Forest Investment Account (FIA) - Forest Science Program
FIA Project Y102049

    Validation of the SIBEC Model for Estimating Site Index in Complex Stands
Project lead: Gordon Nigh (Ministry of Forests and Range)
Author: Nigh, Gordon D.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Research Question and Purpose:
The purpose of this project is to determine whether the SIBEC model for Douglas-fir in the CHWxm2 01 site series needs refining, to make these refinements (if necessary), and to validate the existing site index estimate for Douglas-fir in the CHWxm2 01 site series. The research questions that this project is addressing are: Does the SIBEC estimate for Douglas-fir in the CHWxm2 01 site series need refining? If so, what refinements are necessary? Is the existing estimate of site index valid for Douglas-fir in the CHWxm2 01 site series?

Description of Project:
The Site Index - Biogeoclimatic Ecosystem Classification (SIBEC) model has now been under development for over 10 years. It is part of a suite of tools, which also includes site index models and growth intercept models, that are used to estimate site index in British Columbia. The SIBEC model provides site index estimates based on biogeoclimatic site series and tree species. Since it does not require site tree measurements to estimate site index, it is ideally suited for estimating site index in complex stands, including stands that have been partially cut, have had variable-retention harvesting, contain a mixture of species, or are uneven-aged.

The SIBEC model is a simple model (y = u, where y is the estimated site index and u is the mean site index for a species/site series combination). A site series is a relatively homogenous ecosystem with respect to productivity. However, there is still variability in a site index estimate for a given species and site series (+- 8 m or more). There is anecdotal information indicating that productivity may change in a north-south gradient, in a elevational gradient, or with some other gradient in a biophysical variable. If this is indeed the case, then further refinement of the SIBEC model by capturing some of this variability will improve growth and yield estimates for timber supply analyses and silviculture applications.

I propose to sample 30 Douglas-fir site index plots in the 01 site series in the CWHxm2 subzone. A full ecosystem classification will be done at each site so that we have ancillary data to relate to site index, including UTM coordinates and elevation. I will stem analyze the sample trees using the stem-splitting technique to get the observed site index. An alternative method to obtain a site index is to estimate it with a site index or growth intercept model. However, this introduces a confounding source of variation and the additional variation may obscure any potential relationships. By sampling in the 01 site series, which already has a site index estimate, I will also be able to compare the site indices from the stem analysis to the existing model-based site index estimate. This will show how much additional variation is introduced by estimating site index with a model.

The end product will be a refined SIBEC model (if necessary) for Douglas-fir in the 01 site series in the CWHxm2 zone. More importantly, it will also provide direction for future SIBEC sampling, analyses, and models.

Forest Management Issue:
This project addresses the issue of getting as good as possible estimate of site index for Douglas-fir in the CHWxm2 01 site series. It also addresses the broader issue of whether the simple SIBEC model can be improved.

Application of Results:
The data will go into the SIBEC data warehouse, where it will be incorporated into the next generation of SIBEC site index estimates. The refined (if necessary) SIBEC model can be used in timber supply analyses to generate yield tables for managed stands of Douglas-fir in the CWHxm2 site series.
Related projects:  FSP_Y091049


Extension Note 96 - An analysis of the sibec site index estimates for Douglas-fir in the CWHxm2/01 site series (0.6Mb)

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Updated August 16, 2010 

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