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

Abstract

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. 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. 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. 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. 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). 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.