LAUSR

Atmospheric nitrogen (N) deposition influences tree hydraulic architecture and thus the growth and survival; but the responses of leaf hydraulic traits remain uncertain, and may vary with species or plant functional types. We used the 16-year N addition experiment (10 g N m -2 year -1) on Fraxinus mandshurica (ash, broadleaf angiosperm) and Larix gmelinii (larch, conifer gymnosperm) plantations in northeastern China and examined the effect of N addition on their leaf hydraulics. We measured the leaf pressure-volume traits by the bench drying method and quantified the maximum leaf hydraulic conductance (Kleaf_max) and resistance to embolism (P50leaf) by the timed rehydration method. Larch had higher Kleaf_max and stronger drought tolerance (i.e., lower relative water content at turgor loss point (RWCtlp) and modulus of elasticity (ε), and more negative P50leaf) than ash. N addition increased the leaf osmotic potential at turgor loss (πtlp) and full turgor (π0), and leaf capacitance (Cleaf_mass) for ash but not for larch, indicating that ash is more sensitive to N addition. N addition consistently increased Kleaf_max and P50leaf values for both species. πtlp and π0 were positively while Cleaf_mass was negatively correlated with leaf density (LD) for ash. Kleaf_max was positively but P50leaf was negatively related with LD for larch. There were negative relationships between Kleaf_max and P50leaf for both species. Overall, our findings suggest that long-term N addition decreases the leaf drought tolerance for these two important tree species, which improve the understanding of the tree hydraulic performance under N deposition.