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ŠUMARSKI LIST 1-2/2019 str. 48     <-- 48 -->        PDF

thinning is usually to improve the quality of the final crop (Savill et al., 1997).
Dominant trees form a stronger and more conical stem, while suppressed trees form a cylindrical and weaker stem. In general, the effect of thinning is seen in the form of crown expansion, and as a result, the diameter growth can be further increased faster at the lower parts of the stem than at the upper part. Thus, the stem may become more conical in shape. For this reason, the increment at breast height provides ample information about the growth trend; however, it is not useful for quantitative estimation of stem taper and stem volume change (Smith et al., 1997).
Previous studies have shown that there is a rapid increment in diameter at the base of the stem after thinning (Pukkala et al., 1998; Tasissa and Burkhart, 1998). Subsequently, compared to a non-thinned stand, in a heavily thinned stand there may be substantial increments in the diameter of the upper parts of the remaining tree stem without a temporary standstill in the diameter increment at the base (Hilt and Dale, 1979; Mäkinen and Isomäki, 2004b; Pukkala et al., 1998; Tasissa and Burkhart, 1998). However, this growth response to thinning diminishes over time (Tasissa and Burkhart, 1998).
The response to thinning can be different along the stem. A more conical stem can be formed immediately after the thinning by freeing a tree of competition and by encouraging more xylem production at the base level than at higher levels. Thus, thinning can have a direct impact on the shape and build of the remaining trees (Arbaugh and Peterson, 1993; Larson, 1963; Tasissa and Burkhart, 1998). Kalıpsız (1998) stated that the tree-ring width along the stem depends on the relationship to neighboring trees and that an increment in diameter at the base of the stem would result in a greater diameter increase in the lower part of the stem which would lead to formation of a weak stem. There are different opinions about the effect of thinning on the diameter increments in different parts of the stem (Adegbeihn, 1982; Eler, 1988; Eler and Keskin, 1991; Mäkinen and Isomäki, 2004b; Morris et al., 1994; Weiskittel et al., 2009). As the effect of thinning on the diameter increment at different heights along the stem is not clear, more research is needed (Peltola et al., 2002).
Narrow-leaved ash (NLA, Fraxinus angustifolia) is a tree having great ecological and economic importance for lowland forests because of its valuable timber and its rapid development ability (Çiçek et al., 2013; Drvodelić et al., 2016; Kranjec et al., 2017). In Turkey it grows in riparian areas and is found as scattered trees or in small groups in mixed hardwood stands in mountainous areas (0-2000 m) (Boshier et al., 2005; Davis et al., 1988). Almost all of the NLA-dominated lowland forests in Turkey have been converted to pure NLA plantations over the last 50 years. However, information on the silvicultural practices that should be applied in these plantations, especially on the growth effects of thinning intensity, is lacking. Although there have been numerous studies on the effect of thinning intensity on stem diameter growth in various tree species, there is limited information on the effect of thinning on the diameter increment distribution along the stem.
The purpose of this study, carried out on a 22-year-old NLA plantation, was to evaluate the seven-year results of the effects of different thinning intensities on the diameter and diameter increments at different heights along the stems of the dominant and co-dominant trees.
Study site – Mjesto istraživanja
The study site was established on a pure NLA plantation situated on bottomland in the Sakarya-Hendek region (40º 45 ‘N, 30º 35’ E, 25 m). In 1984, the trees were cut down in natural stands having NLA as the dominant species scattered among Ulmus laevis, U. minor, Quercus robur, Q. hartwissiana and Acer campestre. The General Directorate of Forestry then planted bare-root NLA seedlings (aged 0+1 year) with 3.7 × 3.7 m initial spacing (730 tree ha-1) (Çiçek et al., 2010). The pre-thinning age of the plantation was 22 years, and it contained 544 trees ha-1 having an average height of 24.0 m, average crown base height of 15.0 m and basal area of 24,418 m2 ha-1.
The deep alluvial soil at the site is poorly drained and heavy textured with a pH ranging from 7.0 to 7.8. The ground water level may well reach up to the soil surface during the February-April period. According to data from the Adapazarı Meteorology Station (40º 46 ‘N, 30º 23’ E, 30 m), located about 15 km southwest of the site, the region receives an annual rainfall of 846 mm. However, water deficiencies can occur in the site throughout the summer to the beginning of autumn. The average annual temperature is 14.3 ºC, with the average temperature during the growth season (April - October) being 18.8 ºC. The average relative humidity is around 72% (Çiçek et al., 2010).
In autumn 2005, three-replicated thinning experiments were set up in the given plantation according to the randomized block design (Çiçek et al., 2010). For the experiment, the plot size was chosen as 63 × 63 m (0.397 ha), and a 15-m wide area on the sides of the plots was accepted as an isolation strip. Sampling quadrats of 33 × 33 m (0.109 ha) in the center of the nine plots were used for measurement and evaluation purposes. All the trees in the sampling quadrats were marked, and their diameter at breast height (d1,30) was measured using calipers with mm precision. The