Canopy and understory nitrogen addition increase the xylem tracheid size of dominant broadleaf species in a subtropical forest of China.

Tree xylem anatomy is associated with carbon accumulation and wood quality. Increasing nitrogen (N) deposition can cause a significant effect on xylem anatomy, but related information is limited for subtropical broadleaf tree species. A 3-year field N addition experiment, with different N addition approaches (canopy and understory) and addition rates (0, 25, and 50 kg N ha −1  yr −1 ), was performed beginning in 2013 in a subtropical forest of China. N addition effects on xylem tracheid (wall and lumen) size, vessel, and growth of dominant broadleaf species ( Schima superba Gardn . et Champ . and Castanopsis chinensis ( Sprengel ) Hance ) were investigated. The results showed that The effect of N addition on tracheid size was dependent on the tree species and addition approaches. Canopy N addition did not affect the tracheid size of C . chinensis , while both addition approaches increased the tracheid size of S . superba . The vessel size of both species was not affected by N addition. There was no difference in radial growth or other growth-related variables between the control and N-treated trees. These findings indicated that short-term N addition could significantly affect xylem anatomy, but might not influence tree growth. Meanwhile, understory N addition may pose challenges for mechanistic understanding and forest dynamics projection. [ABSTRACT FROM AUTHOR]