Tree ring evidence of rapid development of drunken forest induced by permafrost warming

Climate warming risks permafrost degradation causing "drunken" trees toppling along with thaw, but drunken forest also occurs with hummocky ground on continuous permafrost. However, no evidence has previously clarified whether tree leaning is activated by climate warming or is part of a natural hummock formation process. We test hypothesis that climate warming accelerates development of drunken forest. Results showed that trees' leaning synchronizes with development of soil hummocks and that tree leaning events are recorded in darkened lignin-rich tree rings. Cell morphology in tree rings shows that tree leaning is caused not by loosening of soil foundation in warming summer but by winter frost heaving. Activities of tree leaning and hummock formation increase with increasing air temperature. Reconstruction of drunken forest development using tree ring record suggests that hummock formation has shifted from periodic events until 1960 to continuous mound rising. Recent warming increases hummock development and soil potentials for carbon storage on continuous permafrost, but further warming risks permafrost degradation and hummock collapse with carbon loss.