Climate change is driving latitudinal and altitudinal shifts in species distribution worldwide(1,2), leading to novel species assemblages(3,4). Lags between these biotic responses and contemporary climate changes have been reported for plants and animals(5). Theoretically, the magnitude of these lags should be greatest in lowland areas, where the velocity of climate change is expected to be much greater than that in highland areas(6). We compared temperature trends to temperatures reconstructed from plant assemblages (observed in 76,634 surveys) over a 44-year period in France (1965-2008). Here we report that forest plant communities had responded to 0.54 degrees C of the effective increase of 1.07 degrees C in highland areas (500-2,600 m above sea level), while they had responded to only 0.02 degrees C of the 1.11 degrees C warming trend in lowland areas. There was a larger temperature lag (by 3.1 times) between the climate and plant community composition in lowland forests than in highland forests. The explanation of such disparity lies in the following properties of lowland, as compared to highland, forests: the higher proportion of species with greater ability for local persistence as the climate warms(7), the reduced opportunity for short-distance escapes(8,9), and the greater habitat fragmentation. Although mountains are currently considered to be among the ecosystems most threatened by climate change (owing to mountaintop extinction), the current inertia of plant communities in lowland forests should also be noted, as it could lead to lowland biotic attrition(10).