Toward a Beech-Dominated Alternative Stable State in Dinaric Mixed Montane Forests: A Long-Term Study of the Pecka Old-Growth Forest

Abstract

In the last century, a synchronous beech expansion has been observed for many mixed mountain forests in southeastern Europe. This change is associated with the interaction of various disturbances. We analyzed structural changes in the Pecka old-growth forest in Slovenia during the last century, using several inventories of the tree layer, regeneration, and site factors. Throughout the observation period, the density of silver fir in the regeneration layer and in the overstory steadily decreased. In 1893, silver fir accounted for about 60% of the growing stock, whereas in 2013 it accounted for less than 13%. This is likely because of silver fir’s decline in the canopy layer due to air pollution, successive windthrows, and overbrowsing. However, climate change may also have played an important role, as silver fir also declined in southeastern European old-growth forests where air pollution was less pronounced and ungulate densities were low. A gradual decline of silver fir in the overstory resulted in a decrease of overall tree density to 231 trees ha–1, while growing stock remained relatively high at 712 m3 ha–1. Median diffuse light at 1.3 m was 3.7% and regeneration density was 19,954 ha–1. Beech was dominant (94%), followed by silver fir (4%), and sycamore maple (2%). No silver fir seedlings larger than 0.2 m were recorded. Silver fir, sycamore maple, and beech regeneration showed 87, 76, and 45% browsing damage, respectively. Regression models indicated some evidence of niche partitioning between silver fir and beech. However, many processes may be masked by the silver fir’s avoidance strategy. Given current red deer densities, climate change, and existing forest structure, the Pecka old-growth forest will likely reach an alternative stable state dominated by beech in a few decades. This calls for immediate reduction of ungulate populations. Despite the interaction of multiple disturbances, the Pecka old-growth forest has maintained a relatively high overall growing stock, a favorable microclimate, and succession pathway with shade-tolerant beech. This indicates the intrinsic resilience of natural forests. The mechanisms discussed here can be applied to the future governance of old-growth and managed montane mixed forests.

Key drivers affecting the spatial heterogeneity of the regeneration process in old-growth beech forests in southeastern Europe

“…Considering that beech is a shade-tolerant species, the occurrence of seedling banks under a closed canopy is a very common phenomenon (Szwagrzyk et al, 2001;Caquet et al, 2010;Orman et al, 2018). This finding is also supported by the positive correlation between small seedlings and the stand basal area (Diaci et al, 2022), as well as their negative correlation with direct light (Rozenbergar et al, 2007).…”

“…In areas where a middle layer of young trees was formed, only the presence of small, usually one-year-old individual seedlings was recorded. On the other hand, the highest presence of tall beech seedlings was noted within seedling banks formed in canopy openings, which is a typical trait of the regeneration in these forests (Wagner et al, 2010;Čurović et al, 2011;Orman et al, 2018;Diaci et al, 2022). Competitive interactions in the regeneration layer seemed to be quite limited in the old-growth beech forests included in these studies.…”

“…The mortality rate of beech seedlings is lowest in seedling banks, where individuals have a better-developed root system and necessary resources for growth (Topoliantz and Ponge, 2000). Furthermore, the largest presence of tall beech seedlings is within seedling banks in canopy openings (Wagner et al, 2010;Orman et al, 2018;Diaci et al, 2022). Accordingly, in beech old-growth forests, a balance is established between seedling mortality and the occurrence of new seedlings, which ensures the continuity of the regeneration process, allowing beech to use any additional influx of light and suppress other herbaceous and tree species.…”

“…Unlike old-growth forests that are characterized by a passive regime, managed forests are in a more favorable position since they can provide support to potentially endangered species through silvicultural treatments such as planting, protection from browsing, competition regulation, etc. (Diaci et al, 2022).…”

Vital rates and their multidecadal trends in the fir-beech old-growth forest of Badínsky prales

“…The fir regeneration was probably affected primarily by ungulates, whose artificially elevated populations in Badínsky prales have been reported since the beginning of the observation (Korpeľ 1958;Saniga et al 2012). Ungulate populations inhibiting fir recruitment were reported from various parts of Europe (Vrška et al 2009;Diaci et al 2011;Nagel et al 2021), and with other anthropogenic factors they led to considerable changes of the tree species composition in European old-growth forests (Diaci et al 2011;Šumichrast et al 2020;Diaci et al 2022).…”