Water potential gradient between sapwood and heartwood as a driving force in water accumulation in wetwood in conifers

Wetwood, water accumulation in the heartwood of tree trunks, is a defect of forest trees and needs to be improved for wood utilization. To understand the mechanism of wetwood formation, differences in water potential between sapwood and heartwood and their seasonal changes were investigated. The water potential of specimens sampled from tree trunks using an increment corer was measured by psychrometry at four-week intervals for 2 years in two coniferous species. A water potential gradient from sapwood to heartwood was identified that could provide a driving force of the radial water movement, and this gradient was found to fluctuate seasonally. The two species studied differed in seasonal change in water potential. The source of the water potential gradient is thought to be heartwood-accumulated potassium and water-soluble heartwood substance. The hypothesis is proposed that water movement from sapwood to heartwood during wetwood formation involves vaporous rather than liquid water and occurs over a limited season.