LANDFIRE ›› Frequently Asked Questions (FAQs)

Q.	What are reference conditions and how are they estimated?

Reference conditions are the characteristic mosaic of vegetation (including composition and structure) and occurrence of disturbances (including frequency and severity) under the natural range of variability. For LANDFIRE, the natural range of variability is defined as the vegetation and disturbance dynamics that would have existed under current climatic conditions prior to Euro-American settlement. It is important to note that reference conditions do not necessarily equal desired future conditions because desired future conditions may integrate other values (such as communities at risk, management of threatened species, or hydrologic considerations). The reference condition succession and disturbance dynamics were developed during a series of regional vegetation modeling workshops, conducted by The Nature Conservancy. For the Rapid Assessment, reference conditions for the area occupied by each succession class within each potential natural vegetation group are estimated using the aspatial Vegetation Dynamic Development Tool (VDDT; see www.essa.com/vddt for additional information). The simulated outputs from VDDT represent the central tendency of the area occupied by each succession class given the disturbance and succession pathways. The outputs do not represent a range of variability that would arise from spatial or temporal interactions between disturbance and vegetation. For LANDFIRE National, reference conditions of the area occupied by each succession class within each biophysical setting are estimated using chronosequences simulated by the spatially explicit vegetation succession and disturbance model LANDSUM (Keane and others 2002). The simulated chronosequences represent the presumed natural range of variability that would arise from spatial and temporal interactions between disturbance and vegetation. The chronosequences are summarized into a single reference condition value for each succession class within each biophysical setting for use in FRCC departure calculations.

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Keane, R. E., R. Parsons, and P. Hessburg. 2002. Estimating historical range and variation of landscape patch dynamics: Limitations of the simulation approach. Ecological Modelling 151:29-49.