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UBC Theses and Dissertations

Modelling the recovery of old-growth attributes in coastal western hemlock forests following management and natural disturbances Gerzon, Michael


Since the early 1900’s logging activities in coastal British Columbia (BC) have reduced the extent of old-growth forest area from its historical distribution of 90% to approximately 55% of the forest landbase at present. Presently, the province employs an age threshold approach to identify old-growth forest throughout BC. While this approach allows for a rapid inventory assessment, there are several factors that limit its efficacy: the age-based classification method does not capture the gradual development of old-growth features, the development of old-growth structure is not always well correlated with age, and it does not work well in multi-aged stands created through variable retention (VR) harvesting. The objectives of this thesis were to: 1) develop a set of structural attributes for assessing the old-growth condition based on a chronosequence analysis, 2) evaluate capability of the FORECAST model to project the temporal trends in structural development observed in the chronosequence, and 3) employ FORECAST to assess the implications of alternative dispersed retention scenarios on the recovery of old-growth structural attributes. A chronosequence study was established including 33 sites varying in age from 60 to 300+ years on Vancouver Island. Data collection from the sites included stand structural attributes, vegetation, soil properties, disturbance history, and stand age. A set of attributes representing stand structure was developed that showed clear trends in the recovery of old-growth forest characteristics. The forest growth model, FORECAST, was evaluated against the chronosequence in terms of its ability to project patterns of development of old-growth structure. The model evaluation showed that FORECAST could be employed to project the impact of different silviculture systems on temporal trends in the development of old-growth conditions for the vh and vm subzones of the coastal western hemlock zone in BC. The analysis of dispersed retention scenarios using FORECAST showed that VR can help to mitigate the effects of harvesting on the maintenance of old-growth structural attributes. The analysis also showed that starting condition prior harvesting influenced the outcome and that the time required for structural elements to recover to old-growth levels was generally not reduced with increasing levels of dispersed retention.

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