Spatial variation in stand structure of selected old-growth forests in the Western Carpathians and the Dinaric Mountains

The research was conducted in three old−growth forests consisted of silver fir Abies alba Mill., European beech Fagus sylvatica L., and Norway spruce Picea abies Karst. located in the southern part of Poland (Żarnówka and Oszast) and the south−eastern Bosnia and Hercegovina (Perućica). The aim of the study was to compare basic stand characteristics and spatial heterogeneity in terms of variation in stand basal area and vertical structure. In each research area, small sample plots (0.015 ha) were localised in a regular 20×20 m grid covering approximately 10 ha. In each sample plot the diameter at breast height (d1.37 cm) and species of all live trees were recorded. For each plot the basal area of live trees and an index of structural diversification were determined. As a measure of structural diversification, the simple variance in tree height was computed and scaled through comparison with a hypothetical variance of the uniform distribution. The spatial patterns of basal area and structural diversification were tested with paired−plot approach. In addition, simulation techniques were used to model variation in the basal area of live trees dependent on spatial scale. The Carpathian and Dinaric stands considerably differed in basal area (ranging from 36.1 to 65.2 m2/ha) and volume (varying from 522 to 1176 m3/ha), but all of them had diameter distributions proximate to a negative exponential model. The basal area recorded on the 0.015 ha plots had a very similar pattern of variation, which could be generalised as a truncated normal distribution. The distribution of the structural diversification index was different and resembled an uniform (Perućica) or a bimodal distribution with modal values at its extremes (Oszast and Żarnówka). However, in the spatial scales above 1,000 m2 the index distri− bution become similar to a normal (Oszast) or a truncated normal (Perućica and Żarnówka) ones with high mean values, indicating the predominance of complex vertical structures. In general, the spatial variability in basal area and structural diversification of live trees tended to be random. These results suggest that the patch−mosaic assumption being fundamental for the developmental cycle hypothesis is inapplicable to the studied primeval forests. Regardless of differences in geographic location and site conditions, the studied stands show a similar spatial pattern of structural heterogeneity, suggesting a close resemblance of disturbance regimes driving its dynamics