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Résumé :
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Wide-spaced tree planting on pastures is the most researched agroforestry scheme at the moment in France and Great Britain. Such plantations could meet the requirements of the new Common Agricultural Policy for the reduction of agricultural surpluses, the production of high quality timber, and the ecologically sound management of the land. High tree growth rates can often be expected. Low treestocks (about 100 stems.ha-1) allow high and sustained inter-row production of fodder for grazing or harvesting. Careful management of the trees is required to obtain a straight knot-free bottom log, 3 to 8 metres tall depending on the tree species and the site index. In such a scheme, the protection of the tree against grazing sheep or cattle is a key issue. The following guidelines may be drawn from our experiments : round plastic tree shelters are effective to protect the trees, provided that they are high enough, that the stakes are adequate, and that the animals are properly managed (e.g. no fodder shortage). Moreover tree shelters allow easy spot-weeding, with significant benefits on tree growth. The growth of trees inside the shelters is however altered in our experiments, wild cherry trees and walnut trees exhibited enhanced height growth, but reduced stem diameter and total biomass increments during the time when they were growing inside the shelter. At the end of the first growing season, the biomass partition to the stem is higher for sheltered trees. An experiment set up in 1992 compared ventilated and unventilated shelters, with two levels of light transmission inside the shelter. 1992 had a very unusual rainy spring and summer, and in these conditions, ventilation proved to increase tree growth, but the result may not be extended to dry years.
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