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Incorporating the effects of windthrow after variable retention harvesting into TASS and TIPSY
Di Lucca, C. Mario
Program theme/topic: This project aligns with the recommended research topics for the Timber Growth and Value program. The most germane high-priority topic is 4.2 Estimating Losses that calls for ?biologically robust, agent-specific models which move beyond models that estimate simple overall proportional reductions?. The team assembled for this project has many years of experience in both growth modelling and windthrow hazard prediction. Table Interpolation for Projecting Stand Yield (TIPSY) was expanded in 2003/04 to include factors that reduce regenerated yields for variable retention (VR) harvesting systems (Di Lucca et al. 2004). A recent timber supply analysis in the Fraser TSA (Ministry of Forests, 2003) using TIPSY indicated that relative to the clearcut base case, leaving about 15 dispersed trees per hectare within openings could reduce harvest volumes by about 3% in the medium-term, and 8% in the long-term. These factors, however, don't account for the impacts of windthrow which often results from this harvesting system. Early results from silvicultural systems experiments show that dispersed retention can lose 20% (D?Anjou 2002) to 50% (de Montigny 2004) of residual trees due to windthrow. Windthrow losses in aggregated group retention is lower due to mutual protection but still approach 10% of the residual trees (de Montigny, 2004). Windthrow losses will affect growth and yield projections for overstory trees. The loss of overstory will also affect regeneration growth rates. We want to capture both effects in our simulations of VR in the Tree and Stand Simulator (TASS) and in TIPSY. TASS is a spatially explicit individual tree model that provides key growth and yield estimates for the forests of British Columbia, and provides the database for TIPSY. The VR data recently added to TIPSY was created by thousands of simulations generated by TASS. TIPSY is used in every timber supply review in the province. The probability of windthrow loss in partial cuts is known to reflect stand, neighbourhood and tree-level attributes (Scott, 2004). A group of Canadian researchers including Dr. Steve Mitchell of UBC and Dr. Jean-Claude Ruel of U. Laval are working with the UK Forestry Commission to add Canadian species to the Forestry Commission's ForestGALES mechanistic windthrow risk model (Ruel et al. 2000). Dr. Mitchell and his team are working on this problem in the FSP-funded project entitled 'Numerical modelling of wind flow in retention system openings'. In this project they are characterizing wind effects using numerical modelling techniques for 2 km grid cells across BC. This project will improve our ability to predict the impact of losses due to windthrow after variable retention harvesting. This proposal is divided into three subprojects: 1. Incorporate preliminary data on the effects of windthrow into TIPSY & BatchTIPSY for Douglas-fir, western hemlock, lodgepole pine and white spruce after variable retention harvesting. 2. Link the spatially explicit growth and yield model TASS with the mechanistic wind risk assessment capabilities of ForestGALES to assess the windthrow yield losses after variable retention harvesting. 3. Incorporate the results from the former subprojects into an upgraded TIPSY & BatchTIPSY version that will also include the addition of the remaining commercially important species in BC. Subproject 1 will incorporate the results of empirical studies conducted by Dr. Stephen Mitchell and his students. For example, they have found that variables such as slenderness, live crown ratio, tree and stand height, site index, and wind exposure (fetch) are strongly related to percent stem loss in VR. A key driver such as a modified edge length will likely provide the link between amount of windthrow and variable retention adjustment factors (VRAFs). For further flexibility, users will also be able to directly input percent or number of stems lost due to windthrow. The linkage between TASS and ForestGALES (su ...
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Executive Summary
Extension Note 77

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