Seasonal and daily variations in stem water relations between co-occurring Larix principis-rupprechtii and Picea meyeri at different elevations

Internal water storage in the stem provides additional water for transpiration and photosynthesis and thus ensures tree hydraulic safety and growth under stressed conditions. Comprehensive understandings in elevation- and species-specific patterns of stem water relations and the underlying mechanism could shed light on the physiological foundation of forest conservation and management in the montane area. Here, stem radius variations of co-occurring Larix principis-rupprechtii and Picea meyeri were monitored at three sites of different elevations (2200 m, 2400 m, and 2600 m a.s.L.) in 2018 and 2019 using point dendrometers. The seasonal cycle was divided into four periods: spring rehydration, summer growth, autumn contraction, and winter dormancy. Tree water deficit-induced stem shrinkage (TWD) during the summer growth and daily stem cycles in each period were extracted and compared between species and among elevations, and their responses to environmental factors were determined. The results showed an elevational trend for averaged TWD in L. principis-rupprechtii instead of P. meyeri. TWD in both tree species had a closer relationship with vapor pressure deficit (VPD), while the TWD showed piecewise changes with species-specific differences as the VPD increased. The daily amplitude of L. principis-rupprechtii was smaller than that of P. meyeri in periods of winter dormancy and spring rehydration, however, the comparison was opposite during summer growth and autumn contraction. Furthermore, the daily amplitude of L. principis-rupprechtii and P. meyeri had different responses to the air and soil hydrothermal conditions in different periods over the year. Our study highlights interspecific water-use behaviors between larch and spruce, indicating their contrasting responses to a global increase in drought stress, especially in low elevations.