Predicting and minimizing windthrow in partial harvest prescriptions continues to be a significant challenge for forest managers in BC. Managers need decision-support tools that enable them to evaluate the potential windthrow outcomes of different cutblock design scenarios and in complex stands. Integrating windthrow predictions with growth and yield modeling will enable more realistic projections of overstory and understory growth and yield.
Since 2005 we have been working with the United Kingdom (UK) Forestry Commission and the BCMOFR Decision Support group to update and localize the mechanistic windthrow risk model ForestGALES and integrate it with TIPSY/TASS. ForestGALES is a non-spatial model that predicts the critical wind speed for tree failure and the probability of this critical wind speed in even-aged stands. We have updated ForestGALES by adding mechanical properties of BC conifers from our winching and wind tunnel studies and have added local equations for stem and crown characteristics (Rudnicki et al. 2004, Byrne 2005). We have also developed an interface, WINDFIRM, which characterizes within-opening wind exposure due to fetch and boundary orientation. We have linked WINDFIRM with TASS-III to account for windthrow propagation and to add directional and spatial components to ForestGALES_BC (Di Lucca et al. 2007, Gardiner et al. 2007). These improvements set the stage for windthrow prediction in complex stands for partial harvesting scenarios.
In our recent research, we have identified several components of the ForestGALES_BC model that need improvement to more accurately represent the variability of wind loading and resistance on a tree-to-tree basis. These modifications are necessary to realistically represent damage propagation in complex stands during peak wind events. Representation of wind loading and resistance for individual trees in ForestGALES is currently deterministic ? based on empirically derived regressions that use crown and stem mass as predictors. We propose two approaches to better represent tree-tree variability. (1) We will incorporate the error associated with the local equations to represent the variability in among trees of the same size. The dendrometric functions used to calculate stem and crown mass from TASS derived tree size attributes will be also be adjusted to better reflect variability in size-mass relationships. These refinements will reduce the deterministic nature of the model. (2) Through our collaborations with international windthrow research groups, we have access to new information sources on wind behaviour in partial cuts and complex stands, including field and wind tunnel studies and results of large eddy simulations (LES). We will incorporate these functions to refine the upwind wind-field and improve model sensitivity to loss of neighbouring trees.
ForestGALES_BC is a complex mechanistic model and to properly test the model we will use validation datasets for stands from a variety of sites and under varied partial harvesting regimes. We have access to variable retention monitoring datasets produced by Weyerhaeuser Limited (now Western Forest Products Ltd.), and long term BCMOFR silviculture system experiments and will use these for validation. We will compare expected outcomes, with actual outcomes for trees in sample plots, and will conduct sensitivity analyses of input parameters and functions to improve predictive accuracy.
The updated and validated ForestGALES_BC and WINDFIRM models will replace the current model in TASS-III. The user will specify stand and harvest design conditions via the TASS-III interface. Trees that are overturned or broken will be deleted from the tree list and will be identified in the TASS stand visualization screen. This will give managers a very interactive way of evaluating the windfirmness of different cutblock design scenarios
Mitchell, Stephen J., Byrne, Kenneth E.; Di Lucca, C. Mario; Polsson, Ken R.. 2009. Incorporating the effects of tree-to-tree variability and upwind windfield conditions on mechanistic windthrow models and growth and yield models. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), British, Columbia
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