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Stand volume per hectare is a primary model output for management purposes.
It is determined based on the stand density, the species composition,
diameters, and the tree heights. Mortality and growth relationships
are used to predict these changes as the stand age increases. Both tree
(e.g. species diameter, height, and relative tree size) and stand characteristics
(e.g. site index, density, and basal area) influence these predictions.
Most variables are maintained separately for each major species group.
Link to Future Development
The MGM stand growth simulator software consists of several Microsoft
Excel workbooks and the user interface. The "MGM.xls" and
related workbooks ("MGM Crop Plans.xls", and "MGM Records.xls")
allow the analyst to compose, execute, and save crop plans and results
of growth simulations for one or more stands or management scenarios.
MGM uses crop plans (Crop
Plan, Design and Test) to control simulations. A crop plan
is a sequence of "events" that determine the
source and characteristics of the stand, the growth schedule, and the
timing and method of other activities like thinning, harvest, and regeneration.
Then, following the growth schedule, the tree list and other stand characteristics
are repeatedly projected 1 year into the future by updating tree diameters,
heights, and densities to reflect increment and mortality. At the end
of each projection period, a stand yield summary report is prepared.
This cycle continues until the growth schedule is completed.
The interface adds an MGM Toolbar to the Microsoft Excel interface.
The toolbar controls the simulation by invoking dialogs to construct
and execute crop plan "events". The interface
also allows you to save crop plans and simulation results for one or
more stands or management scenarios.
In the earlier versions, the model was primarily based on diameter
rather than age and height as is typical with many whole stand (yield
table) models. Age was not used in estimating any stand characteristics
or in predicting growth or mortality. Age was provided only as an approximate
stand age, a calendar year, or as elapsed time since the projection
began. It was the responsibility of the analyst to interpret the meaning
of the age label.
This version includes preliminary relationships that link growth and
mortality to site index as well as the earlier relationships based on
variables other than age. Tree age is maintained for each tree, and
it is not the same as stand age (elapsed time since harvest or disturbance).
Mean annual increment based on stand age is provided as part of the
yield report.
Many growth relationships for older trees are based on variables other
than age (Huang and Titus 1993, Burkhart et al 1972, Curtis at el. 1981,
Wykoff et al. 1982). The seedling (juvenile stand) model allows linking
the age variable needed when producing yield tables with a stand growth
model that may not require age. The juvenile stand model is also useful
in simulating the tree height until it reaches a 1.3 metre height. At
that point diameter at breast height is available and can be used in
other tree growth relationships (Huang and Titus 1993, Arabatzis and
Burkhart 1992, Wang and Hann 1988).
All basic growth relationships were derived from data collected in
Alberta. Tree volume and height-diameter estimation is based on regional
relationships developed by the Alberta Environment, Land and Forest
Service and the British Columbia Ministry of Forests.
Stand simulation input can be a tree list or stand table, or trees
may be simulated based on an assumed tree diameter distribution. MGM
requires both region and tree species variables as input. Three regional
classification are recognized: the Alberta Sampling Region (VSR), the
Natural Region of Alberta, and two British Columbia Bio-geo-climatic
Ecological Classification (BEC) zones, Boreal White Spruce (BWBS), and
Sub-Boreal Spruce (SBS). It uses 16 individual tree species grouped
into 4 major groups: white spruce, pine, aspen, and black spruce Creating
and Importing Stand Data.
The growth simulation process requires a growth schedule and the stand
data to be projected. The growth schedule is a list of the intervals
between yield reports and the stand consists of five information items:
name, year, site, tree list, and treatment parameters
Stand summary reports are generated according to the Schedule and include:
stand age, total density/ha, average DBH, average height, total basal
area/ha, total volume/ha, and mean diameter volume increment (total
volume/ha divided by average DBH) Figure 1.
These characteristics are presented for two broad species groups: coniferous
and deciduous.
Stand summary statistics are computed based on all trees at or above
the minimum utilization DBH. Total volume/ha is estimated using the
tree volume function and merchantability settings. At present, no deductions
are made for defect or breakage. If necessary, regional height/diameter
relations (Huang, et al.1992 and Huang 1994) are used to predict tree
height. MGM yield tables summarize tree and stand characteristics as
species averages and totals.
Link to
Getting Software
MGM generates yield tables for white spruce and aspen, and black spruce
and lodgepole pine mixedwood stands and supporting graphics for:
- stand level crop planning; and
- forest level planning for long term timber supply projections of
boreal mixedwood stands .
MGM 2001B was reviewed and evaluated by exploring model behavior across
a wide range of conditions ( FARNDEN 2002).
Model comparison with remeasured BC MOF PSPs data also indicated model
strengths and weaknesses ( HARPER 2002).
As a result, a number of recommendations for operational use have been
suggested ( BC USE RECOMMENDATIONS - MGM
2001B).
The following information provides additional limitations and model
specifications for the recent MGM 2001B release:
- Growth and mortality relationships are based only on Alberta PSP
data.
- Site index and taper functions are based on BC equations. BEC regions
SBS and BWBS equations are implemented.
- BC species codes are applicable if the regional variant is set to
BC.
- The upper limit of stand data for site index is around 21 m at 50
years.
- The upper limit of stand data for density is slightly less than
20,000 per ha.
- Juvenile and small tree data is limited to a small sample of stands
covering a limited range of conditions. This is indicated in the Establishment
Dialogs.
- All species other than white spruce, lodgepole pine, aspen and black
spruce are pooled into one of these four. See the MGM help files.
- Some growth and mortality relations for black spruce are based on
limited data.
The following are some of the limitations and additional guidance for
usage of MGM in northeastern British Columbia (Titus, 1998):
- Interpretation of MGM simulations should be tempered by professional
experience and knowledge of local conditions.
- Application of MGM in FIZ zones other than "L" is not
recommended.
- Where possible, MGM yield projections should be compared with plot
data.
- Other than taper functions used for tree volume estimation, there
is not differentiation of growth relationships by BEC zone. All growth
relationships were developed with data from outside British Columbia.
- Pure (100%) species composition for spruce and aspen may provide
overestimates of volume. This tendency has not been verified because
of difficulty in obtaining data for completely pure stands of these
species. It is recommended that at least 10% of stand density be included
for the typical complementary species.
- Estimates of yield beyond 150 years are subject to greater uncertainty
than those below that age.
- Starting values for stand characteristics obtained from plot averages
can be inappropriate if the plot averages have been computed only
for merchantable trees. For example, if height data for plots is based
on top height or D/DC height, then the input height should be lower
than this to approximate the average of all trees.
- When comparing plot data summaries with MGM projections, definition
of terms must be consistent or allowances must be made in interpreting
the output graphs and statistics. For example, MGM reports average
height while the VDYP database uses
top height or D/DC height.
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