Thinnings have the double finality of improving the quality of standing trees for future cuts and obtaining intermediate products. Preceding Nature actuation, they avoid the loose of an important fraction of the forest production by death and descomposition of those trees unable to sustain natural competition. In this paper, we present data and results from three inventories (15 years) carried out on two experimental sites in a natural Pinus sylvestris L. stand where different thinning regimes, quantified by the residual basal area, were compared. The analysis of our data show that initially there were no significance differences between the values of the basic variables: number of trees per hectare, mean height, average diameter and basal area in none of both the experimental sites. On the contrary as time went by and induced by thinnings, significative differences of some variables: mean and dominant height, average diameter, volume of the average tree and current basal area growth, became more and more important. These differences are likely to show that differences between site qualities grow with the stand age and the stand response to intensive thinning is better in good qualitied stands. The effect of thinning in each block is significative for some variables: average diameter and volume of the average tree which grow as thinnings are more and heavy. One again, the facts that the total timber yield for a given species and stand is independent from the stand density and that this timber yield does nor vary even with heavy thinnings if the stand density remains within certain limits are confirmed.
