Site aridity has a stronger impact on nocturnal sap flow than species with contrasting stomatal behaviors on the northeastern Tibetan Plateau

Climate change is increasing the frequency and intensity of drought episodes, altering regional aridity levels. Previous studies have focused on the temporary impacts of seasonal drought on nocturnal sap flow, but how long-term site aridity levels and species-specific stomatal behaviors influence nocturnal sap flow remains unclear. Here, we monitored sap flux density and relevant environmental factors for 16 isohydric Qinghai spruce (Picea crassifolia) trees on the north-facing slope and 14 anisohydric Qilian juniper (Juniperus przewalskii) trees on the south-facing slope at four arid to semi-arid sites on the northeastern Tibetan Plateau. We assessed their nocturnal sap flux density (F_n), nocturnal sap flow volume (Q_n), and the proportion of Q_n to daily sap flow volume (Q_n/Q) over the growing season (May through September) in 2019. Our results show that Q_n on non-rainy days accounted for about 83 % of total Q_n on all days in the arid region and about 78 % in the semi-arid region. The average Q_n/Q on the non-rainy days was 8 % for Qilian juniper and 5 % for Qinghai spruce at the arid sites, while it was 15 % and 14 %, respectively, at the semi-arid sites. Differences in F_n and Q_n/Q were more pronounced between aridity levels (2–4 times) than between species (< 1.3 times). In the semi-arid region, atmospheric vapor pressure deficit dominated F_n for both species (r = 0.86 for juniper and 0.78 for spruce), while soil water potential had a more significant impact on F_n in the arid region (r = 0.26 for juniper and 0.57 for spruce). Our findings suggest that site aridity levels have a stronger impact on nocturnal sap flow and its environmental response than species-specific stomatal behaviors in high-elevation dryland ecosystems.