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    Author(s): Donald C.E. Robinson; Sarah J. Beukema
    Date: 2012
    Source: In: Kerns, Becky K.; Shlisky, Ayn J.; Daniel, Colin J., tech. eds. Proceedings of the First Landscape State-and-Transition Simulation Modeling Conference, June 14–16, 2011, Portland, Oregon. Gen. Tech. Rep. PNW-GTR-869. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 123-142.
    Publication Series: General Technical Report (GTR)
    Station: Pacific Northwest Research Station
    PDF: Download Publication  (468.58 KB)

    Description

    Land managers commonly use FVS and VDDT as planning aids. Although complementary, the models differ in their approach to projection, spatial and temporal resolution, simulation units and required input. When both are used, comparison of the model projections helps to identify differences in the assumptions of the two models and hopefully will result in more consistent results across models.

    We used the transition probability matrix as the basis for comparing the two models, using side-by-side simulations with FVS and VDDT to project 250 mixed conifer stands. We designed and carried out a simulation experiment with managed and unmanaged scenarios, to explore the consequences of different approaches to filtering FVS outputs by removing censored and rare observations, as well as smoothing out side-effects such as jitter which result from creating categories from continuous data.

    Our analysis includes verification of the Preside system, comparison of matrix behavior, as well as the behavior of VDDT with FVS runs imported into VDDT. Three useful conclusions are that: (1) including rare transitions from FVS is important to getting reasonable temporal dynamics; (2) initialization and censoring issues can be ignored; and (3) smoothing and jitter can also be ignored.

    One surprising conclusion is that very different assumptions about regeneration cause VDDT and FVS results to be profoundly different for species, size and canopy structure. One nagging question is “how can we tell which model is right?” Field observations would help, and iterative model revision of both FVS and VDDT models is also helpful to a point. Our best advice is to use each model to challenge and improve the assumptions of the other, using each model to illuminate the “blind spots” of the other.

    Publication Notes

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    • This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.

    Citation

    Robinson, Donald C.E.; Beukema, Sarah J. 2012. Through a glass, darkly—comparing VDDT and FVS. In: Kerns, Becky K.; Shlisky, Ayn J.; Daniel, Colin J., tech. eds. Proceedings of the First Landscape State-and-Transition Simulation Modeling Conference, June 14–16, 2011, Portland, Oregon. Gen. Tech. Rep. PNW-GTR-869. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 123-142.

    Keywords

    Forest Vegetation Simulator, Vegetation Dynamics Display Tool, FVS, VDDT, scale, simulation.

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