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The Spread of Disease in Patchily-Infected Forest Stands

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If the incidence of disease in a forest stand is patchy, it is postulated that the stand consists of a mosaic of: (i) infected patches within which a proportion of the trees is diseased; and (ii) non-infected gaps within which none is diseased. The hypothesis may be tested by examining the trees in narrow transects and recording their numbers in alternating runs of healthy and diseased trees. From the observed distributions of the run lengths the proportion of diseased within-patch trees may be estimated; also the proportion of the stand area occupied by patches. If the total number of diseased trees increases with time, the changes in these two proportions indicate disease spread. It is possible to determine whether there has been an increase in the proportion of diseased trees within the patches; also whether the patches have expanded. Data from two plantations of Pseudotsuga menziesii infected with Armillaria root rot, used as examples, were examined at the beginning and end of a 3-year interval. For these plantations both proportions had probably increased; and also, as the patches spread they coalesced, so that the numerous small patches originally present had combined to form comparatively few, large patches.

Document Type: Journal Article

Affiliations: Statistical Research Service, Canada Dept. of Forestry, Ottawa

Publication date: March 1, 1965

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