Effects of weed control on the early growth ofBetula pendulaseedlings established on an agricultural field

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Abstract
Various herbicides (glyphosate, sethoxydim, pendimethalin, chlorthiamid, dichlobenil, ter‐buthylazine) as well as particle board mulch and a cover crop (clover, Trifolium repens) were compared during the first two post‐planting years as weed control means in a silver birch (Betula pendula) plantation established on agricultural soil in southern Finland. Chlorthiamid, dichlobenil and terbuthylazine exhibited good weed control for two years and also increased the height growth of the seedlings by 40–50 cm and much greater relative increase in leaf area and volume as compared to untreated control plots. Weed control had a significant effect on the foliar nutrient concentrations of birch, particularly that of N. As the amount of weed vegetation increased, foliar N, P (second year), K, Cu and B decreased and, respectively, foliar P (first year), Ca and Mg increased. Vegetation control also had a great indirect influence on the state of health of the seedlings. Incidences of vole damage and bark necrosis disease were associated with a high cover‐percentage of weeds, particularly of clover which is much favoured by voles. Particle board mulch seemed to increase vole damage by providing shelter for the voles. The competition by weeds for nutrients, and probably also for water, was much more important than their competition for light. Use of mulch and a cover crop did not reduce root competition as effectively as did the best herbicides.