Climate-Driven Range Shifts in Alpine Plant Communities

Abstract

Alpine plant communities are expected to track rising temperatures through upward and, where topography allows,
poleward shifts in distribution, but the pace and consistency of this response across species and functional groups
remains incompletely resolved. We resurveyed 18 GLORIA-protocol summit sites spanning the Austrian and Swiss
Alps (2,650-3,410 m), comparing standardised vegetation surveys conducted in 1998 against a 2023 resurvey, and
complemented this with dendrochronological and aerial-imagery-based establishment surveys along 12 treeline
ecotone transects spanning 1975-2023. Regional summit weather stations recorded a mean annual warming of
1.6C over the 25-year survey interval. Mean species richness per summit plot increased by 34% (range 8-71%),
and the abundance-weighted optimum elevation of individual species shifted upward by a mean 28.4 m per decade
(95% CI 21.2-35.6), estimated by linear mixed-effects models with summit identity as a random effect. Shift rates
differed markedly by functional group: thermophilous and generalist species advanced at a mean 41.2 m per
decade, more than three times faster than cold-adapted alpine and nival specialists (12.6 m per decade), producing
measurable biotic homogenization of summit floras. Treeline advance averaged 0.82 m per year across transects
(range 0.31-1.94), driven predominantly by Pinus cembra and Larix decidua seedling establishment, but observed
advance lagged consistently behind the elevation implied by contemporary climatic suitability models, indicating a
persistent colonization credit. These findings demonstrate that alpine range shifts are proceeding rapidly but
unevenly, with slow-colonizing treeline species and narrow-niche cryophilous specialists falling furthest behind their
climatic optima, raising extinction risk for high-summit endemics as thermophilous species continue to encroach
from below.