INCREMENT DIFFERENCES OF SILVER BIRCH BETULA PENDULA ROTH AND DOWNY BIRCH B. PUBESCENS EHRH. GROWING IN DRAINED SOILS IN SOUTH LATVIA

According to State Forest Service information for the year 2016 birch occupied 30% of whole forest area (3.38 million ha). Although between these two species have dendrological differences, however in forest inventory it is not taken into account. There is also a variety of wood properties which are not respected in the timber market. The aim of this research study is to analyze growth differences between both birch growing in the same conditions. For collecting of empirical data 2 sample plots were created. Selected stands have been regenerated naturally and 2 times repeatedly drained. The forest stand investigated is 65 years old mixed stand dominated by birch which makes up 90 % of total growing stock. In each plot for all the trees have been measured diameter, height and using Pressler borer have been taken core samples. The widths of tree rings have been measured using Lin TAB system microscope and computer software T-Tools Pro. Analyzing the dynamics of diameter and height growth in both sample plots it has been observed that silver birch shows higher values than downy birch (p<0.05). In the last 30 years both birch species show systematic reduction of the annual increment. Decrease of height growth for both birch species in Vacciniosa mel. - 0.26-0.66 m. Decrease of B.pendula height growth in Vacciniosa turf. mel. - 0.39-0.70 m while B.pubescens - 0.39-0.80 m. Decrease of the stem diameter at breast height (DBH) of B.pendula - 2.7-7.5 mm while DBH of B.pubescens - 2.7-5.9 mm.