Effects of soil properties and high intensity thinning on male flower production in hinoki cypress (Chamaecyparis obtusa Endlicher) forests

Abstract

Pollen of hinoki cypress (Chamaecyparis obtusa Endlicher) causes allergy and forest management reducing male flower production is required. Effects of soil properties and high intensity thinning on male flower production in hinoki cypress were evaluated. Six forests were selected having different soil properties and site characteristics in Kyoto and Kochi prefecture. Within each forest two study plots were selected; one plot was thinned with strong intensity (>50%) and the other was not thinned or thinned with ordinary intensity (<25%) as the control plot. Male flower production of hinoki cypress was measured over 3 years from 2005 to 2007. Male flower production was not different between the Kyoto and Kochi sites although the climatic condition was very different. Male flower production was higher in 2005 than the other two years in most plots. Male flower production of a mast flowering year in 2005 was higher where soil CN ratio was lower. By contrast, there was no significant relationship between male flower production and soil CN ratio in 2006-7. There was no significant difference in male flower production between control plots and high intensity thinning plots. Male flower production per unit area would not decrease after strong thinning in spite of reduction in tree numbers, because male flowers per individual tree located in or adjacent to high intensity thinning plots increased substantially. The results suggest that high intensity thinning is not a useful management practice reducing male flower production and that male flower production of hinoki cypress is higher where soil nutrient availability is higher in a mast flowering year.