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The behaviour and impacts of Armillaria ostoyae in mature stands and plantations in the Shuswap region of British Columbia Woods, Alex J.


Armillaria ostoyae causes considerable loss in forest productivity in both immature and mature stands within the Interior Cedar Hemlock (ICR) and the Interior Douglas-fir (IDF) zones of the southern interior of British Columbia. Two studies concerning the impacts of this pathogen where conducted near Salmon Arm, B.C.; one was within four plantations age ten to twenty-five years on Larch Hills, and the other was within mature stands on Hunter’s Range and Larch Hills. In the plantation study the relationship between the levels of A. ostoyae infection in mature tree stumps and the regeneration was examined. The evidence of past A. ostoyae infection in stumps remains visible on the inner bark for at least thirty years. This evidence may be used to estimate the extent of the disease in the former stand. The relationship between A. ostoyae in stumps and A. ostoyae-caused mortality in regeneration was significant, though not strongly. Three measures of stump inoculum were compared: the proportion of stumps infected, the number of stumps infected, and the basal area of stumps infected. The number of stumps infected was most closely associated with the proportion of regeneration infected. The relative rates of infection incidence were compared among the eight regeneration species present in the four plantations. A quantitative means of comparing the incidence of infection among species was developed. The number of infected Douglas-fir trees was chosen as a standard measure of disease incidence for each plantation. The incidence of infection in the other seven species were then compared to the Douglas-fir standard. The probability of a young tree becoming infected with A. ostoyae did not increase as the distance from an infected stump was reduced. Brushing in one of the twenty-five year old plantations significantly increased the mortality caused by A. ostoyae. The second study was concerned with the impacts of A. ostoyae in mature stands approximately 120 years old. The relative rates of incidence of A. ostoyae infection were compared between species. The incidence of infection for Western larch (Larix occidentalis) was no less than that for Douglas-fir (Pseudotsuga menziesii). The ranking of tree species susceptibility may depend more on site than on inherent differences among species. An A. ostoyae severity rating system was developed. This system assigned plots a rating based on the proportion of conifer trees infected out of the total number of conifers in the plot. This severity rating was then used in analyses to test the relationship between A. ostoyae severity and a variety of site characteristics, including elevation, logging disturbance, site index, and biogeoclimatic site classification. Of these characteristics, past logging disturbance was mostly closely associated with high levels A. ostoyae severity. The relationship between the biogeoclimatic ecological classification system and A. ostoyae incidence and severity was examined. The ICR zone had significantly more A. ostoyae infection than the IDF zone. More detailed analyses using site units within both the ICH and IDF zones did not indicate any significant relationships. The A. ostoyae severity rating was also compared to timber volumes. There was a significant relationship between A. ostoyae severity and conifer volume in 120 year old stands in the ICH zone. The most severely infected plots had significantly less conifer volume than the less heavily infected areas. There was no significant relationship between A. ostoyae severity and conifer volume in the IDF zone. However, there was a clear trend towards lower conifer volumes with increasing A. ostoyae severity.

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