OSM will always attempt to maximize the number of trees cut. To retain certain trees, one or more Retention constraints must be defined.
The following steps are used internally by OSM to select trees for retention when implementing a cut:
1. Order trees by retention priority (reverse of cut priority)
2. Retain trees until each retention constraint is satisfied.
3. Cut all trees that have not been retained.
Note that retention statements are cumulative, so statement order is very important. Consider the following example: retain 50% basal area and then 10 spruce trees. If not all 10 spruce trees were retained under the first constraint, then up to 10 spruce trees will be retained in addition to the 50% BA amount already retained. However, if we instead retain 10 spruce trees and then retain 50% BA, then exactly 50% BA will be retained.
There are two main approaches for retaining / removing trees:
1) Specify the absolute or relative amount of a stand attribute to retain; the rest will be cut. Example:
·
Trees >= 1000 { 1000 of trees will be retained if present }
·
Trees >= 50% { 50% of trees will be retained if present }
2) Specify the absolute or relative amount to remove; the rest will be retained. Example:
·
Trees – 500 { 500 trees will be cut if present and if
not already retained }
· Trees – 50% { 50% of trees will be cut if present and if not already retained }
In both cases above, the amount of trees to retain is calculated by OSM and then, if possible, retained. Retention statements, regardless of operator used (>= OR -) are cumulative and order is again important. You can use both approaches together when defining the harvest prescription.
For example, assume we have a stand of 600 trees. If we state:
· Trees – 500
· Trees >= 50%
, then only 300 trees will be cut because the 2nd constraint is more restrictive than the first (300 trees retained in the 2nd vs. 100 in the 1st). If you want to ensure exactly 500 trees are removed, then you must drop the second statement. Assuming 400 trees are present and only the 1st statement is used, the 1st statement will remove all 400 trees.
Trees cut from trails (see TRAILS command) cannot be retained, but are considered when calculating pre-cut stand variables. Trails are always cut before retention statements are evaluated.
OSM provides a set of predefined stand variables that can be used to construct retention constraint statements. These variables are calculated at the beginning of each cycle, and therefore represent pre-cut estimates.
|
Variable |
Metric |
USA |
Description |
|
Trees* |
#/ha |
#/ac |
Number of trees per unit area. |
|
BA* |
m2/ha |
ft2/ha |
Basal area per unit area |
|
GTV* |
m3/ha |
ft3/ha |
Gross total volume per unit area |
|
GMV* |
m3/ha |
ft3/ha |
Gross merchantable volume per unit area, assuming a 30 cm stump height and 8 cm top diameter inside bark. |
|
CCF† |
% |
% |
Crown competition factor, where 100 means that open grown tree crowns fill the entire growing space, but on average do not overlap. |
|
RD† |
ratio |
ratio |
Relative maximum density ratio, where 1 = maximum theoretical stocking. Definition of RD and self-thinning lines (stand density index) may depend on the Variant. Consult Variant documentation. Important! Using RD > 0.65 has a different meaning than RD > 65%; see calculation differences in the examples below. |
|
STK† |
ratio |
ratio |
Stocking ratio, where 1 = normal (A-Line) stocking (Depreciated – use RD; STK will be removed in future releases) |
* Can be
expressed as an absolute or percent retention.
† Can be expressed as an absolute or percent retention,
but care is needed in formulating these statements because these measures are
already expressed as either percentages (CCF) or ratios (RD & STK). See RD
examples below for clarification.
These constraints can be as simple as maintaining 50% of total stand basal area (BA):
RETENTION
# Retain 65% of
total BA in the stand
BA >=
65%
# Remove 10 m2/ha of total BA
in the stand
BA - 10
Or, quite specific, as shown in the examples below:
RETENTION
# Retain 10 Spruce trees/ha with
DBH >= 30 cm and Height >= 20 m
Trees >= 10 in Spruce with DBH >= 30 HT
>= 20
# Retain all trees <= 10 cm
DBH, where
# % Retention = (Retained number of
trees/ha with DBH <= 10 cm) /
# (Pre-harvest number of trees/ha
with DBH <= 10 cm) * 100
Trees >= 100% with DBH <= 10
# Retain >= 65% of Spruce BA with DBH
>= 10 cm, where
# % Retention = (Retained Spruce BA/ha
with DBH >= 10 cm) /
# (Pre-harvest Spruce BA/ha with
DBH >= 10 cm) * 100
BA >= 65% in Spruce with DBH >= 10
# Retain >= 65% of maximum theoretical
stand density (max stand density index) in the stand
# % Retention = (Retained
Cumulative Density Index) / (Maximum Stand Density Index) * 100
RD >= 0.65
# Retain >= 65% of pre-harvest stand
density (stand density index) in the stand
# Note here that the % sign makes a big
difference in the calculation below compared to above
# % Retention = (Retained
Cumulative Density Index) / (Pre-Harvest Stand Density Index) * 100
RD >= 0.65%
# Ensure that the cumulative
density contribution
# of trees larger than 10 m
sums to >= 65% of maximum stand density index
# % Retention = (Retained
Cumulative Density Index of trees > 10m) / (Maximum Stand Density
Index) * 100
RD >= 0.65 with HT > 10
# Retain >= 65% of Spruce Pre-harvest
Density (spruce density index), where
# % Retention = (Retained Spruce
Cumulative Density Index) / (Pre-harvest Spruce Cumulative Density
Index) * 100
RD >= 65% in Spruce
# Remove 10 Spruce trees/ha with
DBH >= 30 cm and Height >= 20 m
# In other words, retain 10
spruce trees less than the total Spruce BA with DBH >= 10 cm
Trees - 10 in Spruce with DBH >= 30 HT
>= 20
# Remove 10% of Spruce BA with
DBH >= 10 cm, where
# % Retention = 1 - (retained Spruce
BA/ha with DBH >= 10 cm) /
# (Pre-harvest Spruce BA/ha with
DBH >= 10 cm) * 100
# In other words, retain 90% of
Spruce BA with DBH >= 10 cm
BA - 10% in Spruce with DBH >= 10
See conditional statement documentation regarding formulation of IN and WITH conditions.
A special command (J) is available for retaining an ‘ideal’ J-shaped distribution of diameter classes in order to emulate a single-tree selection method harvest.
The J command has the following required arguments:
1) Ideal ratio of trees in the previous DBH class compared to the current class (i.e., q factor).
2) Ideal residual stand basal area
3) Ideal maximum DBH of the distribution
These arguments must be stated in order and with somewhat similar structure to the statements described above:
· J [Q Factor] [Residual BA] with DBH >= [max DBH]
RETENTION
#J distribution (Q = 1.4, Min Residual BA
= 18 m2/ha, Max DBH = 60 cm)
J 1.4 18 with DBH <=
60
Although
this ‘J’ constraint implies that 18
m2/ha of basal area will be retained, this may be an unlikely
result. What we are actually saying here is retain trees that fall under this
ideal residual diameter distribution, but cut everything else that falls
outside of this distribution. If the stand has mostly large trees or a less
than ideal distribution of diameters, 18m2/ha will probably not be
retained. To ensure that at least 18m2/ha is retained, you must
explicitly state this one additional retention constraint, as shown below.
RETENTION
#J distribution (Q = 1.4, Min Residual BA
= 18 m2/ha, Max DBH = 60 cm)
J 1.4 18 with DBH <=
60
# Retain 18 m2/ha of
basal area in trees with a DBH between 9 and 60cm.
BA >= 18 with DBH 9 60
By retaining trees that meet your ideal J distribution and ensuring your target residual basal area is retained, these two constraints should, over multiple stand entries, eventually mold the stand into an ideal structure for single-tree selection method harvest, regardless of the stand’s initial diameter distribution.
Important: The J retention constraint
must be stated before the BA constraint if you want to ensure all available
trees falling under this ideal distribution are retained.
This example demonstrates all commands
required to schedule a single-tree selection method harvest under the following
constraints:
1)
Prefer
retention of tolerant species (SM, EH, YB, RS, WC)
2)
Pre-cut
gross merchantable volume must be >= 120 m3/ha in trees with >= 9cm DBH
3)
At least
5 simulation cycles (or 25 years assuming 5 years/cycle) must pass before the
next cut.
4)
First
cut will randomly remove 20% of volume in new trails
5)
Subsequent
cuts will remove 1% of volume in existing trails
6)
Target a
residual J DBH distribution with:
a.
Q = 1.5
b.
16 m2/ha
c.
Max DBH
= 50 cm
7)
Maximum
basal area removal = 30% b/w DBH classes 9 and 50 cm
GROUPS.TolerantSpecies SM EH YB RS
WC
RANKS.TolerantPriority.TolerantSpecies 1
def SelectionMethodHarvest
OPERABLE.GMV >= 120 with DBH >= 9
LOCK 5
TRAILS 20% 1%
PRIORITY.TolerantPriority -1
RETENTION
J 1.4 16 with DBH <= 50
BA >= 70% with DBH 9 50
Suppose we want to remove no more than 10 m2/ha if the stand contains >= 29 m2/ha of merchantable basal area (prescription A), and remove 33% of stand basal area if the stand contains >= 22 m2/ha (prescription B). In addition, if treatment A or B occurs, a subsequent stand entry cannot occur for 10 or 25 years, respectively.
Because these two prescriptions have different operable conditions, removals, and lock out periods, we need to build two separate actions definitions. We need to be cognisant of the order in which the actions are defined to ensure treatment A is given priority over treatment B. And finally we need to make use of the LOCK statement to ensure these actions are not scheduled at the same time.
Assume 5-year cycles.
def TreatmentA
OPERABLE.BA >= 29 with DBH >= 9
LOCK 2 TreatmentB
RETENTION
BA – 10 with DBH >= 9
Trees >= 100% with DBH < 9
def TreatmentB
OPERABLE.BA >= 22 with DBH >= 9
LOCK 5 TreatmentA
RETENTION
BA >= 67% with DBH >= 9
Trees - 0 with DBH < 9
Because treatment A is stated before B, it will be scheduled first if the stand has >= 29 m2/ha. Both treatment A and B will be immediately locked from occurring for 10 years if Treatment A occurs because treatment B is listed after the LOCK argument in Treatment A; without this explicit lock on Treatment B, treatment B would occur in the same cycle as treatment A.
Note the two
different ways all trees below 9cm DBH are omitted from harvest in each
treatment; both statements have the same effect.