A Comparison of Forest Survey Data with Forest Dynamics Simulators FORCLIM and ZELIG along Climatic Gradients in the Pacific Northwest

By Richard T. Busing and Allen M. Solomon


This report is also available as a pdf.


July 2004


U.S. Geological Survey Scientific Investigation Report 2004-5078




Two forest dynamics simulators are compared along climatic gradients in the Pacific Northwest. The ZELIG and FORCLIM models are tested against forest survey data from western Oregon. Their ability to generate accurate patterns of forest basal area and species composition is evaluated for series of sites with contrasting climate. Projections from both models approximate the basal area and composition patterns for three sites along the elevation gradient at H.J. Andrews Experimental Forest in the western Cascade Range. The ZELIG model is somewhat more accurate than FORCLIM at the two low-elevation sites. Attempts to project forest composition along broader climatic gradients reveal limitations of ZELIG, however. For example, ZELIG is less accurate than FORCLIM at projecting the average composition of a west Cascades ecoregion selected for intensive analysis. Also, along a gradient consisting of several sites on an east to west transect at 44.1oN latitude, both the FORCLIM model and the actual data show strong changes in composition and total basal area, but the ZELIG model shows a limited response. ZELIG does not simulate the declines in forest basal area and the diminished dominance of mesic coniferous species east of the Cascade crest. We conclude that ZELIG is suitable for analyses of certain sites for which it has been calibrated. FORCLIM can be applied in analyses involving a range of climatic conditions without requiring calibration for specific sites.





Models and Field Data

Parameterization of ZELIG

Parameterization of FORCLIM

Types and Sources of Field Data

Model Tests

West Cascades Test

Geographic Transect Test

Forested Ecoregion Test

Actual Vegetation along Test Gradients

Species Pool Effects


Actual Vegetation Patterns

Simulated Vegetation Patterns

Western Cascade Range

Geographic Transect Gradient

Forested Ecoregions





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