Forest Structure and Structural Dynamics of Virgin Beech Forests in Slovakia

Abstract

European beech (Fagus sylvatica L.) forests gain a special interest in Central Europe. The highly competitive species would most likely achieve dominance within a wide range of site conditions. Understanding the natural structure and dynamics of beech forests is of great importance for forest ecosystem research from both a conservation and management perspective. Today, there are only few remnants of virgin beech forests and our knowledge on their structure and especially on structural dynamics is poor. This thesis focuses on gap dynamics processes, their effect on stand structure and the analytical description of stand structures in terms of forest development stages in virgin beech forests. Canopy gap formation is suggested being the main driver of structural processes in in natural beech forests. However, information on spatio-temporal change of gap pattern, released understory structures and their reaction to canopy release are scarce. In the virgin beech forest Kyjov a repeated gap inventory was conducted to quantify differences in gap size frequency and fraction over a 10-year interval (2003–2013). Further, the percentage cover of understory trees and regeneration were estimated in 2013. Gap fraction decreased significantly from 13.6% in 2003 to 8.2% in 2013. Within 10 years the mean rate of gap closure was > 80%. Saplings and trees in lower canopy layers formed a heterogeneous understory in large parts of recently formed gaps. The results indicate considerable variation in disturbance intensity in the past decades. The prevalent heterogeneous tree understory guarantees a high resilience to canopy disturbance. The effects of gap formation on the regeneration process are fundamental for understanding forest dynamics. There are only a few studies from temperate virgin forests on gap regeneration and its natural dynamics over time and results are partly contradictory. The regeneration structure was studied in understory gaps of variable size and age, and under closed-canopy conditions in the Kyjov forest. Further, spatial differences in regeneration structure and height growth within a gap and in the gap periphery were analyzed. Sapling density and cumulative biomass were significantly higher in understory gaps than under the closed canopy. Sapling density was positively affected by high direct radiation but low diffuse radiation intensities, resulting in pronounced spatial differences in sapling density across a gap. Sapling shoot length growth was positively affected by higher levels of diffuse radiation and increased sapling size, while direct radiation was not influential. The results suggest that regeneration in small gaps most likely ends in a suppressed heterogeneous understory while in gaps ≥ 100 m2, regeneration may be capable even at low plant densities to fill the gap center, often forming a rapidly growing cohort-like regeneration layer. V It is suggested that in natural temperate forests gap induced regeneration processes imprint on forest structure in the way that trees establish on the approximate gap area and close the gap over time. This assumption lacks an empirical basis. To verify this assumption neighboring trees of similar size were assigned to tree groups on a continuous 12 ha plot in the Kyjov forest and the group size-frequency of such homogeneous patches was analyzed across diameter classes. The number of tree groups (cohorts) in a particular diameter class decreased exponentially as group size increased. Compared to random distributions, the Kyjov forest exhibited a more clumped distribution especially for small trees. The frequency distribution patterns for group sizes are similar to those for gap sizes reported from the same stand and many others. Even though a direct comparison (number of trees vs. m2) is not possible, the results indicate that gap initiated tree groups may persist through the forest cycle. Researchers defined a succession of forest development stages to explain the diversity of structures encountered in beech-dominated virgin forests. However, existing methods based on empirical stand structural data fail to display structural development comprehensively. A novel approach for determining the extension of development stages is proposed. The Development Stage Index (IDS) assigns living and dead trees to the Initial, Optimum and Terminal stage according to their DBH (7–39 cm, 40–69 cm and ≥ 70 cm, respectively) and quantifies their extension by means of stem density and basal area. It was applied to 40 spaced sample plots (500 m2 each) in three beech virgin forests in East Slovakia (Kyjov, Havešová and Stuižca; 120 plots in total). Based on thorough stand structural analyses in the three virgin beech forests, it is demonstrated that IDS is a promising tool for quantifying the proportion of the three stages on different scales, visualizing the complex mixing of stages and analyzing dynamic changes in old-growth forest structure. The results from the Kyjov forest derived by the IDS are in large agreement with the conclusions on how gap dynamics shape forest structure in this forest. This suggests that it might be possible to infer on past forest dynamics by analyzing stand structure with the IDS.