Growth responses to climate warming and their physiological mechanisms differ between mature and young larch trees in a boreal permafrost region

The effects of tree age-related intrinsic water-use efficiency (iWUE) under climate change such as warming and rising atmospheric CO2 concentrations (Ca) on tree growth are still poorly understood. We measured tree-ring delta 13C, delta 18O, tree-ring width, and their relationships with climate variables, calculated iWUE for relatively young (90 +/- 5 yr) and mature (230 +/- 40 yr) Larix dahurica trees in a boreal permafrost region in northeastern China. The period of tree-ring measurements covering 70 years includes a pre-warming period (1951-1984; base period) and a warm period (1985-2018; warm period). The growth rate of both mature and young trees did not change with time over the base period, while the mature trees but not the young ones significantly increased the growth rate with time over the warm period. The iWUE of mature trees consistently increased from 1951 to 2018, whereas that of the young trees increased only with time during the warm period from 1985 to 2018. The tree-ring delta 13C and delta 18O were not correlated for both young and mature trees over both base and warm periods, except for a significantly positive correlation between tree-ring delta 13C and delta 18O for mature trees during the warm period. We found that mature trees were more sensitive to Ca than young trees. Our results suggest that mature trees have a more 'conservative' (low stomatal conductance (gs), constant assimilation rate (A)) water use strategy than young trees (constant gs, high A) in the permafrost regions in a warming world.