Distribution and Anatomical Characteristics of White Rings in Populus Tremuloides

ʻWhite rings ̓have been reported to occur within trembling aspen (Populus tremuloides Michx.) during years of intense early defoliation by the forest tent caterpillar (Malacosoma disstria Hbn.). This study was undertaken to 1) assess the distribution of white rings within trembling aspen stems and 2) to quantify the anatomical differences between white rings and the ʻnormal ̓rings formed in the previous year. Both stem analysis and image analysis were used. Several cross sections from six trees were dated and the presence of white rings on each disc was recorded. The development of white rings from the base up to about 70% of stem height was found to be uniform. Results from the comparison of eleven white and normal rings indicated that white rings were narrower and less dense than normal rings. The anatomical variable with the most discriminating power was average fiber wall thickness. White rings have smaller fiber diameter with little secondary cell wall thickening resulting in a higher proportion of fiber lumen. Possible mechanisms involved in the formation of white rings are discussed. It is speculated that defoliation affects the growth hormone complex inducing earlier cessation of xylem growth and reallocation of reserves to the production of new foliage. The occurrence of white rings in many diffuse-porous species stresses the potential for identification of periods of intense defoliation episodes. Increased interest in the dendroecology of diffuse-porous species may confirm their widespread occurrence.