retention — optional within block retention definition


Content Model

   <retention> = assign+, features+.  


factorfraction of block to be retained (0.0 - 1.0)Required


<retention> elements assist in the implementation of aspatial retention rules. Spatially explicit retention is defined by block area being pre-apportioned to 'managed' and 'unmanaged' stand components, based on the value of theme variables. Aspatial retention can simulate the implementation of an area retention policy that can not be defined in advance with lines on the ground (for example, retain 5% of block area for wildlife hiding cover, the location of which is unknown until on-the-ground inspection). <retention> elements identify the stand types that are applicable, the percent area that should be widthdrawn from the managed part of the landbase, and an optional set of attribute curves that should be attached to the retained areas (in addition to all of the original block attribute curves). Retained areas are withdrawn from the eligible landbase before the start of the simulation. Retained areas will be subject to succession events.


These elements contain retention: select.


The following elements can occur in retention: assign, features.



This attribute indicates the percentage of the selected record to be retained as unmanaged. The factor is a number between 0.0 and 1.0 (100%).

See Also

select, assign, features.


Retention can distinguish areas in the original inventory that are to be unavailable for management using <select> statements. <assign> elements can then be used to flag the record as unavailable and sent on an unmanaged track. For example,

  <select statement="FU eq 'SB1' and IFM = 'managed'">
    <retention factor="0.05">
      <assign field="IFM" value="unmanaged" />

In the example above, 5% of the managed area in the SB1 type is converted to 'unmanaged'. This may be used to simulate an aspatial reduction factor, such as unmapped riparian buffers. The conversion occurs as the Matrix Builder reads in the inventory records. After the initial conversion the revised stand records are processed as normal for their stratification values, and the retention elements are no longer used.

Aspatial policies may be incorporated into the ForestModel by specifying retention percentages based on stratification values (forest units, ecosites or environmentally sensitive zones). In the next example, 16% of the PWST managed type is retained and assigned a number of additional feature attributes.

  <select statement="FU eq 'PWST' and IFM in managed">
    <retention factor="0.16">
    <assign field="IFM" value="retained" />
        <attribute label="feature.Retention.ByPass">
            <point x="0.0" y="0.125" />
        <attribute label="feature.Retention.Riparian">
            <point x="0.0" y="0.25" />
        <attribute label="feature.Retention.NDPEG">
            <point x="0.0" y="0.625" />

16% of the fragment will be split from the original record and assigned a value of 'retained' in the IFM field. This new record will be tested against other select statements to assign feature attributes. In addition to the regular features appropriate for the stratification values this fragment will also have the 3 additional features we have assigned in the retention element.