Acacia mearnsii (tree, shrub)

Ecology Distribution Management Info Impact Info References and Links Contacts

General Impact The invasiveness of this species is partly due to its ability to produce large amounts of long-lived seeds (which may be triggered to germinate en masse following bush fires) and the development of a large crown (which shades other vegetation). Its leaves and branches may have allelopathic properties. Acacia mearnsii competes with, and replaces, indigenous vegetation. It may replace grass communities, reducing the carrying capacity of the land. By causing an increase in the height and biomass of vegetation Acacia mearnsii infestations increase rainfall interception and transpiration, which causes a decrease in streamflow. Soil under Acacia mearnsii becomes dessicated more quickly (than it does under grass). Acacia mearnsii stands also destabilise stream banks and support a lower diversity of species (Adair, 2002; Sankaran, 2002; Le Maitre et al. 1999; Samways et al 1996). Commercial plantations and invasive stands of A .mearnsii in South Africa reduce surface runoff and decrease water ability, causing an estimated annual economic loss of $US 2.8 million. According to KwaZulu-Natal Wildlife (the governmental agency responsible for managing protected areas in KwaZulu-Natal Province, South Africa) the advance of alien plants (particularly Chromolaena odorata, Lantana camara, Acacia dealbata, and Acacia mearnsii) is the most significant past and future threat to conservation in these areas (De Wit, Crookes and Van Wilgen, 2001; Goodman, 2003)          Location Specific Impacts: Kolukkumalai (India)   Competition: Invades and suppresses native vegetation. Vattavada (India)   Competition: Invades and suppresses native vegetation. Israel   Reduction in native biodiversity: Has the potential to form monospecific stands, reducing biodiversity. Reunion (La Réunion)   Competition Other South Africa   Competition: Competition between invasive wattles and important grazing grasses reduces grass cover; this affects rural communities in grassland areas in the Eastern Cape, Mpumalanga and KwaZulu/Natal provinces. In one study 94% of households reported this as a significant factor. Economic/Livelihoods: Losses due to invasive wattles were estimated to amount to 577 million cubic metres of water annually in South Africa. This corresponds to an economic loss of about $1.4 billion annually (Versfeld et al.., 1998, de Wit, Crookes and van Wilgen, 2001). Ecosystem change: Disrupts important ecosystem processes. Acacia. mearnsii (as well as a number of other Acacia species) has become invasive in South Africa and threatens conservation areas, dune ecosystems and the potential of agricultural land (Dennill and Donnelly, 1991, Hill, Gordon and Neser, 1999). Habitat alteration: The invasion of riverbanks causes deep channelling followed by slumping during floods. In addition, increases in fire intensity results in an increase in soil water repellency and increased erosion. Causes erosion and destabilisation of stream banks. Modification of fire regime: Increases in biomass lead to increases in fuel loads (often by up to a 10 fold). Dense stands of trees prevent access needed for fire management. This leads to an increase in fire intensity and damage (Versfeld and van Wilgen 1986, van Wilgen and Richardson, 1985, de Wit, Crookes and van Wilgen, 2001). Modification of hydrology: Commercial plantations and invasive stands of Acacia mearnsii decrease surface runoff and water ability. This is due to an increase in the height and biomass of vegetation (which increases rainfall interception and transpiration causing a reduction in stream flow). Increases water evaporation and reduces surface stream flow. Dense infestations of black wattle occurring in the riparian zones produce high rates of evaporation. Total annual evaporation may exceed 1500mm. In comparison, annual evaporation from indigenous grasslands and fynbos shrub lands commonly varies over the range of 600 to 850mm. The removal of black wattles is predicted to increase stream flow in an area, especially in areas of high evaporative demand and with low drought stress throughout the year (Dye and Jarmain, 2004). Modification of nutrient regime: The increases in soil nitrogen levels (caused by the nitrogen fixing bacteria that live in symbiosis with leguminous plants) can make habitats unsuitable for indigenous plants and more susceptible to invasion by other exotic species. Other: The commercial value of Acacia mearnsii on the one hand and its invasiveness on the other, give rise to a classic conflict of interest, where the benefits accrue to a number of people (i.e. profits from timber and tannin sales for wattle growers) and while society at large bears the external costs (i.e. a loss in water supply for locals and a national decrease in biodiversity). Physical disturbance: Invasive plants along river banks can reduce access for anglers, canoeists, white-water rafters and swimmers. Reduction in native biodiversity: Species richness of birds is lower in areas with Acacia mearnsii stands. Total plant cover at the ground level is lower in areas invaded by Acacia. mearnsii. Causes a displacement of diverse indigenous plant communities by single species (monospecific) wattle stands. Almost 1900 of the 3435 plant species in the South African Red Data List are threatened at least in part by all the alien invasive plant species. Mgahinga Gorilla National Park (MGNP) (Uganda)   Modification of successional patterns: Lejju (2004) found that the level of regeneration in the Mgahinga Gorilla National Park is influenced by the intensity of cultivation and soil nutrients. The advanced growth beneath the exotic woodlots, especially Acacia mearnsii and Eucalyptus sp. is relatively impoverished. Reduction in native biodiversity: Lejju (2004) suggests that a low level of native species biodiversity is supported by, and able to exploit, exotic tree communities (such as those created by stands of Acacia mearnsii).