Much faster warming-tracking rate of the latitudinal forest line in North America than that in Eurasia under the early 21st century's warming

Abstract

Warming drives poleward expansion of forests over high latitudes. However, it remains unresolved whether the poleward forest lines can track the warming rate at continental scales. To answer this question, we compared latitudinal forest line shifts detected by remote sensing under the early 21st century's warming (i.e., from 2000 to 2019) with shifts of temperature isolines derived from a time-invariant temperature-forest line relationship. The results show that the northward forest line shift in North America (NA) can generally keep pace with changes in temperature isolines; while in Eurasia (EA) it greatly trails that of temperature changes. Therefore, the northward shift of the forest line tracking temperature warming over NA is much faster than that over EA. The slower warming rate over NA allows slow temperature acclimation. In contrast, the warming rate is much faster in EA, which makes other environmental factors (e.g., winter temperature, water stress, and active layer thickness, biotic factors, and land use and land cover change) increasingly important in limiting the northward shift of forest lines.