Climatic control of high-resolution stem radius changes in a drought-limited southern boreal forest

Key message Stem radius changes measured at high temporal resolution in a drought-limited southern boreal forest were primarily related to soil temperature and partly soil moisture variation, but only weakly to VPD. Abstract Forest productivity at the southern fringe of the boreal forest biome in Inner Asia is strongly drought-limited, as is evident from dendrochronological analyses. Using electronic point dendrometers, we studied the climate response of stem radius changes at high temporal resolution (10-min intervals) in a mixed larch–birch forest in northern Mongolia in a drought year and two subsequent moist years. Larch trees showed stronger stem radius fluctuations than birches, and this difference was more pronounced in dry than in moist years. Stem radius changes were most tightly related to soil temperature variation, while soil moisture was the dominant controlling factor only in birch in the dry year. Correlations with the atmospheric vapor pressure deficit (VPD), and even more so with air temperature, were much weaker. While the linkage between radius change and VPD might primarily reflect diurnal transpiration-driven stem shrinkage and expansion, soil temperature is thought to directly affect cambial cell division and elongation during wood formation. We conclude that the phenology of stemwood increment is strongly controlled by soil temperature even in drought-limited southern boreal forests under continental climate due to the combination of cold and dry climate.