Abstract:

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 (K_{leaf_max}) and resistance to embolism (P_50leaf) by the timed rehydration method.

Larch had higher K_{leaf_max} and stronger drought tolerance (i.e., lower relative water content at turgor loss point (RWC_tlp) and modulus of elasticity (ε), and more negative P_50leaf) than ash. N addition increased the leaf osmotic potential at turgor loss (π_tlp) and full turgor (π₀), and leaf capacitance (C_{leaf_mass}) for ash but not for larch, indicating that ash is more sensitive to N addition. N addition consistently increased K_{leaf_max} and P_50leaf values for both species. π_tlp and π₀ were positively while C_{leaf_mass} was negatively correlated with leaf density (LD) for ash. K_{leaf_max} was positively but P_50leaf was negatively related with LD for larch. There were negative relationships between K_{leaf_max} and P_50leaf 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.

Key words: nitrogen deposition, pressure–volume traits, leaf hydraulics, Larix gemelinii, Fraxinus mandshurica