Transitional responses of tree growth to climate warming at the southernmost margin of high latitudinal permafrost distribution.

The marked increase in temperature warming and permafrost degradation has raised apprehensions about the fate of forests of boreal forests in permafrost regions. However, the impact of climate on tree growth is not limited to direct effects but also involves complex interactions with permafrost. The degradation of permafrost poses a threat to forest growth that has received insufficient attention thus far, after analyzing the impact of permafrost degradation and climate on Dahurican larch (Larix gmelinii) growth from six forest sites with two maximum active layer thickness (ALT) classifications (more and less than tree root length) across the southern margin of the permafrost region. We found that accompanying the continued degradation of permafrost, tree growth was inhibited (slope = -0.67, p < 0.05) by the degradation of permafrost and the growth-climate relationship was shifted from positive to negative at maximum ALT less than tree root length sites. However, the growth rate of trees significantly accelerated (slope = 5.46, p < 0.05) at maximum ALT more than tree root length sites. Path analysis indicated that tree growth did not benefit from temperature warming and more stress could be caused by waterlogging due to permafrost degradation at maximum ALT less than tree root length sites, however, enhanced tree growth primarily by reducing the physical spatial constraints and root layer additional water source with permafrost degradation at maximum ALT more than tree root length sites. It also implies that the matchiness between tree root and maximum active layer depth is critical to the effect of permafrost degradation on tree growth. The transitional response to climate warming and the opposite trend of tree growth at two ALT classification sites suggest that future tree growth responds to the different stages of permafrost degradation differently. Our study provides a new insight on permafrost degradation impact on tree growth.