Climate change increased the intrinsic water use efficiency of Larix gmelinii in permafrost degradation areas, but did not promote its growth

The degradation of permafrost caused by climate warming accelerates the infiltration and evaporation of water, destroys the hydrological system, and increases the drought stress of boreal forests. However, how climate warming and permafrost degradation affect intrinsic Water Use Efficiency (iWUE) and tree growth is not fully understood. Using tree-ring width and stable carbon isotope composition (δ13C) data, we analyzed the growth and iWUE of Larix gmelinii on slopes and gullies in three different permafrost degradation areas in Daxing'an Mountains, China. The results showed that the iWUE of L. gmelinii increased significantly in all areas from 1900 to 2015. Radial growth and the ratio of intercellular (Ci) and ambient (Ca) CO2 concentration was significantly negatively correlated with temperature and positively correlated with Palmer Drought Severity Index (PDSI) in severe degradation permafrost areas. Meanwhile, radial growth (basal area increment, BAI) has decreased significantly in recent 40 years, and the decline rate of radial growth in slope topographies was higher than that in gully topographies. Growing season temperature was generally positively correlated with iWUE, but negatively correlated with the radial growth of L. gmelinii. The increase of iWUE was negatively correlated with the radial growth of L. gmelinii. The response of BAI, iWUE, and Ci/Ca to temperature rise and water change was weak in the mild degradation area, and radial growth still increased. In severe and moderate degradation areas, tree physiology (iWUE and Ci/Ca) was driven by temperature and PDSI, indicating that L. gmelinii adopted a conservative water-saving stomatal strategy, and increasing iWUE did not promote tree growth. Although the water use efficiency of L. gmelinii in the southern edge of permafrost area increased with the increase in temperature, it may face more serious drought stress and growth decline in the future.