Injecting glyphosate herbicide into a regrowth Eucalyptus obliqua stem. Image: Ann LaSala

Aerial photo of the treated coupe following pre-commercial thinning. The brown colour is due to the death of treated trees. Stags from an earlier wildfire also stand out clearly. Image: Ann LaSala

The treated stand in the weeks following stem injection, showing the extent of resultant mortality. Image: Ann LaSala

A year-long study investigated the effect of season on tree mortality following glyphosate injection in eucalypt regeneration at a wet sclerophyll and a dry sclerophyll site in Tasmania. Trees of a range of sizes were injected at each site on a monthly basis, and their decline and death monitored using a canopy rating system. Generally, the larger the diameter class, the longer it took for 90% of trees in that diameter class to die, regardless of the season of injection or site. Season of injection affected the speed of kill at both sites; trees were generally killed more quickly at the drier site following winter injections, and at the wetter site following summer injections. Ninety per cent of trees injected in all months were dead within three months at the wetter site and within 12 months at the drier site. Available moisture and metabolic activity are important factors in the efficacy of glyphosate injection and vary from site to site. At the dry forest site, available moisture may limit metabolic activity and growth in summer which may explain the slower mortality from summer injections. The findings are used to make recommendations concerning the scheduling and monitoring of pre-commercial thinning.