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Sarah Chisholm

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  • Sarah ChisholmSarah Chisholm
    Moderator

    Hi there,

    Leonardo, who is our ST-Sim expert, is out of office until Tuesday next week.
    According to our ST-Sim documentation, the spread distance is accumulated over timesteps with the origin point at the center of a cell. Here’s the corresponding excerpt from the documentation:

    “Each timestep the model samples from the Spread Distribution for each neighbor of each cell that is contagious and tracks the accumulated spread distance in that direction. Once the accumulated spread distance exceeds the center to center distance between neighbors, the model will apply a transition from the contagious transition group to the target neighbor.”

    Here’s a link to the Transition Spread Distribution documentation: https://docs.stsim.net/reference/prop_transition_spread_dist.html

    I will confirm with Leonardo when he is back in office next week if I missed any details,
    Sarah

    in reply to: Advanced menu -> Referenc Condition #21059
    Sarah ChisholmSarah Chisholm
    Moderator

    Hi Louis,

    Thanks for reporting this issue. We have identified a bug that has recently been introduced to this feature and we are currently working to resolve it. A new fixed version of the package will be released and we will let folks know when it is ready.

    Thanks for your patience,
    Sarah

    Sarah ChisholmSarah Chisholm
    Moderator

    Hi there, Here’s an example library of worked solutions to your questions, followed by a description of the library below:

    Example Library: https://s3.us-west-2.amazonaws.com/apexrms.com.public/stsim-forum/Forum-Post-2022-06-14.ssim.backup.2022-06-16-at-14-23-45.zip

    Part 1: Repeated fires required to transition to woodland

    There are two approaches to conditionally trigger a transition based on the number of disturbances that occur in a pixel. The simpler approach requires that you create more state classes (see the ‘Definitions – States’ Project in the example library). For example, to trigger a transition from Forest to Woodland only after three low severity burns, you will need three Subclasses of the Forest Class representing 1) unburned Forest cells, 2) Forest cells burned once, and 3) Forest cells burned twice (see the ‘States’ tab in the ‘Definitions – States’ Project). In the Project Definitions, you’ll also need to define a ‘Fire – Low Severity’ transition. The last step is to add the appropriate Transition Pathways to a Scenario (see the ‘[22] Forest to Woodland – Using States’ Scenario). In this non-spatial single cell model, the State Class is initialized as unburned Forest. In the Transition Pathways, an unburned Forest cell has a 0.1 probability of transitioning to a Forest burned once after a low severity fire (note that all transition probabilities in this example are arbitrary). Once in the ‘Forest:Burned Once’ Subclass, low severity fires that occur within 20 years will transition this cell to the ‘Forest:Burned Twice’ and finally to the ‘Woodland:All’ Subclass. Burned forest cells can transition back to unburned forest if a low severity fire does not occur within 20 years following the previous fire (See Result Scenario [23]).

    The second solution requires fewer State Classes, but more Transition Types and a Transition Group (see the ‘Definitions – TST’ Project). In this Project, only two State Classes are defined (‘Forest:All’ and ‘Woodland:All’). Under the ‘Transitions’ tab in the ‘Transition Type’ node, you’ll find that three types of low severity fires indicating the number of previous burns are defined. These Transition Types also belong to a single Transition Group, ‘Low Severity Fire’. In the child Scenario ([1] Forest to Woodland – Using TST), three Transition Pathways are defined, where the different low severity fire types sequentially transition Forest cells to Woodland after a cell has experienced a third burn. Under the ‘Initial Conditions’ tab, the ‘Initial TST Randomize’ node, TST for all Fire types are set to a minimum of 1000 years to set the initial time since a fire to a very large number; in other words, this landscape has not experienced recent fire. Lastly, under the ‘Advanced’ tab, the ‘Transition – Multipliers’ node is used to define at what time since the last fire each Transition Type and Group may occur.

    Part 2: Transition from woodland to forest in the absence of fire, maintenance of woodland with fire

    To model the succession of Woodland to Forest, two State Classes (Forest:All, Woodland:All) and two Transition Types (Fire, Succession to Forest) are needed (see the ‘Definitions – Woodland Succession’ Project). In the ‘[25] Woodland Succession’ baseline Scenario, the landscape is initialized in the Woodland State Class under the ‘Initial Conditions’ tab. Two Transition Pathways are defined, where 1) Woodland cells transition to Forest via succession after 10 years, and 2) Woodland is maintained if there is a fire. Under the ‘Initial Conditions’ tab, the ‘Initial TST Randomize’ node, time since transition is randomized for Fire when cells are initialized so that all cells have had a Fire between 0 and 10 years ago. Lastly, under the ‘Advanced’ tab in the ‘Time Since Transition Group’ node, the Succession to Forest Transition Type is associated with the Fire Transition Type. This sets the probability for Succession to be contingent on the time since a fire. In the ‘[28] Woodland Succession – Double Fire’ Scenario, a Transition Multiplier (under ‘Advanced’) is used to double the probability of fire, resulting in a slower progression out of Woodland to Forest (see Result Scenarios [27] and [29] to compare).

    You can find a similar question posted to the forum here: https://syncrosim.com/forums/topic/using-tst-to-conditionally-set-transition-probabilities/
    For more information on the Time Since Transition Group Datasheet, visit the online ST-Sim Reference pages: https://docs.stsim.net/reference/prop_time_since_transition_groups.html

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