Nitrogen addition to the canopy of Castanopsis chinensis (Sprengel) Hance promoted xylem formation in a subtropical forest in China

Abstract

Key message: Nitrogen addition during 4 years to the canopy of Castanopsis chinensis (Sprengel) Hance promoted xylem formation, mainly by increasing radial growth during the early growth season. Context: Increased atmospheric nitrogen (N) deposition caused by intensive anthropogenic activities resulted in inconsistent effects on the xylem formation of trees, in N-limited boreal and temperate forests. However, the effects of N deposition on xylem formation of trees in subtropical forests are still poorly understood. Aims: The aims of the study were to (i) characterize the dynamics of xylem formation in C. chinensis in a subtropical forest in China and (ii) determine the effect on xylem formation of adding N to the canopy. Methods: From 2013 to 2016, 50 kg N ha−1 year−1 was applied over the canopy of C. chinensis at the end of each month (April to October) in a subtropical forest in southern China. Then, the dynamics of xylem formation were monitored during 2015 and 2016 using a microcore sampling approach. Results: Xylem formation of C. chinensis lasted from February to December and the growth rate peaked during April–May. Adding N to the canopy promoted xylem width, by 21% in 2015 and 20% in 2016, mainly by increasing the growth rate during the early growth season (February to May in 2015 and February to April in 2016). Conclusion: Our study suggests that in a subtropical forest, canopy N addition could significantly promote the xylem ring width of C. chinensis. However, the response of tree growth to N addition showed significant difference between the early and late growth season. Therefore, further studies are needed to more comprehensively address the effect of N deposition on tree growth under global climate change.