From pine plantations to natural stands. Ecological restoration of a Pinus canariensis Sweet, ex Spreng forest
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We evaluated silvicultural thinning of pine plantations in order to determine the extent to which plantations treated in this way showed a greater structural similarity to natural stands. Specifically, we tested for differences in community structure (increase of DBH, increase of height and canopy height) and regeneration (seedlings and saplings <1, 1–2 and >2 years old) in response to thinning treatments (20% and 50% removal of density). We compared the variables of the thinned plots with those of the control plots (no thinning of living trees). Comparison of the structural variables between any treatments is of limited value due to the high intra- and inter-plot environmental variability (both slope and orientation affect tree growth to a significant degree). We therefore used ordination methods (Redundancy Analysis, RDA) to monitor covariation and to select non-redundant explanatory variables. We tested for differences between control and managed plots using Monte Carlo tests for the eigenvalues of the obtained axis of the RDA. Of the two treatments, only the 50% thinning treatment was significantly different from the control plots (in which only dead pines were thinned). In ten years, the basal area of pines showed a 10% increase in 50% thinned plots in comparison with the control plots. The number of saplings >2 years old was also significantly higher in 50% thinned plots. The control plots typically had an appreciably higher density of dead trees and a greater number of seedlings. Fifty percent thinning is having a positive effect on the naturalization of the stand but subsequent management will be needed to ensure establishment of advance regeneration.