Finnish Forest Research Institute  Metla

Press release 29.11.2006

Vegetation control necessary for successful stand establishment on former agricultural land

Successful post-planting control of ground vegetation on stand establishments on former agricultural land increases the growth and decreases mortality of Scots pine. Large Norway spruce seedlings can survive even in thick vegetation, but their growth is also slowed. The study, conducted by the Finnish Forest Research Institute (Metla), showed also that the most effective herbicide is terbuthylazine, which has already been recalled from the market. During the 11 years of follow-up, the stand volume on pine plots treated with terbuthylazine was almost double that from untreated reference plots.

Field experiments were set up and data was gathered over 6-11 years of follow-up to determine the effects of competitive ground vegetation and various weed control methods on the development of Scots pine and Norway spruce seedlings. The seedlings were planted in spring 1991 on ploughed and harrowed agricultural mineral soil in Vilppula, southern Finland. The effects of the following soil-active herbicides were examined: terbuthylazine, hexazinone and pendimethalin, as well as a mixture of terbuthylazine and glyphosate. Of the foliar-active herbicides glyphosate, tested using both spray and contact applications, as well as pendimethalin were included. Mulch (fibreboard, 50 cm x 50 cm) and a cover crop of white clover (Trifolium repens L.) were used as alternative methods.

Competing vegetation retarded the height and volume growth of Scots pine and increased mortality. The mortality of pine started a rise only after the vegetation coverage had reached the level of 60 %. For Norway spruce, vegetation coverage did not affect the mortality. The best result was achieved with herbicides with at least 2-3 years efficacy. The efficacy of glyphosate preparations was only one growing season, after which weeds, mainly annuals, colonized the plot.

After 11 years, the stand volume of Scots pine treated with terbuthylazine was 32 m3/ha and 17 m3/ha on untreated reference plots. The corresponding volume obtained with glyphosate treatment was 23 m3/ha. The Norway spruces were damaged by frost during three springs, so the effect of the treatments on the height and volume growth of spruce could not be reliably evaluated. Norway spruce did, however, indicate signs of significance of weed control, since increased vegetation coverage reduced the growth of the basal diameter of Norway spruce. White clover represented as much competition as weeds. It may also attract voles to the site and thus, increase the risk of vole damage. The mulch boards used were too small in size to reduce competing vegetation significantly. The spruce seedlings used were good-sized 4-year-old bare-rooted seedlings, which are rather competitive against weeds.

According to forest certification criteria, chemical herbicides can only be used when absolutely necessary, for example, for prevention of ground vegetation on forest regeneration sites. Effective control of ground vegetation is especially important on former agricultural land, because the soil contains large amounts of viable weed seeds after decades of farming. The use of chemical herbicides in forestry has been almost totally discontinued. Cycloxydim was only recently accepted for forestry use and in addition to glyphosate, it is the only herbicide allowed for coniferous trees. Glyphosate is usually applied on plantations as a single treatment, but the efficacy of a single treatment is not sufficient on former agricultural fields. For successful results the treatment should be repeated during the following summer. However, if applied with negligence or at the wrong time, glyphosate may damage seedlings and slow down their growth.

To save costs in stand establishment, many forest owners choose to plant small-sized seedlings that are not prepared to meet the competition on former agricultural land without effective vegetation control. Since the means of chemical herbicides are becoming scarce, there should be other ways to secure the early development of seedlings. For example, choosing more developed seedlings could improve the afforestation results. Development of alternative methods should also be continued.

Publication: Jylhä, P. & Hytönen, J. 2006. Effect of vegetation control on the survival and growth of Scots pine and Norway spruce planted on former agricultural land. Canadian Journal of Forest Research (36): 2400-2411.

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