The impact of uncontrolled weeds on the rehabilitation success of Nabarlek uranium mine in Arnhem Land, Northern Territory

摘要

Nabarlek is a decommissioned uranium mine in the Northern Territory 270 km east of Darwin. The minesite was revegetated predominantly with native species in 1995, with the objective of achieving a 'self-sustaining woodland that blends in with the surrounding savannas'. The success of revegetation was assessed in the 2003 dry and 2004 wet seasons. The impact of weeds on vegetation composition and ecological function was investigated. Eleven grass weed species and 17 forb weed species were found in thewet season and comprised 48% of all species on mine sites with a ground cover of 55%. Only three weed species were found on adjacent natural reference sites. Species richness and abundance (% cover) of native ground cover plants were negatively correlated to richness and abundance of weeds. Weed abundance (% cover) decreased with increasing density of trees and shrubs. Grass biomass on the minesite was twice that of reference sites (5.0 cf. 2.5 t ha~(-1) ODW) in the late dry season, substantially increasing the risk of fire. Grasses on the minesite were mostly weeds such as mission grass (Pennisetum pofystachion (L.) Schult. and Pennisetumpedicellatum Trin.), para grass (Urochloa mutica (Forssk.) T.Q.Nguyen) and Rhodes feather top (Chloris virgata Sw.). Woody seedlings were common on reference sites but none were found on mine sites. Weeds persist on the minesite because of the huge build up of a soil seedbank (mean 1518 weed seeds m~(-2)) whereas reference sites has few weed seeds in the soil (5 m~(-2)). Weeds had a major impact on vegetation composition and possibly on ecological function.