Leaf litterfall and decomposition of different above- and belowground parts of birch (Betula ermanii) trees and dwarf bamboo (Sasa kurilensis) shrubs in a young secondary forest in Northern Japan

In many Japanese forests, the forest understory is largely dominated by dwarf bamboo (Sasa) species, which compete with overstory vegetation for soil nutrients. We studied the rate of leaf litterfall, and decomposition and mineralization of carbon (C) and nitrogen (N) from various components (leaf, root, wood, and rhizome) of overstory and understory vegetation in a young Betula ermanii forest from 2002 to 2004. Total litterfall was 377 g m⁻² year⁻¹, of which the overstory vegetation contributed about two thirds. A litter decomposition experiment conducted for 770 days indicated that mass loss of different litter components varied significantly, except for Sasa kurilensis wood and rhizome. Relative decomposition rates were significantly greater in the first growth period (June to October) than the dormant period (November to May) in most cases. Rainfall was the most important abiotic variable, explaining 75–80% of the variability in mass loss rates. Concentrations of ethanol soluble substances and N were significantly positively correlated (r=0.77 to 0.97, P<0.05) with mass loss at an early stage (41 days). The ratios of lignin/N and C/N were found to be negatively correlated with mass loss rates at all stages of litter decomposition. C stock loss was similar to that of mass loss, whereas N stock loss was slower, except for S. kurilensis fine root litter. The evergreen understory species S. kurilensis exhibited greater N use efficiency than B. ermanii, suggesting better competitive ability that might favor the production of a high biomass and invasion under tree species like B. ermanii.