Effects of disturbances in a subalpine forest on its structural indicators and bird diversity

Due to harsh climatic conditions, a limited number of tree species, and a predominantly single-story structure, subalpine forests are characterized by low faunal diversity. While the habitats of species associated with this altitudinal zone are strongly modified by abiotic and biotic factors, the forest management procedures implemented in the aftermath of disturbances typically focus on sanitary hazards and economic losses rather than benefits to biodiversity. The study involved three areas in different stages of forest development resulting from natural disturbances: mature (closed undisturbed stands), break-up (stands with ongoing break-up processes), and growing-up (stands with advanced post-disturbance regeneration). Forest structural indicators for each stage were evaluated on sample plots. A bird census was conducted during breeding and non-breeding seasons to determine species composition, density, and ecological groups (nesting guilds). Actual species diversity, the maximum Shannon-Wiener diversity index, and the evenness index were calculated for each forest development stage. Finally, similarly indices were computed to compare the various stages in terms of bird species. The processes of stand mortally, deadwood accumulation, and regeneration initiation were the driving forces for the development of new habitat conditions with a substantial expansion of potential nesting and foraging sites and shelters. The forest structural indicators were greatly varied. The mature stage revealed the highest density of breeding bird pairs, but the species composition was simplified, with the lowest evenness indicator. The largest number of nesting bird species was found in the break-up stage. During the non-breeding season, the break-up stage was not preferred by birds; the greatest number of species was observed in the growing-up stage, and the highest density in the mature stage. The bird species composition and similarly indices showed substantial differences between the various stages of forest development. Given the continuity of forest development processes and the related changes, it appears that the various development stages should not be viewed as more or less desirable than others from a conservation perspective. The findings show benefits to biodiversity brought about by disturbances in subalpine forests and the importance of each stage. Thus, management practices should be modified to better focus on the protection of the microsites created by disturbances, which are sparse in undisturbed stands.