Stand structure and temporal variability in old-growth beech-dominated forests of the northwestern Carpathians: A 40-years perspective

The study presents the results of long-term observations in seven old-growth beech-dominated forest reserves of the northwestern Carpathians. Investigated forests cover a variety of forest sites within the natural range of common beech (Fagus sylvatica L) in Central Europe. The main goal of the research was exploring the magnitude and the character of the changes of basic stand characteristics over time. Only those old-growth beech forests were included in the study, where the proportion of beech was higher than 90% according to the last measurement. In each investigated reserve, usually three rectangular plots of 0.5 ha were located and measured periodically every 10 years. Within each plot, we recorded all living and dead (standing and lying) trees. Basic stand characteristics (stem density, basal area, growing stock, volume, and proportion of deadwood) were calculated for each old-growth forest and measurement year. The temporal variability was quantified by the relative change (%) between subsequent decades. In addition, the differences of diameter distributions between particular reserves and measurement years were analysed. The long-term average stem density ranged in the investigated old-growth forests from 226 to 401 ha(-1) and the average growing stock from 452 to 744 m(3) ha(-1). The highest temporal variability was recorded in stem density (mean relative change of 9-17%), whilst the changes of basal area and growing stock were significantly lower (less than 10%). The long-term average deadwood volume fluctuated amongst the reserves between 91 and 345 m(3) ha(-1) and the deadwood proportion between 17% and 51%. The mean relative change of the deadwood volume reached 7-42%. The comparison of diameter distributions between the reserves showed a high variability of diameter structures, whereas no identical distributions were observed. In individual reserves, the diameter structure was relatively stable over time and it remained unchanged for at least three decades in most cases. The majority of analysed diameter distributions had a bimodal shape and the best fit was performed by a finite mixture of two Weibull functions. The results showed that old-growth beech forests of northwestern Carpathians can be considered very stable ecosystems and therefore the return of beech to the native sites can promote the overall stability of managed forests in Central Europe. (C) 2011 Elsevier B.V. All rights reserved.