Divergent diel increment patterns and drivers among co-occurring conifer species in semi-arid mountain forest

Although interannual variation in tree radial growth and its climatic drivers has been widely studied in semi-arid forests, diel stem dynamics and their environmental controls remain poorly understood. During the 2024 growing season, we used high-resolution dendrometers to monitor stem-diameter variations at 30-min intervals in two individuals of each of three co-occurring conifers— Picea crassifolia , Pinus tabuliformis , and Juniperus rigida —in the Helan Mountains. The zero-growth model was used to partition stem variations into potential radial growth (GRO) and tree water deficit (TWD), and their relationships with environmental variables were examined across 1-, 3-, and 7-day windows. Diel radial increment patterns differed among species and individuals. Most P. crassifolia and P. tabuliformis individuals reached peak GRO in the early morning, with P. tabuliformis showing the strongest nocturnal allocation, whereas J. rigida showed the lowest nocturnal allocation and greater extension of increment into the morning. After normalization by individual diameter at breast height, J. rigida exhibited the highest size-normalized TWD, whereas P. tabuliformis exhibited the lowest. TWD–VPD hysteresis was greatest in P. crassifolia , intermediate in J. rigida , and smallest in P. tabuliformis , indicating species differences in the lag between atmospheric demand and stem water deficit. Across all temporal windows, precipitation was positively associated with GRO, whereas vapor pressure deficit was negatively associated with GRO and was the strongest positive predictor of size-normalized TWD. These findings reveal contrasting diel hydraulic patterns among co-occurring conifers, suggesting greater dehydration tolerance in J. rigida and more conservative water-use regulation in P. tabuliformis .