Effects of forest dieback on wood decay, saproxylic communities, and spruce seedling regeneration on coarse woody debris

Abstract Picea is one of the most dominant conifer genera in the Northern Hemisphere and includes species which require coarse woody debris(CWD) as a seedbedfor regeneration. To understand the future of forest distribution under global climate change, it is important to investigate regeneration mechanisms in Picea forests on the borders of its distribution. In the present study, we evaluated the biotic factors affecting the establishment of Picea jezoensisvar. hondoensis seedlings on CWD in one of its southernmost populations in central Japan, where there is dieback of Picea forest. Amplicon sequencing of the fungal ITS1 region of rDNA obtained from wood samples showed that forest diebackincreased the frequency of brown rot fungi in CWD. The frequency of brown-rotted wood, in which wood holocellulose is decayed, increased with dieback intensity. The domination of brown-rotted wood in dieback forests was negatively associated with bryophytecover which was positively associated with Picea seedling density. Forest diebackitself also had other strong negative effects on bryophytes. Thus, linkages between dead wood and spruce seedlings via bryophyteshad collapsed after the dieback event, which may partly be a reason that the spruce forest shifted to and is staying as open grassland.