Abstract
Projections of wood flow in the Nelson Forest Region indicate that there may be a near-term fall down in timber due, in part, to cutting constraints and green-up requirements. To alleviate some of the impacts of this projected fall down, partial cutting has been suggested as an option that will provide timber in the short term. Planners therefore need forest growth and yield tools that will allow them to assess the possible outcome of partial harvesting on a regional basis.
To project potential timber yields in unmanaged stands. the Ministry of Forests currently uses a growth and yield model (VDYP version 6.3b) (Anonymous 1995a) linked to a forest estate simulation model (FSSIM). However, neither of these tools has been designed for simulating or evaluating partial harvesting scenarios. For example, VDYP is not able to model growth release or ingrowth following stand entry because it is based on empirical tables of stand growth in unmanaged stands. In the same way, FSSIM is designed to work with clear-cut harvest projections and is not well able to project stand entries with partial removals.
As an added complication, root disease is common in many ecosystem types in the Nelson Forest Region. Armillaria root disease centres often expand in response to partial stand entries. The utility of partial cutting as a tool to alleviate near term timber supply problems therefore depends, in part, on stand dynamics after an initial entry. VDYP does not consider the impacts of root disease on stand growth and mortality.
As an alternative growth model. Prognosis (version IDF 1.0f, recently renamed the SI variant) (Stage 1973, Wykoff et a!. 1982) is able to simulate tree growth in a way that is more naturally able to incorporate the stand dynamics that result from a mixture of tree species and partial harvests. With the Western Root Disease Model extension (WRDM3; Beukema and Kurz 1996), Prognosis is able to incorporate the effects of Armillaria root disease and its responses to stand entry. Although it is not yet calibrated for the region, it is possible to use the self-calibration features of Prognosis (validating the results with VDYP) to give reasonable projections that are sensitive to partial harvesting, ingrowth and root disease.
This project is based on discussions arising from a scoping meeting held in the Nelson Regional office on May 30th, 1996. Two major tasks were identified at this meeting:
creation of partial harvesting volume-age curves with Prognosis for 24 combinations of BEC, Timber Type and Site Productivity in the Cranbrook and Boundary districts, linking those curves to the FSSIM model: and
an experimental analysis of Prognosis-based estimates of mean annual increment over a combination of partial harvesting options and root disease levels.
The FSSIM forest estate model requires volume-age curves that reflect projected yields for partial harvest scenarios across biogeoclimatic (BEC) subzones, Timber Type and Site Productivity strata. The selected curves cover stand types that are considered to be potential candidates for partial cutting, and exclude, a priori, stands that are known to be at risk by merit of high levels of root disease, or stands that would be put at risk if a flush of root disease were triggered by a stand entry.
The analysis of predicted Mean Annual Increment (MAI) is a simulation study in which an experimental block design is used to evaluate productivity over many different partial harvesting options and levels of root disease. Because it is computationally intensive, the MAI analysis is restricted to a total of four of the strata. In addition, the intensive analysis of the MAI response has been supplemented by an additional sensitivity analysis that examines possible changes in system behaviour under an increase and decrease of 10% in the species-multipliers used by Prognosis.
This report is not intended to provide a comprehensive explanation of the methods by which the many simulations were carried out. Neither is it intended to provide an analysis of the application of Prognosis to FSSIM in the Timber Supply Review process. Rather. this document is intended to provide a concise description of the way in which the Prognosis model has been integrated into the decision making process. Hopefully, it demonstrates the strength of the model and the way in which its projections can be incorporated into and work alongside existing forest planning tools.
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Updated April 12, 2007
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