The Mixedwood Growth Model (MGM) is a deterministic, distance-independent, individual tree-based stand growth simulation model. The general modeling approach is similar to that of the Forest Vegetation Simulator, formerly known as Prognosis.
Four primary species, spruce, pine, aspen, and black spruce, are currently the focus for component relationships. Black spruce has received much less attention during development since there is much less data available for black spruce.
Limited regional variations are included in MGM. These relate mainly to species codes for data input, site index, and tree volume estimation equations. Growth relationships are based on Alberta data and are currently the same for all regions. A simple ratio-of-means calibration feature can be employed if local diameter increment data is available for some trees in the stand at establishment.
The stand may be established in several ways. Simulation of annual change in stand conditions is based on tree-level relationships predicting annual in-growth, juvenile growth, height increment, diameter increment, and mortality.
Juvenile trees less than 4.0 cm DBH
In-growth can be predicted for trees reaching a 1.1 cm DBH limit using a two-stage approach that predicts both the probability of annual in-growth and the number of trees/ha assuming in-growth occurs.
Separate juvenile sub-models predict height increment, diameter increment, and survival.
Mid-rotation trees ≥4.0 cm DBH and <80% of "maximum height"
"Maximum height" is defined as the maximum height value (asymptote) of a height - DBH curve fit for site trees in Alberta natural subregions. Trees below 80% of this value and equal to or above 4 cm DBH are considered mid-rotation.
Height increment is a predicted based on the site index for the species and the competition that the tree is experiencing. Competition is measured by the sum of DBH (cm/ha) of trees of each species that are larger in diameter than the diameter of the subject tree. This competition index was chosen since it is closer to approximating sapwood area (and therefore leaf area) than the conventional basal area/ha. Competition from deciduous species, pines and spruces is considered separately.
Diameter increment is predicted based on height increment, the regional height - DBH curve for site trees, and the same measure of competition as for height increment (though with different coefficients).
Mortality is predicted using a logistic regression model that predicts probability of survival. In addition, survival is constrained based on the biological limits of size and density as well as theoretical expectations for small trees growing under the canopy of other trees.
Old-growth trees (not currently applied to pine or black spruce)
Height increment is determined as for mid-rotation trees.
Diameter increment is based on a constant basal area increment (BAI) for site trees, set at the transition from mid-rotation to old-growth phase. As the bole perimeter grows larger in future years, the diameter increment gets smaller to maintain this BAI, a commonly observed feature of older trees. For trees that are thinner than site trees, the diameter increment predicted for site trees is modified by the same competition indices and coefficients as for diameter increment of mid-rotation trees.
Mortality is predicted as for mid-rotation trees.
Validation of the Model
Several approaches have been taken to validation of the model, including comparison of short term re-measured plots with MGM predictions and longer term projections compared to temporary plots selected over a wide range of stand age.