Passive protection is widely assumed to preserve biodiversity and ecological integrity, yet the evidence for long-term vegetation stability in protected temperate forests remains inconclusive. We resurveyed two deciduous forests in SW Poland after 30 years of strict protection to assess temporal changes in their understory vegetation, functional structure, and habitat conditions. Using paired phytosociological relevés (n = 40), collected using the Braun-Blanquet method, we compared baseline (1989–1991) and recent (2022) data with respect to species frequency, Ellenberg indicator values, basic functional traits, and functional diversity. Species composition proved highly stable: only 10% of vascular plant species exhibited significant changes in frequency in particular layers, largely reflecting the vertical redistribution of woody species rather than species turnover. Habitat conditions showed no significant temporal changes. In contrast, the functional structure of the herb layer changed markedly, with significant increases in community-weighted means of seed mass, plant height, and specific leaf area, accompanied by a significant rise in functional diversity. These shifts were partly driven by the increasing abundance of woody species and some opportunistic and invasive species. Our results demonstrate that functional traits may reveal directional ecological changes in passively protected forests even when species composition and habitat indicators remain unchanged, highlighting the importance of trait-based approaches for long-term forest surveys.
Reczyńska et al. (2026) studied this question.