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One of the most widely used methods for assessing the growth of a tree species is based on the dominant height-age relationship and is termed the site index (Savill et al. 1997; Avery and Burkhart 2002; Fontes et al. 2003). The dominant height of a tree species on a given site is insignificantly influenced by the thinning intensity (Hamilton 1981) hence being good indicator of the species potential productivity on that particular site (Cailliez and Alder 1980). Based on Eichhorn’s hypothesis (Eichhorn 1904) the total production from a fully stocked stand, which is the volume of currently standing trees plus anything removed in previous thinnings, is a function of its dominant height (Savill et al. 1997). One of the major needs in forest management planning is to predict forest stand development under various treatment alternatives. In this respect, a thorough knowledge of tree growth on different sites is critical, and an aid to successful forest management and silviculture. Hence, construction of site index curves is a fundamental task.
No literature data has been found to suggest that site index curves have been developed in the region of South East Europe in order to determine the growth of European chestnut (Castanea sativa Mill.) along the site gradients in regions where it grows. Accordingly, the objective of the study is (i) to model the age − dominant height growth relationship and (ii) to elaborate site index curves for chestnut population on the northern slopes of Belasitsa mountain, Southwest Bulgaria.
Materials and methods
Materijali i metode
Study area – Područje istraživanja
Currently, chestnut dominated and codominated stands in Bulgarian part of Belasitsa mountain occupy an area of 1700 ha which is approximately 20 % of the total forested area in the mountain. Chestnut dominated forest alone cover 648 ha, 93 % of them located between 176 000 and 186 000 meridians (Velichkov et al. 2010). Chestnut forests grow at elevation between 350 to 950 m a.s.l., on often steep slopes with predominantly northern exposures. The total growing stock of chestnut is 96 000 m3 out of the total of 1 850 000 m3 (PFEMP 2010). Currently, blight disease caused by Cryphonectria parasitica has spread into most chestnut stands, disease incidence ranging from 18% to 100% of the trees in the stands, and mortality caused by the fungus ranging from 2 % to 80 % (Zlatanov et al. 2011). The current spatial and age structure of chestnut dominated forests in the mountain is considered to be a result of the management regime alteration (land abandonment) since the first half of the last century. The spatial structure of the stands is generally horizontal (simple). Most stands are composed of two age classes. The first class/cohort includes rarely spaced overmatured chestnut trees (density 20–40 trees/ha; age close to 100 and above). The second cohort originated shortly after the land abandonment (40–60s of the last century) under the sparse chestnut canopy at that time. It is composed of chestnut with participation of European beach (Fagus sylvatica L.) and/or sessile oak (Quercus petraea Liebl) in most plots. As a light demanding species, chestnut discontinued its further recruitment in the study area after canopy closure of the stands. Management trials, were initiated again since (80s) 90s of the last century in the form of clearcuts on small areas (less than 1 ha).
Due to the latitudinal position of Belasitsa mountain the climate on its northern slopes is not as strongly influenced by the Mediterranean as the climate of the surrounding territories. The average annual precipitation of the nearest climatic station (climatic station Petrich, 200 m a.s.l.) for the years 1965−2010 is 580 mm, predominantly occurring in the period November − February (240 mm). The driest period is June – September (150 mm). The altitudinal increase in precipitation and decrease in temperature in Belasitsa mountain average 30 mm and 0,7 °C per each 100 m respectively (Galabov et al. 1973). The soil is loamy-sand Eutric Cambisols with depth mostly varying between 40 and 80 cm (PFEMP 2010).
Data collection – Prikupljanje podataka
Data for the construction of the chestnut age − dominant height relationship and site index curves was collected from a systematic network of temporary sample plots in 2010. As a part of the systematic sampling approach, a grid was drawn between the 176 000 and 186 000 meridian lines (coordinate system: UTM 35 North, WGS 1984) across the most recently updated forestry map of Belasitsa mountain, Southwest Bulgaria (PFEM 2010). The interval between grid lines was set to 250 m in both longitudinal and latitudinal direction. A total of 67 grid intersections fell within the boundaries of chestnut dominated stands as depicted in the Petrich Forest Enterprise Management Plan (PFEMP 2010). Grid intersections were positioned on the field by GPS (Trimble Juno SB) navigation. Accordingly, 67 temporary sample plots sized 0.125 ha (40 m in diameter) were installed, plot centres coinciding with the grid intersection points. The plots extended over the altitudinal belt from 400 to 900 m a.s.l.
Two steps procedure was applied in order to select dominant chestnut trees for analyses. At the first step the three tallest chestnut trees per a plot were selected. The total number of dominant trees selected at this stage was 201. At the second step all selected dominant trees characterized by presence of blight disease symptoms were removed from the selection. Accordingly, 97 trees were finally selected from the systematic network of temporary plots. The lack of dominant chestnut trees aged less than 35 in the representative network of sample plots was an important restriction in the initial data set of the study. In order to achieve better coherence of the sample along the age gradient, and following the same approach for selection of dominant trees, 32 dominant chestnut trees aged 10 to 33 were selected in younger stands at various altitudes. Finally, a total of 129 dominant trees were chosen for further analyses. The height of the trees was measured by Vertex IV heightmeter with accuracy of 0.1 m. Increment core samples were