Intra-annual Dynamics of Xylem Formation in Liquidambar formosana Subjected to Canopy and Understory N Addition

Increasing N deposition caused by intensive anthropogenic activities is expected to affect forest growth. However, the effects of N deposition on trees are still controversial due to the wide variability in results and experimental methods used. We conducted an experiment involving both canopy and understory N addition to investigate the effects of N-addition on intra-annual xylem formation of Chinese sweetgum (Liquidambar formosana) in a warm-temperate forest of Central China. Since 2013, 50 kg N ha(-1) year(-1) (2.5 times the current natural N deposition) was applied monthly from April to December. In 2014 and 2015, the timing and dynamics of xylem formation were monitored weekly during March–December by microcoring the stems of control and treated trees. Similar dynamics of wood formation were observed between canopy and understory N addition. Xylem formation of all the experimental trees started in March and lasted for 119–292 days. Compared to the control, no change was observed in the timing and dynamics of wood formation in N-treated trees. Tree ring-width ranged between 1701 and 4774 μm, with a rate of xylem production of 10.52–26.64 μm day(-1). The radial growth of trees was not modified by the treatments. Our findings suggest that short-term N addition is unable to affect the dynamics of xylem formation in Chinese sweetgum in Central China. The effects of N on tree growth observed in previous studies might be related to the duration of the experiment or the imbalance between the amount of natural deposition and N added during treatments.