succession — specifies overstory breakup timing, renewal timing and change in forest characteristics


Content Model

	    <succession> = assign*



breakup The age when breakup occurs (a number) "0"
renew The new age after breakup "0"
initialagelimit The minimum number of after the start of the simulation to defer incipient succession"-1"


Succession elements are used to mimic the natural changes that take place in overstory conditions by defining an age of breakup, age of renewal and post-successional stratification values.

Succession elements are contained within select statements, allowing stratification value query expressions to limit the stand types that these rules apply to.

Succession elements provide a mechanism for describing the stand-replacing action of overstory senescence and emergence of a new cohort. As a spatial model, Patchworks is restricted to modeling deterministic stand development on each polygon. Polygons are initially classified with a single set of stratification values, and can only transition to another single set of stratification values. The selection element allows the post-successional change of the stratification values to single new values. Dividing polygons in to multiple types based on a successional proportion is not permitted.

Changing the characteristics of the stand at a certain age is easily accomplished with an assign element.

The breakup attribute defines the stand age when the succession event will occur. The renew attribute defines the age given to the post renewal condition. Post-succession characteristics are specified using the assign element. Assign elements only need to be specified for stratification values that are changing. Unchanged values may be omitted.


Since the succession rules are deterministic the rules will be applied as soon as the breakup age is reached. A special situation occurs when the initial inventory contains stands that are already older than the specified breakup age. In this case the succession rule is immediately applied as the stand record processing is started, and the breakup/renewal age calculation is applied with a retroactive age. For example, if a stand with an initial age of 125 matches a succession rule having a breakup age of 115, then the stand will immediately transfer to the new type and renewal age, and will then be aged another by 10 years. These values will be assigned as the initial age and condition of the stand when the Patchworks model is initialized in time 0. If the stand is sufficiently older than the breakup age then it may be possible that several succession events may be triggered until the stand age stabilizes below all relevant breakup limits. As a consequence of the initial succession processing it is possible that the initial Patchworks inventory summary will differ from the initial input inventory summary.

The timing of initial breakup behaviour described in the above note can be modified by using the 'initialagelimit' attribute. This attribute specifies the number of years after time 0 to defer the successional transfer of a stand, in cases where the stand would already be old enough to undergo succession. The method works by changing the initial stand age to be younger than the breakup age by the set number of years. If the breakup age is 115 and the stand is already older than this age, a initialagelimit value of '10' would reset the initial stand age to 105, so that it would not undergo succession until the 10th year of the simulation.


Note that with the use of the initialagelimit attribute the initial Patchworks inventory summary might still differ from the initial input inventory summary, as excessively old stands will have their ages changed to be younger than the breakup age.

One method that can be used to ensure that the initial Patchworks inventory summary is the same as the input inventory summary is to set up a number of succession rules with increasing breakup ages, and use a supplemental stratification value to assign elderly stands to these supplemental 'late breakup' rules. These rules can be used to prevent the old stand from immediate breakup. After the succession event occurs the supplemental stratification value can be reset to subsequently follow normal succession ages.


These elements contain succession: select.


The following elements can occur in succession: assign.



The age at which the stand breaks up, allowing a new generation to succeed.


The new age of the stand, specified as a numerical attribute in double quotations.


The minimum number of years after the start of the simulation to defer incipient succession. If the 'initialagelimit' parameter is less than 0 then the succession deferral methodology is not applied.

The breakup and renew attributes are numeric values, and can be specified as numbers or as a Patchworks Query Language expression. The expression may make use of numbers, operators, defined values and functions. The expressions will be evaluated in the context of the stand condition that the succession rule is being applied to.

See Also

attribute, assign.


In the following example a succession rule is defined. The select statement indicates that any inventory record that contains the value 'AW' in the 'theme2' field will adhere to the contained succession rule. At age 150 the stand will break up and will renew again at age 25. The stratification values are not changed so the stand renews to the same condition.

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

In this following example an assign element has been added that changes the theme2 statification value to 'SW' in the post-successional condition. The premise for this rule is that when the aspen overstory senescences and breaks up the white spruce advanced growth in the mid-canopy will dominate the new stand.

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

The following illustrated example helps to show how the succession processing works.

  <!--Visual quality attribute set.  Stands appear visually disturbed
	  till age 30.-->
  <attributes id="Visual">
    <attribute label="feature.Visual.disturbed" cycle="false">
      <curve id="Visual.disturb">
        <point x="0" y="1.0" />
        <point x="29" y="1.0" />
        <point x="30" y="0.0" />

  <!--  Yield curves for yield strata AW -->
  <attributes id="Yield.AW">
    <attribute label="feature.Yield.managed.Decid">
        <point x="5.0" y="0.0" />
        <point x="20.0" y="0.17" />
        <point x="30.0" y="0.85" />
        <point x="35.0" y="1.47" />
        <point x="45.0" y="3.29" />
        <point x="70.0" y="9.37" />
        <point x="80.0" y="11.38" />
        <point x="90.0" y="12.73" />
        <point x="95.0" y="13.13" />
        <point x="105.0" y="13.39" />
        <point x="115.0" y="13.03" />
        <point x="125.0" y="12.19" />
        <point x="180.0" y="5.18" />
        <point x="195.0" y="3.71" />
        <point x="215.0" y="2.27" />
        <point x="235.0" y="1.32" />
        <point x="260.0" y="0.64" />
        <point x="290.0" y="0.25" />
        <point x="325.0" y="0.18" />

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

2 attributes are defined in the above code: one for visual disturbance and one for AW deciduous yield. The visual disturbance attribute has the parameter 'cycle="false"'. The yield curves for the theme2='AW' type are shown in the figure below. Also show are the hypothetical curves for the theme2='SW' stand types.

The succession statement is identical to the one examined in previous examples. The Matrix Builder will construct feature curves that splice the pre- and post-successional stand characteristics. As the stand breaks up and renews the theme2 value is change to 'SW'. The resulting feature curves for stands that match this select statement would follow the first 150 years from the AW curves and then pick up from year 25 of the SW curves. Since the Visual attribute has the 'cycle="false"' attribute, the Visual curve is only included from the initial condition, and ignored in the post-successional condition. In this case the Visual curve descends to 0 and remains that way until the end of the simulation horizon. If the original curve had extended beyond the breakup age then these values would remain present in the post-successional condition.

Figure 188. Example of feature curves using succession rules

Example of feature curves using succession rules