Trees
In this document:
Tree background
Tree life history stage transitioning - growth and death
Tree parameters
The basic unit of data in the model is the tree. Each tree is a discrete individual with a location in space and attributes which describe its size and shape. More information about how tree size is measured and how different aspects of tree shape are calculated can be found in the Allometry topic. In addition, a tree may carry data added to it by Behaviors to help simulate different processes.
Tree life history stages
Tree life history stage (also referred to as tree type), along with species, is the basic way to classify trees. When you set up behaviors for a run, you tell each behavior which trees to act on by species and type. There is support for seven tree life history stages in the model:
- Seed. The seed life history stage is currently not used by any behavior, but is available if there is a need to model seeds as individuals.
- Seedling. Seedlings are defined as trees less than 1.35 meters in height (and thus with no DBH). Their primary size measurement is the diameter at 10 cm height.
- Sapling. Saplings are defined as having a DBH greater than 0 and less than the Minimum adult DBH defined in the tree parameters. Seedlings and saplings are sometimes referred to collectively as "juveniles".
- Adult. Adults are defined as having a DBH equal to or greater than the Minimum adult DBH defined in the tree parameters.
- Stump. Stumps are saplings or adults that have been cut by the Harvest behavior. Stumps exist only for the timestep in which they were created, and then disappear.
- Snag. Snags are standing dead trees. They can be produced when saplings and adults die due to normal tree mortality or a disturbance event, such as disease. Only adult trees become snags. See below for more on how trees become snags.
- Woody debris. Woody debris comes from fallen snags. Currently, no behavior uses woody debris and it is not actually created.
Tree life history stage transitioning - growth and death
Trees transition between life history stages as they grow. When a seedling reaches 1.35 m in height, it becomes a sapling. The diam10 value is converted to a DBH value, which is then used to calculate the rest of the sapling's new dimensions. Since height is re-calculated with a different equation and input parameters, there may be a discontinuity in height values right around the seedling/sapling transition point. If a species uses different allometric relationships for its saplings and adults, another discontinuity may occur at the time of this transition as well. For more on the allometric relationships and how they are calculated, see the Allometry topic. (The automatic updating of these allometric relationships during the growth phase can be overridden. For more, see the Growth behaviors topic.)
Death also produces tree life history stage transitions. Behaviors can request to a tree population that a tree be killed. How the tree population responds to this request depends on the type of tree, the reason for death, and the type of run. The reasons why a tree is killed are natural causes, harvest, insects, fire, and disease (there may not be behaviors set up to create tree death for all of these reasons).
There are life history stages for dead trees, but a run may not be set up to handle them. The tree population takes this into account. It examines the run to see if any behaviors directly deal with stumps and snags. If either is the case, the run is classified as "stump aware" and/or "snag aware".
Here's what happens to a tree to be killed in different situations:
- If a tree is a seedling, it is deleted from memory no matter why it died.
- If a tree is a sapling or adult killed in a harvest, and the run is "stump aware", the tree is converted to a stump.
- Saplings killed for any other reason, or by harvest in a run that is not "stump aware", are deleted from memory.
- If the tree is an adult killed by harvest and the run is not "stump aware", it is deleted from memory.
- If the tree is an adult killed for any reason other than harvest, and the run is "snag aware", the tree is converted to a snag.
- If the tree is an adult killed for any reason other than harvest, and the run is NOT "snag aware", the tree is removed from memory.
- If the tree is already a snag, it is removed from memory.
Tree organization
Trees are organized by location and size in what is called the tree population. The tree population indexes trees into 8 m by 8 m areas, lined up by height. The population acts as a librarian for behaviors that wish to find certain sets of trees upon which to act.
Tree population initial conditions
When setting up a simulation, a key part is to define the initial tree population. There are two ways to add trees at the beginning of the run, and they can be used together or separately. The first is to ask the model to create trees for you by describing a particular pattern of densities by species and size range (size class). You define size classes in the Tree setup window, and enter densities in the Parameters window. The second way is to directly list a particular set of trees in a tree map within the parameter file.
- Initial Densities The density of trees, in number per hectare, for that size class. Trees are randomly placed within the chosen size range, which starts at the value of the size class beneath (or 0 if there is none) and ends at the value for the current size class. The aim is for an even distribution. You can also set up a size class for seedlings. Seedlings set up this way are given a slightly randomized value of "New seedling diameter at 10 cm", below. You can further specify seedling initial densities using the seedling height class parameters. You can set up size classes in the Tree setup window.
- Initial Density (#/ha) - Seedling Height Class 1 Number of seedlings per hectare to create in the first seedling height class (not required). The lower bound of this class is 0 cm and the upper bound is the value in the Seedling Height Class 1 Upper Bound, in cm parameter.
- Initial Density (#/ha) - Seedling Height Class 2 Number of seedlings per hectare to create in the second seedling height class (not required). The lower bound of this class is the value in the Seedling Height Class 1 Upper Bound, in cm parameter and the upper bound is the value in the Seedling Height Class 2 Upper Bound, in cm parameter.
- Initial Density (#/ha) - Seedling Height Class 3 Number of seedlings per hectare to create in the third seedling height class (not required). The lower bound of this class is the value in the Seedling Height Class 2 Upper Bound, in cm parameter and the upper bound is 135 cm (the tallest possible seedling height).
- Minimum Adult DBH The minimum DBH at which trees are considered adults. (See more about tree life history stages, above.)
- New Seedling Diameter at 10 cm The average diameter at 10 cm height value for newly created seedlings, when another size is not specified. Actual values are randomized slightly around this value.
- Seedling Height Class 1 Upper Bound, in cm The upper bound of the first seedling height class, in cm, for specifying seedling initial densities (not required). The lower bound of the size class is 0.
- Seedling Height Class 2 Upper Bound, in cm The upper bound of the second seedling height class, in cm, for specifying seedling initial densities (not required). The lower bound of the size class is the Seedling Height Class 2 Upper Bound, in cm parameter. There is a third size class, whose lower bound is this parameter's value and whose upper bound is 135 cm.
In addition to the values listed in the parameter window, the
tree population also keeps the list of species and size classes.
These can be edited in the Tree setup window.
Last updated: 28-Apr-2005 10:19 AM