INITIAL TWIG INJURY TO HIGH ELEVATION PICEA RUBENS IN EASTERN NORTH AMERICA CAUSED BY INADEQUATE PERIDERM FORMATION

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Apical dieback is the predominant injury symptomatology associated with growth declines of high elevation Picea rubens Sarg, trees in North America. Histological observations of uninjured tissues and of initial injury to tissues were made to understand the mechanism of injury to this species. Observations were made of hundreds of grab samples of apparently uninjured tissues and of uninjured twigs from trees growing on mountains of the Adirondack Mt., NY; Mount Mansfield, VT; Mt. Mitchell, NC; and Clingman's Dome, TN, from March 1985 to April 1986. In the normal growth pattern of red spruce, three buds elongate from each twig terminus during spring. These buds expand into shoot increments during the growing season and three new buds will form at the tip of each of the three elongated increments. The timing of developmental events varied markedly among buds of individual trees and among trees. Bud break occurred between mid‐June to the end of July. Most shoot elongation was completed and periderm formation began near the end of August or in early September. A phellogen, one cell thick, formed and a phellem layer developed from phellogen derivatives during autumn 1985. Many twig samples taken in October and November had produced only one, or at most two, phellem layers external to the phellogen during the relatively short growing season. In some samples, three or more phellem layers were present between November 1985 and March 1986. In some cases a distinct phellem was not developed at all. Usually a phelloderm one cell thick developed in autumn of the first year. Tissue necrosis occurred in twigs during their first overwintering period. Injured twigs with necrotic tissues had only one or two continuous or discontinuous phellem layers. In samples that exhibited initial injury, necrotic tissues consisted mostly of cortical cells and phloem subjacent to this meager periderm. Frequently, necrotic tissues developed initially near the bases of needles and at branch “nodes” (transition zone tissues with older twigs). In contrast, twigs of healthy appearance had two or more continuous phellem layers external to the phellogen.