Influence d'une éclaircie sur la croissance et le bilan hydrique d'un jeune peuplement de Douglas (Pseudotsugamenziesii (Mirb.) Franco)

Abstract

Systematic thinning (alternate rows) was done in a 19-year-old Douglas-fir plantation. The principal microclimatic effect was to increase light at ground level (less than 1% in the unthinned versus 7.3% in the thinned stand) and also in the crowns (crown bases: 1% in the unthinned compared with 17.6% in the thinned stand). The other microclimatological parameters were less influenced.With reference to the water balance, the results indicate that thinning half the trees diminished the interception by only 13.1% (43.4% water intercepted in the unthinned versus 30.3% in the thinned stands). Indeed, the influence of the thinning on the actual evapotranspiration was relatively weak: the observed decrease was only 16.7% compared with the unthinned stand.The measurements also show that, as of the 1st year, the thinning increased the rate of tree growth substantially; the increase in basal area of the thinned stand was 101% of that of the unthinned stand. In other words, there was no production decrease the year of thinning.Essentially thinning benefits codominant trees. As a result of thinning, modifications to the crown cover became evident: the lower whorls of the tree crowns, remaining after the thinning, exhibited a greater mass of foliage compared with the unthinned stand owing to fewer old needles dropping.