Regeneration of Cryptomeria japonica seedlings on pine logs in a forest damaged by pine wilt disease: effects of wood decomposer fungi on seedling survival and growth

Evaluating the environmental factors that affect seedling regeneration on deadwood is important for predicting the forest recovery process after dieback events. Pine wilt disease (PWD) has killed many adult trees of Pinus densiflora in recent decades and consequently created a huge accumulation of deadwood in lowland forests in Japan. These deadwoods are now providing nice seedbeds for variety of tree species including Cryptomeria japonica, which is the most important timber species in Japan. In the present study, we tested the effects of various factors on Cryptomeria seedlings growing on decayed Pinus logs in Japan. In total, 126 individuals of Cryptomeria seedlings (height <50 cm) colonizing on 22 pine logs were investigated. During a 5-month research period from May to October 2015, the survival rate of the seedlings was 87.3%, and the maximum height growth was 32.0 cm. Analysis using generalized linear mixed models revealed that delignified white-rot wood in the sapwood of the logs negatively affected seedling survival and growth. These results suggest that type of wood decay, reflecting fungal wood decomposer activity, would strongly affect survival and growth of Cryptomeria seedlings, although logs were important microsites for their establishment after pine wilt disease.