Abstract:

Aims Roots play an important role in the acquisition of soil resources and ecosystem processes. In this study, roots of nine subtropical tree species were studied to: 1) examine root diameter, specific root length and tissue N concentrations of different root branch orders in various tree species; 2) identify seasonal patterns of root N concentration; and 3) test the hypothesis that roots of lower orders have more marked seasonal patterns of N as they belong to a foundational module that is functionally different from higher order roots.
Methods Nine tree species in Dinghushan, Guangdong were studied. Roots were sampled from at least three trees of each species in the same plot at six different times during one year, and individual roots were separated according to branch orders. We measured root diameter, specific root length and tissue N concentrations of each order of all nine species. Variations of root N concentrations among orders in each species were analyzed by one-way ANOVA.
Important findings There were significant differences in root N concentration among different branch orders, with first and second orders having consistently higher N concentrations than higher orders, as expected. However, contrary to our hypothesis, the first two orders, which should belong to the rapid-cycling fraction of tree root systems, did not demonstrate the most marked seasonal changes in N concentration. Thus, the marked seasonal patterns in N concentrations in lower order roots found in temperate trees may not occur in subtropical trees. We suggest this contrast between temperate and subtropical trees may be related to N storage patterns in roots in temperate trees due to distinct phenology and the lack of distinct seasonality in subtropical trees. More study is needed to identify specific mechanisms that regulate N concentrations in roots and other plant tissues in subtropical vs. temperate trees.

Key words: Dinghushan, root order, seasonal variation, tissue N concentration