Beyond management: Proforestation enriches tree-related microhabitat diversity, but forest types determine their composition across Mediterranean and Alpine forests

Proforestation, defined as the long-term protection of existing forests to allow the self-development of old-growth attributes, is increasingly promoted as a nature-based solution for biodiversity conservation. However, its effects on tree-related microhabitats (TreMs), key indicators of forest structural complexity, remain unevenly documented across forest types. Most existing studies have largely examined TreM richness and abundance, while effects on TreM composition across contrasting forest contexts have rarely been addressed. We investigated how time since abandonment (TSA), as a proxy for proforestation duration, influences TreM richness, abundance, and composition across three different forest categories spanning Mediterranean to Alpine environments. Within each category, we compared actively managed forests with stands under proforestation for more than 20 and more than 60 years. TSA generally promoted TreM richness and abundance, albeit to a lesser extent in the Mediterranean forest site. In contrast, responses of individual TreM groups were strongly context-dependent, with some groups increasing and others decreasing along the proforestation gradient. TreM composition differed significantly among abandonment stages in all forest categories, although the magnitude was generally modest, with long-term proforested stands supporting distinct assemblages primarily driven by turnover and associated with increasing deadwood availability. The pattern of these compositional shifts varied among forest categories, reflecting differences in environmental conditions and management history. Our findings demonstrate that proforestation could not only enhance TreM availability but also influence TreM assemblages through long-term structural dynamics. Integrating long-term proforested stands into forest planning can represent an effective conservation strategy to promote biodiversity-related forest structures.