Inter-annual variations in leaf litter production in a conifer plantation: effects of thinning and climate

摘要

Inter-annual variations in leaf litter production were evaluated for 9 years, from 1991 to 1999, in a mixed coniferous plantation of hinoki cypress (Chamaecyparis obtusa Endlicher) and Japanese cedar (Cryptomeria japonica D. Don) in Kochi, southernJapan. The aim of the present study was to evaluate the effect of thinning and climate on the production and phenology of leaf fall. Thinning was conducted in March 1993 and the percent of tree removal was 30 percent by tree basal area. Leaf litter production in hinoki cypress before thinning (1991-92) was 330 g m~(-2) yr~(-1), but had decreased to 197-250 g m~(-2) yr~(-1) three years after thinning. Decrease in litter production was equivalent to the percent of the tree basal area that had been removed. After 4 years, leaf litter production had recovered to the level before thinning. The time of 10 percent annual leaf fall from 1 July obtained by the Dixon's model was within a 33-day range from 23 September to 26 October during the study period. The time of 10 percent annual leaf fall was positively correlated with temperature during the growth period (March-October) (r_s=0.75, P<0.05), suggesting that trees retain their leaves longer and have higher productivity in warmer years. These results indicated that the changes in environmental conditions after thinning were not stressful for the remaining trees and the recovery of the crown was rapid in the study site.