Adding Succession to the ForestModel

See also <succession> element reference in ForestModel References

By adding succession to the ForestModel, Patchworks can mimic natural changes that take place in the overstory over time by defining the age of breakup, new age and what the stand will become after breakup. In the ForestModel, succession elements fall under a separate bundle of select statements, so that unique succession rules can be applied to only certain selected records.

To add succession, we could use the following lines of code in the existing file (within the <ForestModel> element).

    <select statement="theme2 eq 'AW'">
      <succession breakup="150" renew="25">

As with other select statements, inventory records are tested against the statements in sequential order. Those records that satisfy the statement query are associated with the succession rules within the select element. The sucession element contains only two attributes: breakup and renew.

breakup refers to the age at which the stand breaks up. renew refers to the new age of the stand after breakup. In this particular succession element, the stand we have selected (AW) will breakup at age 150 and the new stand (understory) will be at age 25. The rest of the stand characteristics will remain the same (ie. an AW stand will return to an AW stand).

In the natural world however, an AW stand may not succeed to another AW stand. The succession element can be used to assign new stand characteristics to the selected stand after breakup. To do this we use an element we have already used: assign.

    <select statement="theme2 eq 'AW'">
      <succession breakup="150" renew="25">
        <assign field="theme2" value="'SW'"/>

By including the line <assign field="theme2" value="'SW'"/> in the element, we have changed what type of forest unit the stand will succeed to. In the above example, AW stands break up at age 150 and become 25 year old SW stands. Numerous assign statements can be used within the succession element to change other characteristics as well.

Attributes can also play a role in succession by specifying whether or not the curves should 'cycle' with the defined succession rules. Earlier in this example we defined a visual disturbance attribute;

    <attributes id="Visual">
      <attribute label="feature.Visual.disturbed" cycle="false">
        <curve id="Visual.disturb">
          <point x="5.0" y="1.0" />
          <point x="25.0" y="1.0" />
          <point x="26.0" y="1.0" />
          <point x="30.0" y="0.0" />

This particular attribute specifies cycle="false", whereas the other attributes of our example did not. This indicates to the ForestModel that the visual disturbance attribute is not supposed to succeed to something else. In the case of the visual disturbance attribute, natural succession does not cause the stand to exhibit any additional visual disturbance qualities.