Climate‐driven compositional shifts across a temperate–subtropical island ecotone

Climate change is expected to drive substantial shifts in species distributions, yet ecological responses are often delayed under disequilibrium dynamics, limiting the effectiveness of range‐based indicators for detecting ongoing change. Compositional approaches have been proposed as an alternative; however, conventional comparisons based on gradient shape or pointwise differences may fail to detect change when the overall gradient structure is conserved, because small and spatially heterogeneous differences cancel out across the gradient. Here, we address this limitation by presenting a direction‐sensitive analytical framework that integrates signed differences along environmental gradients. Using a comprehensive dataset of island floras across a temperate–subtropical transition in South Korea, we quantify temporal changes in the relative representation of warm‐temperate and subtropical plant (WTSP) assemblages. By emphasizing proportional changes along latitudinal gradients, we detect directional redistribution within communities even in the absence of pronounced changes in gradient shape. Our results reveal patterns consistent with northward redistribution of WTSP composition across survey periods, despite minimal changes in overall gradient form, indicating that community‐level redistribution can occur prior to detectable range shifts or richness changes. These findings highlight the sensitivity of cumulative, direction‐sensitive compositional metrics for detecting early ecological responses to climate warming. More broadly, this study provides a generalizable framework for detecting compositional redistribution through cumulative inference, even when conventional approaches may have limited sensitivity under some conditions, and for identifying early signals of climate‐driven change across gradient‐structured ecological systems. Transitional systems may therefore serve as particularly sensitive contexts for detecting incipient biogeographical change under ongoing warming.