Century‐long trends in plant diversity of temperate mountain vegetation are modulated along elevation gradient

Abstract Understanding how multiple global change drivers interact to shape forest plant communities requires a long‐term perspective that extends beyond the last few decades. Mountain forests, with their strong climatic and management gradients, enable the assessment of how global and local drivers jointly shape long‐term plant diversity patterns. Our objective was to detect trends in taxonomic diversity and species distributions along the elevational gradient in response to three main drivers of global change: climate change, nitrogen deposition and historical human management. Therefore, we resurveyed 56 vegetation plots first recorded in the 1920s in temperate montane forests in the Tatra Mountains (Poland), spanning a ~650‐m elevational gradient. We quantified changes in species richness and Shannon diversity, community indicator values, species turnover and species elevational optima. Plant diversity increased over the course of the century. The increases were strongest at lower elevations and weakened upward, with little change or even declines at the highest sites. Community indicator values shifted towards higher soil moisture and nitrogen and lower grazing pressure. Nitrogen‐demanding generalist plants increased in frequency, while several oligotrophic or acid‐tolerant taxa typical of species‐poor spruce stands declined. We found no consistent warming signal (thermophilization) in the understory communities. Community change was more strongly correlated with soil reaction and human management legacy than with direct climatic trends. The patterns match the cessation of intensive pastoralism in the mid‐20th century and the recovery of stands previously converted to spruce plantations at lower elevations. Species‐level responses included frequent downward shifts of elevational optima among species typical for broadleaved forests, consistent with reduced grazing pressure and changing stand structure. Synthesis . The century‐long changes in these montane forest plant communities were primarily governed by former human management and its cessation after establishment of the national park, which outweighed the direct community‐level effects of recent climate warming. This study advances ecological understanding by showing that reliable interpretation of biodiversity trends requires very long data records and explicit accounting for management history alongside contemporary drivers such as climate change and atmospheric deposition.