Effects of heavy thinnings on the increment and stability of a Norway spruce stand and its trees between the ages of 32 and 50

The paper studies effects of two heavy thinnings on the increment and slenderness of various categories of trees and stability of the stand as a whole. The research was conducted on a permanent experimental plot in an Norway spruce (Picea abies /L./ Karst.) monoculture in Serbia. This monoculture was established with 5,000 seedlings per hectare on the site of mountain beech forest and the effects of heavy thinnings were investigated in the 33-40 and 41-50 age periods. To determine the thinning effects we compared current diameter increments (i(dt)) and current height increments (i(ht)) of dominant trees (D-100 and D-400) obtained by a detailed analysis of trees and of mean stand dominant trees (D-100 and D-400). At the stand level, we compared the current diameter (i(d)), basal area (I-G) and volume (I-v) increments of all trees and of the same collective of aspirants in two periods after the thinnings, between the ages of 33 and 40, and between the ages of 41 and 50. The first thinning was carried out at the age of 32 when the dominant trees were 15 m tall and the next at the age of 40 when the dominant trees were above 20 in in height. They were both low (q(d) lt 0.85) and heavy selective thinnings (34-36% of the volume). A more significant increase in the diameter increment was recorded after the second thinning between the ages of 41 and 50. It amounted to 29.1% in aspirants and 36-42% in dominant trees (D-100 and D-400) compared to the period after the first thinning, i.e., between the ages of 33 and 40. The thinnings further contributed to the establishment of more favorable relations in diameter and height increments of the trees in the studied culture and thus improved their stability