Vertical Decoupling and Community Disassembly: Projecting the Risk of Thermophilization in Quercus mongolica Communities

ABSTRACT Questions Do dominant tree species and understory diagnostic species migrate synchronously under climate change? Can we spatially quantify the risk of a cryptic community collapse where the dominant canopy persists but the diagnostic understory is extirpated? Location Cool‐temperate deciduous forests ( Quercus mongolica communities) in Northeast Asia, specifically the Korean Peninsula and the Japanese Archipelago. Methods To address the “truncated realized niche” problem of the peninsula, we employed an “extended niche” strategy by integrating occurrence data of vicariant species from Japan. We utilized a weighted ensemble model (WEM) combining Random Forests and XGBoost to predict the future distributions (2041–2060) of the dominant canopy species ( Q. mongolica ) and eight diagnostic understory species under the SSP2‐4.5 and SSP5‐8.5 scenarios. We evaluated spatial mismatch by identifying “Thermophilized Communities,” defined as areas where the canopy persists but diagnostic species richness drops below 50% of the current reference baseline. Results Our projections indicated a distinct vertical divergence in habitat shifts between forest layers. The dominant canopy species exhibited high structural resilience to macroclimatic warming, retaining approximately 91.4% of its current suitable habitat even under the extreme SSP5‐8.5 scenario. In contrast, the diagnostic understory guild showed severe habitat contraction and fragmentation. Due to this migrational asynchrony, up to 42.1% of the remaining Q. mongolica stands are projected to transition into “Thermophilized Communities” by the 2050s, leaving the canopy structurally intact but the understory community degraded. Conclusions The structural persistence of long‐lived canopy trees can mask a cryptic reorganization of forest biodiversity that neither single‐species distribution models nor canopy‐focused monitoring are well suited to detect. By combining community‐level diagnostic species selection with species‐level distribution modeling, our framework identifies Thermophilized Communities as an early warning signal of transition toward novel community assemblages. Although the projected magnitudes are provisional and require further validation, our results support shifting conservation from preserving dominant species toward maintaining community integrity, including the protection of high‐elevation refugia and the monitoring of understory dynamics.