Intra-annual dynamics of xylem growth in Pinus massoniana submitted to an experimental nitrogen addition in Central China.

In recent decades, anthropogenic activities have increased nitrogen (N) deposition in terrestrial ecosystems. This higher availability of N is expected to impact plant growth. However, the effects of N deposition on tree growth remain inconclusive due to the wide variability of experimental methods used. This study aimed to test the effect of short-term N addition on the intra-annual wood formation of Chinese red pine (Pinus massoniana Lamb.) in a warm-temperate forest of Central China. From 2013, solution containing 25 kg N ha-1 year-1 was applied monthly to the understory of experimental plots from April to December to double the current natural N deposition. Each week from March to December in 2014 and 2015, cambial activity and the timings and dynamics of xylem formation were monitored by collecting microcores from stems. Xylem formation lasted from March to November, producing an average of 19 and 33 cells for all studied trees in 2014 and 2015, respectively. No difference in xylem cell production was observed between control and N-treated trees. Moreover, N-treated trees had similar timings, rates and durations of xylem formation as control trees. These findings indicated that short-term N addition was unable to affect timings and dynamics of xylem formation in Chinese red pine of warm-temperate forest.