Factors affecting inter-tree root contact and the transmission of Phellinus weirii in young-growth Douglas-fir Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ht24wn299

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  • Groups of second-growth Douglas-fir (Mirb.) Franco were excavated in plots of two to six trees on two sites in the vicinity of Cowichan Lake, Vancouver Is., British Columbia. Eleven excavation plots provided 36 trees which could be studied in pairs (a total of 50 pairs) in order to ascertain the relation of root attributes and tree, stand and site characteristics to the extent of root contact occurring between pairs of trees. Regression analysis was used to relate the combined root attributes of two trees jointly occupying the same soil space to the number of root contacts. Three contact types were recognized: tenuous (Type I), substantial (Type and apparent grafts (Type IV). Type I contacts were found to be positively related to root length and negatively related to root volume. Type II-III contacts were positively related to the number of roots. Type IV contacts were positively related to both root number and cross-sectional area. Discriminant analysis was employed to determine the relation of various tree, stand and site factors to the probability of root contact. Three contact groups were defined: 1) no contact, tree-pairs for which no contacts occurred; 2) low contact incidence,tree-pairs with only Type I contacts and 3) high contact incidence, tree -pairs having both Type I and higher order root contacts. Inter-tree slope distance, DBH, effective rooting depth, soil gravel content and percent slope were all found to be significant. The results of the discriminant analysis were incorporated into a model for spread of laminated root rot. The goal of the spread model is to characterize the behavior of the system in order to evaluate the influence of silvicultural options and environmental constraints on the ability of laminated root rot to spread from tree to tree. The model consists of four submodels: tree growth and survival, root contact, infection and spread. Twenty-seven simulation runs were made to evaluate the effects of tree spacing, stand age and soil depth on the cumulative distribution of earliest time to bole infection under conditions of constant soil gravel content and percent slope. Three principle areas are identified in the model where better or more complete information is necessary before the model can be considered adequately specified. These are: time to fungal growth cessation following bole infection, the shape and location of the potential contact zone and the distribution of contacts within this zone and certain features of the spread behavior of Phellinus weirii. Numerical solutions for the relation between the tree, stand and site factors and the cumulative distribution of time to earliest bole infection were not attempted, consequently. A qualitative analysis of the laminated root rot spread system's behavior was undertaken, however. The salient features of this analysis are: 1) the cumulative distribution function of earliest time to bole infection is a composite function consisting of two functions, one for each of two recognized phases in the distribution function. 2) the phases of the composite function can be physically interpreted as two relatively distinct infection waves; the first wave corresponds to primary infection from a source while the second represents secondary infection from adjacent trees. 3) the functional form of the phase I relation is given by the single process law while the form of phase II is given by a simple exponential expression and 4) all of the initial conditions studied influence the shapes, slopes and asymptotes of the two phases.
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