The influence of a five-year nitrogen fertilization treatment on hydraulic architecture of Pinus sylvestris var. mongolica in a water-limited plantation of NE China

The increase in nitrogen (N) deposition is expected to have strong influences on forest ecosystems through its direct and indirect effects on tree performance. One way that N deposition can greatly affect tree performance is by altering plant hydraulic architecture and water relations especially in water-limited habitats; however, the magnitude and direction of response are shown to be species and habitat specific and the underlying mechanism requires further investigations. In the present study, we looked at the potential impact of N deposition on stem xylem hydraulics and leaf level water relations of Pinus sylvestris var. mongolica (Mongolian pine), using a five-year N fertilization experiment at a typical plantation site in NE China. Our results showed that N addition significantly enhanced tree growth and altered tree hydraulic architecture in the upper canopy. Hydraulic architecture responses to N addition were mainly attributable to changes in allometric relationship between leaf and stem rather than through its impact on xylem conductive efficiency as indicated by the non-significant differences in stem hydraulic conductivity and decreased leaf area to sapwood area ratio in N-fertilized trees. The N-fertilized Mongolian pine trees exhibited characteristics simulating plants from more water-limited habitats, such as lower leaf osmotic potentials at full turgor and turgor loss, which indicates a less favorable water status related to higher nitrogen availability in a water limited habitat. Moreover, a tendency of reduced leaf lifespan may make the N-fertilized trees more susceptible to carbon imbalance. Our results suggest that, despite tree growth was enhanced in the present experiment, N deposition may negatively affect Mongolian pine plantations through its negative impacts on plant water relations, which may likely deteriorate in more water-limited conditions. Management measures aimed at reducing water use, such as tree thinning, may mitigate the negative influence of N deposition on survival of Mongolian pine trees in water-limited plantations of the vast areas of Northern China.