Stable water isotopes reveal the onset of bud dormancy in temperate trees, whereas water content is a better proxy for dormancy release

Earlier spring growth onset in temperate forests is a visible effect of global warming and affects global water and carbon cycling. Therefore, it is crucial to accurately predict the shift in spring phenology under projected future warming. However, current phenological models lack physiological information and are rarely experimentally validated. Therefore, twig cuttings of five deciduous tree species were sampled at two climatically different sites throughout the winter of 2019/2020. Twig budburst success, thermal time to budburst, bud water content, and short-term ²H-labelled water uptake into buds were quantified to link bud dormancy status with vascular water transport efficacy. We found strong ²H-labelled water uptake into buds during leaf senescence, followed by a sharp decrease that we attributed to the initiation of dormancy. However, we did not find increasing ²H-labelled water uptake into buds with progression of winter, whereas all species showed a linear relationship between bud water content and dormancy status. Our results show that short term ²H-labelled water uptake appears to be a poor tracer of dormancy release, but could be a promising method to track dormancy induction of deciduous trees, whereas bud water content seems to be an inexpensive and more reliable indicator of dormancy release.