The vulnerability of four main tree species in European forests to seven natural disturbance agents: lessons from Slovenia

Understanding tree species' vulnerability to various disturbance agents is crucial for climate-smart forest management. Using a dataset on salvage cutting in Slovenian forests from 1995 to 2022 (n = 48,124,386 trees), we examined (1) the main trends in forest vulnerability to seven natural disturbance agents (insects, fungi and diseases, windthrow, snow-break, ice-break, fire, and others); (2) the vulnerability of Norway spruce, Scots pine, European beech, and sessile oak to these agents; and (3) the impact of tree diameter on vulnerability. We assessed vulnerability using a vulnerability index (VI), defined as the ratio of salvage cut volume to total growing stock for a given year, agent, and species. During the observed period, overall vulnerability increased, with insects being the predominant agent. Tree vulnerability to insects, windthrow, and diseases and fungi showed a positive linear trend, while vulnerability to snow-break, ice-break, and fire fluctuated stochastically, with extreme climate events being more frequent in the second half of the study period. Norway spruce was found to be the most vulnerable species, being 6.5 times more susceptible to disturbance agents than Scots pine, and 6.8 times more susceptible than European beech and sessile oak. Significant differences in vulnerability across diameter classes were observed for all species pairs except European beech and sessile oak. Overall, tree vulnerability increased with diameter up to 30 cm and then decreased. Insect outbreaks mainly affected Norway spruce trees above 30 cm in dbh, while larger trees (especially over 30 cm in dbh) of all species were more susceptible to windthrow, and thinner trees were more vulnerable to ice-break.