Grassland ecosystem susceptibility for invasive of the black wattle (Acacia mearnsii De Wild.) in the Rio Grande do Sul State, Brazil.

Abstract

This paper aimed to evaluate the frequency, the intensity and the pattern of black wattle invasion on field environment, and to propose methodologies for its monitoring and control. Seven environments located less than 30 m from young (1 year) and mature (5 years) plantations were evaluated. Black wattle invasions depended on the degree of soil disturbance. A Permanent Preservation Area (APP) in a recuperation phase, the most disturbed environment studied, presented 152 plants/0.1 ha, the highest number of invasive plants observed. Less than 11 plants/ha were observed on environments with low disturbance rates, such as native grassland, swamp areas and undisturbed APP’s. Environments near young black wattle plantations were more susceptible to invasions (50 plants/0.1 ha) than environments near mature plantations (10 plants/0.1 ha), suggesting that the disturbance was caused by machineries used in harvesting and reforestation operations. New invasions during the following growth cycle were not significant (<1 plant/ha). Mortality in a one year period was of 80.4; 14.0; 6.8 and 5.5 %, for plants <0.5 m; 0.5 m to 1.9 m; 2.0 m to 4.0 m and > 4.0 of height, respectively, showing that the populations of invasive plants were significantly reduced during the following years. Thus, black wattle invasion to neighbouring plantations was conditioned to soil disturbance rates, and as such, it was concluded that the black wattle is a causal invasive species in a field environment.