UBC Theses and Dissertations
Mechanistic modelling of windthrow in spatially complex mixed species stands in British Columbia Byrne, Kenneth Earl
Windthrow is a common problem in forest management that can be predicted or simulated with the use of modelling tools. A hybrid mechanistic model, WindFIRM/ForestGALES_BC, was developed to quantify component windthrow processes for individual trees in heterogeneous stands and in variable retention harvesting scenarios. Harvesting scenarios can have complex shapes, sizes and distributions of cutblock openings and within cutblock retention of aggregated and individual trees. This model accounts for irregular openings and stand characteristics and is able to simulate the propagation of windthrow during storm events. It is also sensitive to wind direction which is evident from spatial outputs of model simulations. WindFIRM/ForestGALES_BC was also successfully integrated with a growth and yield model, TASS (Tree and Stand Simulator), to quantify the effects of openings created by windthrow on stand yields. The flexibility of WindFIRM/ForestGALES_BC to substitute trees and modify crowns in spatial tree lists is demonstrated and a validation of the model was completed using field data from the STEMS (Silvicultural Treatments for Ecosystem Management in the Sayward) research installation on Vancouver Island. The results of the non-parametric tests show that the pattern of simulated windthrow through size classes is consistent with field plot data. However, contingency tables indicate that the model underestimates windthrow which is due to some suspected and unknown causes. The cumulative proportion of modelled windthrow has a consistent pattern with the cumulative proportion of actual windthrow from the field data. The advances in WindFIRM/ForestGALES_BC make it a useful platform to integrate other process models (e.g. large woody debris recruitment in streams) and incorporate new functions obtained in future research. It will also serve as a good framework to guide data collection needs and priorities (e.g. high resolution wind data and simulation, soils and complex topography) related to the understanding of windthrow processes. Further refinements related to the effects of wind on tree stability in complex terrain, soils and tree sway dynamics will improve the model as a predictive tool. This will allow forest managers to simulate a range of scenarios to identify block and retention layouts, locations and individual tree characteristics that pose the highest windthrow risk.
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