VDDT has been used extensively by LANDFIRE to define reference condition models for the entire US. It is being used by The Nature Conservancy, Parks Canada, the US Forest Service and others to help define the pathways and transition probabilities for the different terrestrial vegetation communities.
How is the landscape characterised?
VDDT assumes that the landscape has been stratified into units with similar transition pathways. The default convention is that this stratification identifies a unique state and transition model. Within each model, vegetation states are defined as combinations of the predominant cover type and structural stage, called state classes. Movements between classes are described by two types of pathways: changes driven by probabilistic transitions (e.g., fire, climate) and deterministic changes due to the passage of time (e.g., regeneration, growth, or self-thinning).
State classes are shown with boxes; transitions between classes are marked with arrows.
Probabilistic transition pathways specify, for each class, the type of transition, its probability (which defines the return frequency) and its impact on vegetation. Changes due to deterministic transitions are defined by the time a cell remains in a state class and by the new state class to which it will move after this time has elapsed. VDDT translates the information on transition pathways into a diagram on the screen.
How does it work?
Each VDDT model simulates the changes that occur within one type of vegetation community. This community is represented by a number of cells, each initially assigned an age and state class. Using the pathways and probabilities defined for that vegetation community, the model simulates the probability of each cell being affected by one of the transition types, and if a transition does occur, moves the cell to the appropriate class. Disturbance probabilities depend on the current state of the cell, defined by its state class. They are independent of the state of the neighbouring cells and their transition history. Thus, unlike TELSA, the model does not simulate contagion in space (e.g., wildfire) or time (e.g., invasive plants).
What does a user need to define?
Users need to define the state classes, pathways and transition probabilities for each model. A model may have more than one set of probabilities defined to represent different management regimes or ecological conditions. Creating a database with this information may easily be done entirely within VDDT. VDDT comes with a sample database and model. Users may use this sample dataset when creating their own model or they may create an entirely new model from scratch.
What options are there?
VDDT is a flexible tool for examining the effects of different probabilities or pathways on landscape change patterns. Options available within VDDT include:
- showing only certain pathways (those with non-zero probabilities, those going to and from a state class, those for certain disturbances only, etc.)
- adding or deleting pathways
- changing the probability or transition type for a pathway (e.g., to simulate fire suppression policies)
- disabling some transitions for the current simulation only
- adding, deleting or changing information for state classes
- adding external attributes (such as fuel loads and habitat quality) to classes
- changing the initial cell distribution between classes (including using the ending results from a previous simulation or using the actual distribution in the landscape)
What results are produced?
Model results can be viewed as the changes in the distribution of the cells in different categories (such as state class, structural stage, cover type or area affected by different transition types) at a single point in time.
Graphs over time can be created, showing the change in the proportion of cells in a given class (upper set) or the percentage of cells that were affected by disturbance or succession (lower set).
Additionally, results can be printed to a *.CSV file, suitable for use in external statistical or graphics packages.