Ecological niche dynamics of treeline birches (Betula spp.) under pleistocene - holocene - future climate change in the Caucasus

Treeline birches (Betula spp.) are key components of subalpine forests in the Caucasus, providing important ecosystem services but facing growing threats from climate change. We applied ecological niche modeling (Maximum Entropy – MaxEnt) to assess birch habitat dynamics across multiple temporal scales: the Last Glacial Maximum (LGM), the Mid-Holocene (MH), the present and future projections for 2100 under three Shared Socioeconomic Pathways (SSP126, SSP370, SSP585), using MIROC and CMCC-ESM2 climate models. Model performance was high (Area Under the Curve (AUC) > 0.97). Thermal variables, particularly the mean temperature of the warmest (Bio10) and coldest (Bio11) quarters, were the dominant predictors, complemented by seasonal precipitation (Bio18, Bio19). Paleoclimatic reconstructions showed confinement to refugial areas in Colchis and Hyrcania during the LGM, with expansion during the wetter Mid-Holocene. Present conditions represent the ecological optimum, supporting continuous treeline belts along the Greater Caucasus and fragmented patches in the Lesser Caucasus. Future projections revealed strong scenario-dependent shrinkages: SSP126 maintained relative stability, whereas SSP370 and SSP585 indicated losses exceeding 80-95%, leaving only small, isolated refugia at high elevations. These results demonstrate a refugia – recolonization – optimum – shrinkage trajectory and identify the Caucasus ecoregion as both a long-term refugial zone and a climate-sensitive hotspot. Conservation strategies should prioritize refugial strongholds, ecological connectivity and climate-adaptive management to safeguard birch treelines under future climate warming.