Experimental Thinning Trials in Douglas-fir Stands in Tuscany

1989

30 Abb., 19 Tab., 32 Lit. Ang.

Unselbständiges Werk

4341

200033986

Experimental thinning trials where carried out in 9 different stands, representative of the conditions in which Douglas-fir has been planted in Tuscany. Initial stocking varied between 1600 and 3100 trees/ha. Age of first thinning was 15-27 years. The following thesis were compared: 1) row thinning with removal of every third row; 2) geometric thinning with removal of every third tree on each row; 3) combined thinnings with removal of every (sixth) row and thinning with 400 crop trees/ha; 5) thinning from below with different grades: s% (Hart-Becking distance factor) after thinning equal to 16-20% ("moderate" - "heavy" thinning); 6) control: no thinning (in all stands). A second thinning was carried out in all plots thinned from below 5- 12 years after first thinning. All thinning methods, compared to control plots, reduced mortality from suppression and snow damage. Few trees (1-4% of total) were uprooted in stands planted at 2500 trees/ha and thinned at 18 years with removal of whole rows. After 8 years, for all thinning methods and grades, total production (standing volume plus volume of thinned trees) and current annual volume increment of standing trees were not significantly different from control plots. Comparing different grades of thinning from below, "medium-heavy" thinnings (s% after thinning =18) performed the best. Second thinning, with this thinning grade, should be carried out not before 8-10 years after first thinning. Thinning, regardless of method, favoured increased DBH increment, compared to control plots, but trees of the smaller size classes responded the best. Dominant trees in plots thinned from below grew at the same rate as those in control plots. Row thinnings and crop tree selection thinning, in young and scarcely differentiated stands (15 years, 1600 trees/ha initial stocking), both increased growth rate of dominant trees compared to thinnings from below. Width and variability in growth rings was also examined. Different thinning regimes are suggested in view of different management options.