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

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 ~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. 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.