Unfavourable microsites, competing vegetation and browsing restrict post-disturbance tree regeneration on extreme sites in the Northern Calcareous Alps

Changing natural disturbance regimes threaten forest functions in the Northern Calcareous Alps, with steep, sun-exposed sites on shallow soils at particular risk due to inhibited recovery. Natural tree regeneration after severe disturbances may fail due to extreme microclimate, dense layers of competing understorey vegetation and herbivory. In order to gain insight into regeneration patterns and dynamics, chronosequences of disturbed forest sites were selected along a longitudinal section of the Austrian Northern Calcareous Alps. Regeneration densities of trees, cover of competing vegetation and microsite characteristics were recorded on a total of 19 disturbed sites and in the respective adjacent forest stands. Although high densities of germinants and small seedlings (≤0.1 m) were recorded in the forest stands, recruitment establishment (trees >0.1 m) frequently failed on both disturbed sites and adjacent stands. In fact, half of the disturbed sites were found to be without sufficient regeneration and no significant increase of regeneration density was detected with time since disturbance. Overall, regeneration densities reflect a very critical situation along the disturbance chronosequence. Even if seedling banks are composed of individuals smaller than 0.1 m, seedling mortality is high. General linear mixed models revealed positive effects of convex microsites and thick organic layers on Picea abies (L.) Karst. establishment, while dense litter and grass cover impaired spruce regeneration. Regeneration of other tree species was scarce. The results corroborate the urgent need for establishing seedling banks of larger individuals (>0.1 m) and for reducing ungulate browsing. They also underline the need for comprehensive long-term studies to better understand the dynamic processes, driving resilience of disturbed sites.