Finnish Forest Research Institute  Metla

Media release 21.10.2005

Weed control improves afforestation results for former agricultural fields

Effective weed control in conjunction with afforestation measures applied to former agricultural fields increases seedling growth of silver birch (Betula pendula) and reduces mortality and vole damage. These findings were reported in a study made at the Finnish Forest Research Institute. Dichlobenil turned out to be the most effective of the currently used herbicides. It is possible that the use of herbicides will become more restricted than today. Thus it is essential to continue research into alternative weed control methods.

The effects of competing ground vegetation and various herbicides on seedlings' growth, mortality and vole damages were studied during 6 to 11 post-planting growing seasons. During spring 1991 a field experiment was set up on a mineral-soil-based agricultural field in Vilppula, southern Finland. Following complete soil preparation (ploughing and harrowing) the birch seedlings were planted within a fenced area. The effects of the following herbicides were examined: soil-active terbuthylazine, chlorthiamid, dichlobenil and pendimethalin; and foliar glyphosate and sethoxydim. A cover crop of clover (Trifolium repens L.) and fibre board mulch (50 cm x 50 cm) were used as alternative methods.

When the amount of ground vegetation increased, seedling growth was significantly retarded. Seedling mortality started a drastic rise once the vegetation coverage had reached the level of 60%. Terbuthylazine, chlorthiamid and dichlobenil were the most effective agents in reducing ground vegetation. Test areas treated with glyphosate, pendimethalin and sethoxydim did not show different results from the untreated control plots, nor did the cover-crop or mulch plots. Successful control of ground vegetation increased seedling growth. The chlorthiamid-treated plots produced the highest stem volume, the mean stem volume being 34 m3/ha, i.e. almost three times the volume of the control plots (12 m3/ha).

The highest seedling mortality was found on the cover-crop plots and on the plots treated with sethoxydim. After the sixth growing season, 46% and 44%, respectively, of the seedlings on these test plots had died. On the mulch plots only 2% of the seedlings died during the first six growing seasons. Mortalities were very low also on the plots treated with chlorthiamid, dichlobenil and terbuthylazine.

Abundant ground vegetation also increased vole damage. Vole attacks were especially intensive on shorter-than-average seedlings. Effective control of ground vegetation can accelerate seedling growth and thus reduce the time over which they are vulnerable to vole damage.

According to forest certification criteria, chemical herbicides can only be used when deemed necessary, for example, for prevention of ground vegetation on forest regeneration sites. The development of ground vegetation is particularly intensive in afforestation areas established on former agricultural land. The use of chemical herbicides has dramatically decreased and several products have been withdrawn from the market. Of the agents included in this study, only glyphosate and dichlobenil are currently approved for forestry use. It is possible that further limitations will be imposed on using chemical herbicides due to environmental concerns. Thus, the research into alternative methods should be continued. In this study clover as the cover crop plant turned out to be as bad a competitor as actual weeds. Additionally, it attracted voles to the site. Using mulch did not increase seedling growth, but mortality was small on the mulch plots.

Since 1969, 240,000 ha of former agricultural fields have been afforested in Finland. Afforestation reached its peak during 1992-93, when more than 17,000 ha of agricultural fields were afforested. In 2003 the area of afforestation was less than 2000 ha.

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