The formation of xylem tissues and secondary cell walls is diminished by severe and consecutive insect defoliation.

Abstract

Insect defoliation causes unusual wood structures in forest trees. However, the characteristics of the unusual wood structure of hardwoods remain unclear. The aim of this study was to clarify the characteristics of the wood structure in trunks produced under severe insect defoliation in Betula maximowicziana trees. Samples from the trunks were taken from two sites (Naie and Furano) in central Hokkaido where insect defoliation by Caligula japonica occurred during 2006-2012. The cross-dating and microscopic observations with histochemical staining were done. White rings with thin-walled wood fibers and drastic reduction in annual ring width in the subsequent year were observed in both samples. From these results, the formation year of white rings were determined and severe defoliation was confirmed to trigger white ring formation. The characteristics may prove useful for the detection of the formation year of white rings. Then, we clarified the cell morphology of wood fibers in the white rings. Scanning electron microscopy and histochemical analyses indicated that the thickness of the S2 layer of wood fibers decreased, but xylan and lignin were still deposited in the cell walls of wood fibers in a white ring. However, the cell-wall thickness of wood fibers recovered after defoliation period. Our results suggest that B. maximowicziana would respond to a temporary lack of carbon due to insect defoliation by regulating the thickness of the wood-fiber S2 layer. For B. maximowicziana, insect defoliation during the late growing season has serious deleterious effects on wood formation and radial growth. This article is protected by copyright. All rights reserved.