has occurred
to determine the efficacy of control operations or the response of both native and alien
vegetation to the removal of IAPs. Thus, the major aims of this project were to assess the
efficacy of the WfW programme in clearing invasive alien plants in the KNP, and to
assess the extent of natural ecosystem recovery after clearing operations.
Vegetation from twelve sites along the Sabie River within and adjacent to the
KNP ( Mpumalanga, South Africa) was sampled before and after an annual clearing
operation by WfW. Results were compared with additional data derived from several
historical studies in the area to assess the longer-term efficacy of WfW.
The occurrence of dense stands of IAPs comprising up to 97% of the vegetation
density along the Sabie River within the KNP were likely a result of the significantly
elevated rainfall in the respective growth season. These dense stands of IAPs were
associated with a decline in several diversity measures of native vegetation with a basal
stem diameter > 1 cm (P < 0.05). However similar negative associations were not
observed with understorey vegetation (P > 0.05). The annual clearing operations by
WfW were highly effective and reduced invasive alien plant densities from 3508 ± 1113
plants per ha to 343 ± 156 plants per ha (P = 0.002), translating to an average reduction of ± 80%. After clearing, indigenous vegetation density increased (P = 0.02), despite the
reduced rainfall received in that year’s growing season. Herbaceous and graminoid
growth forms showed the greatest increase in previously densely invaded transects.
In general, alien species richness has increased slightly in the KNP over the last
ten years. However, the invasion intensities have remained exceptionally low, even in the
face of several large disturbance events that are usually associated with a sharp increase
in alien plant invasions. This is more than likely attributable to the continuous annual
clearing operations by WfW in the KNP. Continuous clearing acts to effectively limit the
establishment and spread of many IAP species despite the ever present threat of invasion
from upstream. Furthermore, the continuous clearing of IAP stands in the KNP ensures
that stands are relatively short-lived, preventing long lasting negative impacts on the
ecosystem. Removal of IAP species reduces their disproportionate competitive influence
and facilitates the natural re-establishment of native vegetation.
This study presented important information with regards to the vital need for
assessment and monitoring of WfW operations. It also provided KNP management with valuable results pertaining to research and management objectives.