BARK MOISTURE RELATIONSHIPS IN WESTERN HEMLOCK BRANCHES INFECTED BY DWARF MISTLETOE

Abstract

Investigations of bark moisture in branches of western hemlock, Tsuga heterophylla (Raf.) Sargent, infected by western dwarf mistletoe, Arceuthobium campylopodum Engelm. forma tsugensis Gill, were carried out in good and poor growing sites on the Campus Forest and Research Forest, respectively, of the University of British Columbia. Bark moisture was determined in terms of relative turgidity.Major changes in the moisture content of non-invaded bark occurred in both sites, but the changes were more pronounced in the poor site than in the good site. The relative turgidities of non-invaded bark were less than 80% for 5.9% of the sampling period, June to February inclusive, in the good site, and for 33.3% of the time in the poor site. Bark moistures were more sensitive to changes of precipitation and temperature in the poor site than in the good site, because of the shallow rooting habit of trees and the propensity of the soil in this site to drought.Dwarf mistletoe did not affect bark moisture in the non-invaded portions of branches, but the moisture content of invaded bark was variously affected depending upon the season of year and the age of the infection. When bark moistures were generally low, for example, in April in the good site and June in the poor site, the moisture content of bark that had been invaded for less than 10 years was highest at the centers of infections and lowest at the periphery. At the same time of year but in bark that had been invaded for more than 10 years, the moisture content was highest at the periphery of infections while at the centers it was similarly low to that of the non-invaded bark of the same branches. When bark moistures were generally high, for example, in July in the good site and early August in the poor site, the moisture content of bark in the young infections remained high throughout the entire invaded area. At the same time of year but in the old infections, bark moisture was lowest at the center of infections, and at these times the only portion of an infected branch having a bark moisture content of less than 80% relative turgidity was the center of infection. This may explain the successful establishment of parasitic fungi at these locations.