Design and evaluation of a forest landscape change model for western Oregon

Abstract

This article describes the design and evaluation of a forest landscape model, called LandMod, developed by scaling a forest gap model to operate at a coarser resolution. LandMod is a spatially explicit, stochastic model designed to simulate forest dynamics in the west-central Oregon Cascades over long time frames (500+ years) and large spatial extents (≥18,000 ha) at a relatively fine grain (0.04–1 ha). LandMod tracks diameter growth, death, and regeneration of individual tree species in 5-cm size classes at a 5-year time step. Demographics are modeled using simplified procedures from the PNWGap gap model and statistical abstractions of gap-model behavior. LandMod was parameterized for the three predominant forest types of the western Oregon Cascades. Performance of the underlying equations of LandMod was assessed by comparison of predictions with those of the PNWGap model over an elevation and thinning gradient, and with field observations. Landscape-scale performance was assessed by comparing LandMod predictions of potential natural vegetation with empirically based estimates for an 18,000-ha watershed. Results of performance assessments indicated reasonable predictions with LandMod. Compared to PNWGap predictions and observed stands, percent critical errors ( α=0.05) of predictions for dominant tree species and stand-level measures with LandMod ranged from 1.4 to 29% with the majority of critical errors less than 15%. LandMod predictions of potential natural vegetation closely matched empirical estimates, with an average overall fit of 94% (S.E.=0.01). Reasons for prediction error included under-prediction of canopy-stem size in old-growth stands and of mean size of sub-dominant species. Also, simplified light calculations in LandMod resulted in the under-prediction of stem growth under canopy structures induced by certain thinning strategies. Enhancements are recommended to improve model predictions. Intended applications with LandMod include ecological assessments of land-use strategies and research assessments of landscape pattern–process interactions that require explicit consideration of forest structure.