Multiple dimensions of forest resilience to compound disturbances in a mixed sub-montane forest landscape

Compound disturbances are anticipated to increase under global change, potentially resulting in unpredictable forest recovery dynamics, especially in managed forests where salvage logging is often routine, yet studies quantifying forest resilience to compound disturbances are still scarce. We investigated resilience to an ice storm event, followed by a spruce bark beetle outbreak and subsequent salvage logging in a temperate sub-montane forest landscape in Slovenia. The study landscape was characterized by a gradient in pre-disturbance proportion of spruce, an important species for the forest economy, ranging from mixed fir-beech-spruce forest to spruce monocultures. Given that the bark beetle only affected spruce, this gradient in spruce proportion provided an opportunity to investigate forest resilience across a severity gradient ranging from undamaged areas to complete canopy removal over patches of several hectares. We assessed multiple dimensions of resilience across this disturbance severity gradient, namely i) post-disturbance forest structural and species diversity, ii) radial growth of surviving trees, and iii) post-disturbance regeneration, including measurements made in deer exclosures. The results highlight the low resilience of spruce-dominated stands in terms of low structural and species diversity of the recovering forests, once spruce is removed due to disturbance. However, measures of post-disturbance structural diversity were higher in stands were spruce made up < 50 % of the tree composition. This result may help forest managers to plan how much spruce to maintain in mixture with species less vulnerable to climate change. We also showed that surviving canopy trees with damaged crowns can sustain high radial growth resilience, which calls into question the efficacy of immediate post-disturbance salvaging of damaged trees. Finally, we quantified the detrimental impact of ungulate browsing on the recruitment of post-disturbance regeneration, particularly for highly palatable species, which showed significantly lower aggregate height, a metric combining density and tree height, outside of deer exclosures. We stress the importance of actively controlling ungulate populations for successful post-disturbance recruitment of silviculturally important palatable species, such as silver fir and sycamore maple.