Weed management options for
Weed control around newly planted trees is the single most important silvicultural practice aimed at maximising timber yield. The New Zealand forestry sector relies on herbicides for cost-effective weed management. This chemical dependency conflicts
with environmental principles of chemical reduction and avoidance endorsed by the dominant forest eco-certification body, Forest Stewardship Council (FSC). Research undertaken by Scion will help the industry to identify an integrated approach to weed management that strikes a balance between economic and environmental impacts.
Two of the most commonly used herbicides in New Zealand forestry, hexazinone
and terbuthylazine, are defined as hazardous by FSC. Scion conducted a
review of vegetation management in New Zealand plantation forests to provide
information on alternative methods (manual, mechanical, biological) for
the control of the major weeds in New Zealand forestry. This information is
useful to FSC-certified forest growers as a benchmark for assessing the implications of certification (and vegetation management) for the profitability of their business.
Scion calculated the cost to the industry of five hypothetical alternative (to current) vegetation management regimes. These options included two potential chemical alternatives (that used either FSC-compliant herbicides or spot weed control) and three non-chemical alternatives (manual methods, mechanical methods and the use of weed mats). The study showed that (if proven effective) using FSC-compliant herbicides could mean a marginal increase in direct costs with overall greater financial impacts for marginal land than for highly productive land. The results do however assume no adverse effect of the chemicals
on tree growth and further research should be undertaken to determine the magnitude of long term phytotoxic effects before these FSC-compliant herbicides are widely used.
The assessment of non-chemical vegetation control alternatives showed that combinations of fire, mechanical and manual control methods (including weed mats) are prohibitively expensive, incurring a more than 100% increase in
direct vegetation management costs. For sites with average Pinus radiata yields very large increases in timber volume (20% to 70%) would be required to offset the high treatment costs of non-chemical control at discount rates of 6 to 8%. When expressed as an impact on the internal rate of return (IRR) forest companies could see a potential decrease in IRR of between 2.0 to 2.5%.
This study showed that removal of terbuthylazine and hexazinone from the forester’s vegetation managementtoolbox in FSC-certified forests will have
direct cost implications for the New Zealand forestry sector. Non-chemical
vegetation control is not a cost-effective option and, if enforced, is likely to have
severe consequences for forestry in New Zealand. Spot control and the use of FSCapproved herbicides could provide feasible alternatives depending on site factors.
It is likely that the best way forward for vegetation management in forestry is
to research and develop a management approach that integrates cultural, biological and chemical methods. The integrated approach can be designed to have minimum environmental impact at a cost that does not undermine the practice of forestry.