Brenda R. Baillie

A survey of herbicide use and a review of environmental fate in New Zealand planted forests

BackgroundThis paper examines current herbicide use in New Zealand planted forests. Compliance of key herbicides with existing Forest Stewardship Council (FSC) standards, the key environmental certification body within New Zealand, is also reviewed.MethodsInformation obtained from a survey of six forest companies operating in New Zealand was used to identify major herbicides used by the New Zealand planted forest industry, estimate quantity of herbicides used on an annual basis and also determine changes in weed management practices motivated by certification.ResultsGlyphosate was the most widely used active ingredient in pre-plant weed control with terbuthylazine and hexazinone used most widely for post-plant weed control. Together these herbicides comprise 90% of the estimated 447 tonnes of active ingredient that is annually used. Average aerial application rates for these three active ingredients were estimated at 3.3 kg ha-1, 7.0 kg ha-1 and 1.8 kg ha-1, respectively.Use of terbuthylazine and hexazinone is restricted on FSC-certified forests subject to derogation. Environmental certification has resulted in a shift from broadcast application of terbuthylazine and hexazinone to greater use of spot weed control in the first year after tree planting. Spot weed control can reduce the amount of active ingredient used by up to 89%. Non-chemical weed control is not widely used by the forest industry as it is not as cost-effective as current herbicide regimes.A review of the literature indicated that, when used operationally and according to label registrations, these herbicides are unlikely to have any negative impacts on the planted forest environment. Although they have been detected in groundwater, under multiple land uses, concentrations were at levels below documented safe drinking standards. There are limited data for forest soil and no data on the effects of these herbicides on aquatic biota in New Zealand.ConclusionsAt present time there is insufficient information to support or refute the prohibition of terbuthylazine and hexazinone in New Zealands planted forests. This has highlighted a need to conduct field studies to determine the fate and behaviour of terbuthylazine and hexazinone in planted forests in New Zealand.

Water quality in New Zealand’s planted forests: a review

This paper reviewed the key physical, chemical and biological water quality attributes of surface waters in New Zealand’s planted forests. The purpose was to: a) assess the changes in water quality throughout the planted forestry cycle from afforestation through to harvesting; b) compare water quality from planted forests with other land uses in New Zealand; and c) identify knowledge gaps and opportunities for future research. Afforestation of pasture land significantly improved a wide range of water quality attributes such as stream temperature, nutrient and sediment concentrations and microbial contamination within 4-6 years of planting. Water quality in mid-rotation to mature forests, a large proportion of the forestry cycle, was highly variable but characterised by cool water temperatures, low concentrations of sediment and nutrients, with aquatic invertebrate communities indicative of high water quality. Impacts of timber harvesting on water quality attributes were greatest when clear-cut harvesting up to the stream edge. Harvest impacts were mediated by the retention of intact riparian buffers and to a lesser extent by retention of moderate quantities of logging slash across small stream channels. Temporal and spatial factors influenced the magnitude of response to harvesting activities and duration of the recovery period. Land-use comparisons generally showed improving water quality from pasture to planted forest to indigenous forest. Continued research to identify management systems that mitigate impacts on water quality, particularly during harvesting, remains a priority. Consistent approaches to water quality monitoring will improve the ability to report on water quality in planted forests. Future water quality research in New Zealand’s planted forests needs to encompass emerging contaminants of national and international concern such as pesticides and pharmaceuticals. As New Zealand moves toward increasing productivity from planted forests, use of potential interventions such as intensified herbicide and fertiliser use, will need to be within sustainable freshwater limits. A future challenge is to quantify the economic value of freshwater ecosystems services provided by New Zealand’s planted forests. This review showed that with continued prudent stewardship and ongoing improvements in management practices, particularly during harvesting, New Zealand’s planted forests can provide a sustainable source of well-maintained and high quality water resources.