Formation of xylem tissues and secondary cell walls is diminished by severe and consecutive insect defoliation

Insect defoliation of trees causes unusual changes to wood anatomy and slows radial growth that decreases tree value; however, the characteristics of these anatomical changes in hardwoods remain unclear. The aim of this study was to characterize the anatomy and histochemistry of the wood in trunks of Betula maximowicziana trees after severe insect defoliation.

Secondary xylem tissues were sampled from trunks that had been defoliated by Caligula japonica at Naie and Furano in central Hokkaido during 2006-2012, then cross-dated and examined microscopically and stained histochemically to characterize anatomical and chemical changes in the cells.

White rings with thin-walled wood fibers and greatly reduced annual ring width in the subsequent year were observed in samples from both sites. From these results, the year that the white rings formed was determined, and severe defoliation was confirmed to trigger white ring formation. The characteristics may prove useful to detect the formation year of white rings. Scanning electron microscopy and histochemical analyses of the white rings indicated that the thickness of the S₂ layer in the wall of wood fiber cells decreased, but xylan and lignin were still deposited in the cell walls of wood fibers. However, the walls of the fibers rethickened after the defoliation.

Our results suggest that B. maximowicziana responds to a temporary lack of carbon inputs due to insect defoliation by regulating the thickness of the S₂ layer of the cell wall of wood fibers. For B. maximowicziana, insect defoliation late in the growing season has serious deleterious effects on wood formation and radial growth.