THE INFLUENCE OF ARTIFICIAL SHADING OF THE GROUND VEGETATION ON THE NUTRITION AND GROWTH OF SITKA SPRUCE (PICEA SITCHENSIS CARR.) IN A HEATHLAND PLANTATION

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Abstract
A marked stimulus has been obtained by shading the heather vegetation surrounding the trees by means of wooden laths. The increased growth was accompanied by significant increases in the concentration of nitrogen, ash, and manganese in the needles, indicating a substantial increase in the availability of these nutrients to the trees. A similar shading experiment on plots from which the heather ( Calluna vulgaris Hull) had been previously removed by screefing produced only doubtful responses. Heather plants growing beneath the laths were found to contain appreciably lower concentrations of nutrients in the foliage than unshaded plants. Whilst the response of the trees to shading of the ground vegetation is to be attributed largely to reduced competition by the heather for available nutrients in the soil, especially nitrogen, the mulching effect of the heather in maintaining satisfactory moisture conditions during dry periods must also be taken into account. These findings offer an explanation for the improvement in the growth of spruce in mixture with nurse trees when the latter form a closed canopy and suppress the ground vegetation.

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