The snag model (SIMULATION.MODEL.SnagModel) estimates
the:
1)
Proportion
of dying trees or of existing snags at the end of each simulation cycle that
will fall;
2)
Probability
that a snag that did not fall will transitions to a lower decay class.
See OSM_TreeList: Died column for instructions on
how to represent snags measured in your survey in your OSM tree list table.
If
standing dead tree observations were excluded from the field survey, the
variant snag model may be able to estimate the probability of occurrence of
snags and the diameter distribution of them at the beginning of simulation. If
snag imputation (see IMPUTE) is turned on and no snag observations are present
in your tree list, then the predicted distribution of snags by size class and
species is added to the tree list before the simulation begins.
The snag model is optionally defined by
the variant model developer, so not all variants may be able to model snags.
Likewise, snag prediction approaches and capabilities may differ for each
variant. If a snag model is not defined, then snags (standing dead trees) in
tree lists, and also snags generated during simulation, will be ignored by OSM.
Snags
are represented in OSM as a separate tree list. Snag properties include
species, DBH, height, year of death, decay class, tree factor, and other tree
properties that may have been carried forward from the live tree list. The timing of snag change operations
(imputation of initial inventory, fall rates, and decay class transition
predictions) is controlled by the underlying OSM API, so timing of these
operations is the same across all variants.
When
snags are present, snag records are written after live tree records in tree
reports.
SnagModel
MinDBH { DEFAULT = 9999; i.e., snags not
tracked }
MaxAge { DEFAULT = 50; i.e., max years post
death tracked }
Impute TRUE or FALSE { DEFAULT = FALSE }
Compress TRUE or
FALSE { DEFAULT = TRUE; i.e., snag
records are compressed }
Minimum
size (DBH in centimeters) of a snag to track. Default min DBH is set to 9999 by
OSM. When set to 9999, OSM will not model snag dynamics or track snags in the
snag list. Tracking snags can increase simulation time, so only lower this
threshold when you want to track snag dynamics. If you are only interested in
snags over 30 cm DBH, then set to 30. Consult the variant model documentation
on the minimum snag DBH that can be tracked (predicted) by the Variant;
generally a 10 cm DBH minimum is used because snags < 10 cm.
SIMULATION.MODEL.SnagModel.MinDBH 10
Maximum
number of years after death a snag is tracked in the snag list. This threshold
is set to 50 years by OSM and is used to remove snags from the snag list
following each cycle that should have fall or decayed to a state where tracking
them is no longer useful. Reducing the number of snags in the snag list help
improve simulation performance. Most users will not adjust this property unless
they are only interested in tracking fresh snags; e.g., <= 20 years old.
SIMULATION.MODEL.SnagModel.MaxAge 50
If
set to true (default = false), then snag records will be imputed (predicted)
before the simulation begins. Imputation is a one-time operation that occurs
after loading tree records. Imputation of snags is useful when the field survey
did not inventory them, yet current inventory of snags is important to know.
The
probability of snag occurrence within a DBH class is typically evaluated first,
and if probable, then the abundance of snags is predicted for that DBH class.
Occurrence probability and abundance estimates may vary as a function of stand
species, structure, management, site, and variant you are using. Snag species
is typically assigned as a random draw from the frequency of tree species in
live tree records. If Impute = true
and snags already exist in the freshly loaded tree list, then no snag
imputation will occur.
This
capability may not be possible in all variants and methods of snag generation
will vary.
SIMULATION.MODEL.SnagModel.Impute TRUE
Snag
compression is important to maintain fast OSM simulations. After each cycle,
some fraction of most live tree records are transferred to the snag list as
mortality. This can quickly cause the number of snag records to grow large,
resulting in substantial loss in OSM simulation speed. By default, OSM tries to
compresses snags at the end of each cycle if number of snag records exceeds
100. During compression, snag records are combined if they are same species,
DBH class (2 cm), decay class, and year of death.
Compression
is an internal OSM algorithm common to all variants and is on by default.
SIMULATION.MODEL.GradeModels.Compress FALSE