Estimating decay rates of dead wood by changes in wood density in coniferous plantations in Japan

Abstract

Decay rates of dead wood after non-commercial thinning, i.e., fallen logs, stumps, and snags, in plantation forests were estimated from changes in wood density over time. Decay rates were compared using the decay constant k of the exponential function (D = a exp (-kt), where D : wood density, a : a constant, t : time, k : decay constant). The plots examined were in 15 prefectures from the north to the south of Japan, and contained five dominant planted species : Cryptomeria japonica, Chamaecyparis obtusa, Larix kaempferi, Picea glehnii, and Abies sachalinensis. Decay rates of fallen logs and stumps were estimated separately for two diameter classes : 5-15cm and 15-40cm. The decay rates of P. glehnii, A. sachalinensis, and L. kaempferi, which are planted in relatively cool regions, were slower than those of C. japonica and C. obtusa. No significant difference in decay rates was found between C. japonica and C. obtusa planted in a similar climate range. The decay rates of snags were very slow, followed by stumps and fallen logs. The decay rates of fallen logs with a diameter of 5-15cm had a significant positive relationship with cumulative temperature and actual evapotranspiration, suggesting that smaller-diameter fallen logs decay faster in warmer, humid regions. However, no clear relationships between decay rates and climate conditions were found in fallen logs of 15-40cm diameter or in stumps, but very slow decay rates were found in cool regions.