THE EFFECTS OF THE HERBICIDE TEBUTHIURON ON SEEDLINGS OF MIMOSA-PIGRAAND NATIVE FLOODPLAIN VEGETATION IN NORTHERN AUSTRALIA

We studied the seedbank of floodplain vegetation in three major tropic al river systems in northern Australia, which had been variously invad ed by the tropical woody weed Mimosa pigra. The sites selected had not previously been treated with tebuthiuron, a herbicide which is widely used in northern Australia to control Mimosa. We collected soil seedb ank samples from two floodplain vegetation types (Melaleuca swamp and sedgeland), and, within each type, from areas in which Mimosa was eith er present or absent. The effects of treatment with tebuthiuron at 15 kg ha(-1), twice the usual recommended rate, was subsequently assessed in the laboratory on the soil-seedbank samples. Ordination of the spe cies composition of seedlings which emerged from the soil seedbank sam ples showed no effect of (i) the vegetation community from which the s amples were collected, (ii) the presence of adult Mimosa, or (iii) tre atment with tebuthiuron. The effect of tebuthiuron on the emergence an d mortality of seedlings from four functional groups (grasses, sedges, forbs and Mimosa) was also tested on the seed bank samples. Emergence was significantly decreased by tebuthiuron only for forbs from Melale uca swamps. The mortality of Mimosa was significantly higher than that of the other functional groups, but there was some mortality of forb and grass seedlings. Sedges, however, were unaffected. The impact of t ebuthiuron on Mimosa depended on soil clay content-in the soils with l owest clay content, tebuthiuron was the most effective in killing Mimo sa seedlings. Mortality in forb and grass seedlings, in contrast, was not affected by soil clay content. Tebuthiuron was therefore selective against Mimosa seedlings. However, even at twice the recommended rate of application for killing adult Mimosa, under ideal conditions for d istribution of the herbicide through the soil, 43% of Mimosa seedlings survived. Given the size of the Mimosa seedbank under held conditions (similar to 10 000 seeds/m(2)), tebuthiuron can therefore not be cons idered an effective herbicide against Mimosa seedlings.