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Stand structural characteristics and development patterns in old-growth interior cedar hemlock forests in southeastern British Columbia MacKillop, Debra J.

Abstract

Old-growth forests have been defined and described in many ways including stand age, stand structure, and stand population dynamics. This thesis uses these key approaches to describe the characteristics and stand development patterns of old-growth forests in the moist warm Interior Cedar Hemlock biogeoclimatic subzone of southeastern British Columbia. In total, 16 sites ranging from an estimated 130 to 630 years old were sampled in the Arrow and Kootenay Lake Forest Districts. Stand age plays an important role in defining and describing old growth. However, trees with internal decay were common in the study area, and in many cases, estimates were the only source of data. Age estimates using linear extrapolation and regression analysis were tested then applied to tree cores with internal decay. Using regression analysis, age-on-diameter relationships were assessed on a species-specific and a site-specific basis. Site-specific regression models provided the most accurate means of estimating missing rings. However, the data required for their development can be difficult and tedious to obtain, and although more accurate than other methods, can still be wrong by up to 100 years. Errors associated with linear extrapolation were tested using the mean growth rates from intact portions of tree cores to extrapolate ages to missing portions. However, if 40% or less of a tree's radius was used, estimates were unreliable and errors were as large as 172 years for Tsuga heterophylla ((Raf.) Sarg. (western hemlock) and 513 years for Pseudotsuga menziesii ((Franco Mirb) Douglas-fir). Where more than 40% of the tree radius was used for estimating missing ring counts, accuracy was improved, but decreased as tree size and age increased. Based on the standard deviation of the mean difference between estimates and counts, estimates for Tsuga cores over 50 cm dbh and with more than 40% of the radius included were generally accurate to within 55 years. Large Thuja plicata ((Donn ex D. Don) western redcedar) and Pseudotsuga I Larix occidentalis ((Nutt.) western larch) were accurate to within 37 and 83 years, respectively. With actual tree age estimates evaluated, old-growth forests were defined using stand ages from forest cover inventory planning data and measured ages from tree cores extracted in the field, from age class frequency distributions and tree size class frequency distributions, and by using principal components analysis (PCA) to analyze multiple stand structural attributes. The various methods of defining old growth led to different classifications of study stands. The PCA analysis was based on measures of live and dead trees as well as understory vegetation and coarse woody debris, and provided the most reliable definition of old growth. The PCA analysis resulted in an 'index of old-growthness' in which individual attributes can be measured in the field then compared to calculated thresholds to determine a 'score' of 'old-growthness' for older forests. To assess stand development patterns in old-growth stands, dendroecological techniques were used at six of the old-growth sites. Relative increases in growth were interpreted to reflect releases, while relative declining growth reflected suppression. Small-scale disturbances that create canopy gaps were the dominant disturbance in the old-growth forests studied here and both Tsuga and Thuja, the dominant trees in the study area, were found to rely on gaps for canopy ascension. Over 80% of the Thuja currently in the lower canopy had experienced one or more releases without reaching upper canopy positions. In contrast, almost half of the Tsuga trees currently in lower canopy positions have never released, but 77% of Tsuga trees that are currently in upper canopy positions required one or more release while in the understory to reach their dominant or codominant position. In contrast, 86% of Tsuga trees in lower canopy positions have undergone periods of suppression. These patterns of suppression and release were interpreted to mean that more understory releases are required for both Thuja and Tsuga trees to reach upper canopy positions.

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