Numéro #176

15 décembre 2003

Numéro non thématique

Valorisation et gestion sylvo-pastorale de boisements naturels de pin sylvestre

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Dans les Préalpes sèches, les élevages se sont concentrés sur les
meilleures terres. Le pin sylvestre a envahi les parcours et la rentabilité de la sylviculture est aléatoire. Une remise en valeur sylvo-pastorale permet d'articuler éclaircies fortes et pâturage, voire le ressemis des pins, et d'améliorer le cadre de vie en favorisant le multiusage.

Dans les Baronnies, une forte déprise agricole a entraîné l'enfrichement des parcours et l'apparition d'accrus denses de pins sylvestre. Un aménagement sylvo-pastoral par éclaircie très forte a été expérimenté à Laborel (Drôme). Trois densités, 120, 240 et 400 tiges/ha au lieu des 3 000 initiales, ont été
testées pendant 10 ans pour leurs effets sur la croissance des arbres et la productivité de l'herbe en sous-bois. L'optimum, alliant la production de billes de bois valorisables et une production d'herbe égale à 60% de celle des prés voisins, combine 240 tiges/ha et la fertilisation pendant 5 ans. Le sursemis
améliore fortement la réponse. Les arrière-effets restent importants pendant 5 ans. La germination printanière des graines de pin est favorisée par le pâturage, mais le pâturage estival suivant entraîne une très forte mortalité.

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Valorization and sylvopastoral management of natural Pinus sylvestris (Scots pine) stands

The Baronnies, in the Pre-alpine Highlands, is a limestone region
with shallow and steep soils and with dry summers despite an annual rainfall of 750 mm. There are 125 days of frost per year. In such a difficult environment, there has been a decline in sheep farming and crop cultivation, concomitant with a large depopulation of the land by the poorer farmers. As a result, the number of sheep declined and grazed rangelands were replaced by scrub, increasingly invaded by young Scots pine trees (Pinus sylvestris) ; sheep farming became concentrated in the valleys and on moderate slopes. Classical forest management by moderate
thinning is too costly with regard to the poor quality of the trees. In order to study the potential of sylvopastoral management, three levels of severe thinning were compared at the village of Laborel (about 850 m a.s.l.), to test their effects on tree growth, grass production, and the grazing potential. The pine trees were thinned to obtain three stem densities : 120, 240, and 400 stems per hectare. Pasture management consisted in full grazing every year for ten years, either 1) without fertilization or 2) with NPK fertilization applied during the first five years on the voluntary vegetation or 3) with the same fertilization but after an initial sowing of a grass-legume mixture. The most balanced result, combining the production of timber of acceptable quality with the production of some herbage, was obtained with 240 stems per hectare. With initial sowing and fertilization, herbage production was about 75% of that of the control (cultivated meadows) ; with fertilization of the native sward, it was 60%. During the five years following the last fertilizer application, herbage production declined progressively, to stabilize at 30-40% of the control. With a density of 120 stems per hectare, some 40 stems per hectare broke down ; with a density of 400 stems, too many poor trees
were maintained, resulting in a prospect, ten years after thinning, of timber of heterogeneous quality. Sheep grazing had a mixed effect on tree seedlings : a better contact between seed and soil surface resulted in a better germination in spring than if there had been no grazing, but entailed a very large mortality later on. It was concluded that sheep grazing could be practised to initiate pine tree seed germination ; then the sheep should be excluded for some years to allow the seedlings to develop.
Hay meadows, sylvopastoral plots and protected areas for forest renovation are complementary in land management.

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