Negligible Response of Transpiration to Late-Summer Nitrogen Fertilization in Japanese Oak (Quercus crispula)

Increased atmospheric nitrogen (N) deposition, caused by anthropogenic activities, has various effects on forest ecosystems. Some reports have investigated the responses in tree transpiration to N addition, but few studies have measured the short-term response of mature tree transpiration to N fertilization. This study aimed to clarify the short-term transpiration response in 27-year-old deciduous hardwood trees to an increase in N availability. We established two plot types (control and N-fertilized plots) in Quercus crispula plantation stands in Hokkaido, Northern Japan. We measured sap flow density (SFD; cm3 m−2 s−1) using a thermal dissipation method for three months during the growing season. In the N-fertilized plot, we added 50 kg N ha−1 yr−1 of ammonium nitrate (NH4NO3) to the forest floor in the middle of the measurement periods. For daily mean SFD, we did not find a significant difference between the control and the N-fertilized plots. Leaf N contents did not differ between treatments, implying a negligible difference in physiological responses and transpiration rates. The slight difference between treatments could be because the trees had already foliated before applying the N fertilizer to our deciduous hardwood trees. The present results indicate that the potential increase in N deposition during the growing season does not immediately alter tree transpiration.