The aim of this bachelor 's thesis was to investigate the influence of extreme site conditions on the diameter growth of tree species in the Krokar old-growth forest. Diameter growth was compared between two permanent research plots with different site conditions. Plot 1 is located at the edge of the old-growth forest on a rockier and more extreme site, whereas Plot 2 is located in the interior of the old-growth forest in more favorable site conditions. The analysis was based on diameter at breast height (DBH) measurements from 1985, 2012 and 2022. Data for 2017 was excluded due to measurement errors when using diameter tape which were in some cases greater that the actual diameter increment. In total we analysed diameter growth of 397 trees, including 51 silver firs, 337 European beeches and 9 sycamore maples. For each tree we calculated the mean annual periodic diameter increment. Differences between plots, tree species and time periods were analyzed using descriptive statistics and analysis od covariance (ANCOVA), with the initial DBH included as a covariate. Marginal mean annual diamater increments were higher in the period 2012–2022 than in 1985–2012 for all studied tree species. In the first period European beech had the highest diameter increment, while in the second period silver fir had the highest diameter increment. Adjusted diameter increments of European beech were significantly higher on the interior plot in both periods. For silver fir, a significant difference between the two plots was found only in the 1985–2012 period. Sycamore maple was excluded from the analysis of covariance because of the small sample size. The results show that site conditions affect tree diameter growth and that the response to site conditions differs among tree species.
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