MIXEDWOOD GROWTH MODEL (MGM)

For every year of modelled growth, MGM predicts an annual height increment for each tree, relative to site index assumptions, local competition, and/or other variables. Different height increment models are used for trees ≥ 4cm DBH and trees < 4cm DBH.

For every year of growth modeled by MGM, it predicts an annual diameter increment for each tree > 1.3 m tall, relative to height increment, local competition, and/or other variables. Different diameter increment models are used for trees ≥ 4cm DBH and trees < 4cm DBH.

For every year of modeled growth, MGM predicts an annual survival probability for each tree, relative to tree size, local competition, and/or other variables. Then, tree-level survival probability is multiplied by tree factor (# of trees per hectare that are represented by each tree in a tree list, based on sample plot area) to determine the number of surviving trees per hectare. 

Tree-level survival is summed across each stand to determine stand-level survival in trees per hectare. Different survival models are used for trees ≥ 4cm DBH and trees < 4cm DBH.

Age solver is a supporting module that overcomes the difficulties behind obtaining tree age by estimating breast height age and total relative age relative to tree height and site index. 

Age solver estimates tree age under 4 cases:

1. Trees > 1.3m are initialized with an observed total age (>0) and without a breast height age (-1 placeholder). Breast height age is estimated using Equation A1.
2. Trees > 1.3m are initialized with an observed breast height age (>0) and without a total age (-1 placeholder). Total age is estimated using Equation A2.
3. Trees > 1.3m are initialized without a total age (-1 placeholder) or a breast height age (-1 placeholder). Breast height age is estimated using Equation A3. Total age is estimated using Equation A2.
4. Trees ≤ 1.3m are initialized without a total age (-1 placeholder). Total age is estimated using Huang et al. (2009) for Alberta (Equation A4) and a quadratic age-height relationship for Saskatchewan, Manitoba, and northeastern British Columbia (Equation A5)

BHageijk = Tageijk – RegYearsToBHij (Equation A1)

Tageijk = BHageijk + RegYearsToBHij (Equation A2)

BHageijk = SiteCurvej(Hijk, SiteIndexij) (Equation A3)

Tageijk = Huang2009Tagej(Hijk, SiteIndexij) (Equation A4)

Tageijk = (RegYearsToBHij / 1.32) × (Hijk)2 (Equation A5)

Where:

Tageijk = Total age (years) of tree k of species j in stand i

BHageijk = Breast height age (years) of tree k of species j in stand i

RegYearsToBHij = Regional years to breast height assumption (years) for species j in stand i. RegYearsToBH scales with site index, using Equations A6 or A7.

SiteCurvej = Site index curve for species j that is rearranged to solve for BHAgeijk. Equivalent to height increment Equation H8.

Hijk = Height (m) of tree k of species j in stand i

SiteIndexij = Site index (m@50 years breast height age) of species j in stand i

Huang2009Tagej = The Huang et al. (2009) site index curve for species j that is rearranged to solve for Tageijk, given Hijk and SiteIndexij. The Huang et al. (2009) site index curves are based on total age.

Trees ≤ 1.3m that are initialized with an observed total age are not modified by the Age Solver. Trees > 1.3m that are initialized with an observed breast height age and an observed total age are not modified by the Age Solver.

In some cases, users may not have full tree lists for MGM user but may have summary statistics (average height, domainant height, average diameter, and density for each species) for the stands they want to simulate. The MGM tree list generator (TLG) supports the use of this kind of data in MGM by estimating tree height and diameter distributions for each species using plot summary information. Then, it creates individual trees using these distributions.

We acknowledge the contribution by Ken Stadt and Zhili Feng in the initial development and application of the models used in the TLG. 

*please see research note #2020-3 for more information on the Tree List Generator*

The current validation of MGM (MGM21 VS8.2.21.39 / Rev6378) are available below. These validations assess whole-model performance.

Validation of the Mixedwood Growth Model (MGM) against the Alberta Provincial Growth and Yield Initiative (PGYI) Dataset – April 27, 2021

Validation of the Mixedwood Growth Model (MGM) against the 2020 Saskatchewan Provincial Permanent Sample Plot dataset – December 15, 2022 

Published Documents

• Oboite, F.O. 2018. Growth of understory spruce following mountain pine beetle attack and recalibration and validation of the Mixedwood Growth Model for black spruce. PhD Thesis. University of Alberta, Edmonton, AB. 181p.
• Strimbu, V.C., M. Bokalo, and P.G. Comeau. 2017. Deterministic models of growth and mortality for jack pine in boreal forests of western Canada. Forests. 8(410):1-17.
• Bokalo, M., K.J. Stadt, P.G. Comeau, and S.J. Titus. 2013. The validation of the Mixedwood Growth Model (MGM) for use in forest management decision making. Forests. 4:1-27.

Other Documents

• Bokalo, M., P.G. Comeau, and K.M. Johnson. 2019. Validation of the Mixedwood Growth Model (MGM) against the Provincial Growth and Yield Initiative (PGYI) dataset. Mixedwood Growth Model: (MGM18 VS1.1.18.37 / Rev6115). Prepared July 19, 2019.
• Bokalo, M., and P.G. Comeau. 2015. Validation of the Mixedwood Growth Model (MGM) against the Provincial Growth and Yield Initiative (PGYI) datasets. Report submitted to Alberta Environment and Sustainable Resource Development. Submitted August 3, 2015.
• Strimbu, V., Bokalo, M., and P.G. Comeau. 2015. MGM Calibration for Jack Pine in Alberta, Saskatchewan and Manitoba. Report submitted to Alberta Environment and Sustainable Resource Development, Saskatchewan Enviroment and Alberta Pacific Forest Industries. Submitted April 1, 2015.
• Bokalo, M., and P.G. Comeau. 2013. Lodgepole Pine and Conifer Mixedwood (including pine) Validation Results for MGM. Report submitted to Alberta Environment and Sustainable Resource Development. Submitted June 26, 2013.

Recent MGM submodel validations are listed below. These submodel validations may include a whole-model validation. 

• Oboite, F.O. 2018. Growth of understory spruce following mountain pine beetle attack and recalibration and validation of the Mixedwood Growth Model for black spruce. PhD Thesis. University of Alberta, Edmonton, AB. 181 pp.
• Cortini, F., P.G. Comeau, V.C. Strimbu, E.H. Hogg, M. Bokalo, and S. Huang. 2017. Survival functions for boreal tree species in northwestern North America. Forest Ecology and Management. 402:177-185.

The yields worksheet summarizes the tree list to obtain the periodic results of the MGM projection. The yields worksheet data dictionary describes the conditions that are appled to each field during the compilation of the tree list.

The MGM Make Stands workbook preps tree data for MGM, identifies common errors in data, and creates MGM stand worksheets. The User's Guide includes instructions and formatting rules for data preparation before use in MGM

The MGM Record Extractor workbook converts MGM Records into a tabular format for data analysis. It is designed to help with executing crop plans using MGM's Graphical User Interface (GUI). Users of MGM Batch Macro don't need the MGM Record Extractor. For help using this workbook, please see the User's Guide

The MGM Batch Macro workbook allows MGM to run thousands of projections using plot summary information and MGM Stand Worksheets. It also records long-form MGM outputs in a tabular format for data analysis. For further information on using this workbook, please see the User's Guide

First major release of MGM18. Download available June 30, 2019 to July 11, 2019.

New Features:

1. New climate-sensitive survival functions for white spruce, black spruce, lodgepole pine, jack pine, trembling aspen, balsam poplar, and balsam fir (Cortini et al. 2017).
2. New height increment and diameter increment functions for jack pine (Strimbu et al. 2017) and black spruce (Oboite 2018).
3. The full implementation of the GYPSY / Huang et al. (2009) site index curves for Alberta.
4. The retirement of the Huang (1997) and Huang et al. (1997) site index curves for Alberta.
5. The deactivation of the “stand breakup” Mortality Adjustment (i.e. “MAFlag”).
6. An interim update to the juvenile Treelist Generator (Trees < 25 years).
7. A streamlined crop plan “Options Event” (i.e. fewer user “flags”).
8. An executable installation package and updated installation procedure.
9. An installed Excel “AddIn” that significantly improves model performance.
10. Digital certificates for the installation package and Excel “AddIn” to enhance security.

Minor update. Includes all of the features in MGM18 (VS1.0.18.37 / Rev6115). Download available July 15, 2019 to April 26, 2021.

Updates:

1. Update to integrate the jack pine diameter increment function (Strimbu et al. 2017) with the GYPSY / Huang et al. (2009) site index curves.
2. Update to address how pine competition is passed to the survival functions (Cortini et al. 2017) and black spruce growth functions (Oboite 2018).
3. Update to address how black spruce competition is passed to the jack pine growth functions (Strimbu et al. 2017).

Beta release of MGM21 that is under consideration for use in Forest Management Planning in Alberta. MGM21 is a major update from MGM18 (VS1.1.18.37 / Rev6115). Download available April 27, 2021 to present.

New Features:

1. A climate and composition sensitive maximum size-density function for trees ≥ 4cm DBH (Comeau 2021b).
2. An updated self-thinning system for trees ≥ 4cm DBH.
3. Revised survival functions for balsam fir, jack pine, lodgepole pine, and black spruce without a tagging limit variable (Comeau 2021a).
4. Updated height increment and diameter increment functions for black spruce (Oboite and Comeau 2021).
5. Added white spruce competition to the height increment and diameter increment functions for juvenile white spruce < 4cm DBH (Comeau and Bokalo 2020).
6. Updated site index functions for Saskatchewan, Manitoba, and northeastern British Columbia.
7. A routine that estimates missing tree ages within MGM (i.e. Internal Age Solver).
8. Improved tree age estimates on regenerated trees.
9. A modified approach to estimate tree age for white spruce.
10. The updating of tree age estimates after stand establishment or thinning.
11. Updated diameter estimates in MGM’s Tree List Generator for untended deciduous trees.
12. Streamlined crop plans with fewer inputs.
13. Faster runtimes when using large Stand Workbooks.
14. 32-bit support for Microsoft Office 2016, Microsoft Office 2019, and local installations of Microsoft Office 365.
15. 64-bit support for Microsoft Office 2019 and local installations of Microsoft Office 365.