Abstract

A well-established and widely used correlative climate-vegetation model (Holdridge Life Zone model) was compared to three mechanistic simulation models (BIOME2, Dynamic Global Phytogeography Model (DOLY), and Mapped Atmosphere-Plant-Soil System (MAPSS)) for the conterminous United States under contemporary climate and a set of future climates prescribed by three Global Circulation Model experiments.  Output from the mechanistic models was from the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) intercomparison.  Holdridge modeling approaches, using a "Simple" implementation (vegetation distribution based on biotemperature and precipitation alone) or a "Full" implementation (vegetation distribution based on biotemperature, precipitation, altitudinal region, latitudinal belt, and transitional vegetation zones), represented current potential natural U.S. vegetation poor to fair, respectively.  The more sophisticated mechanistic models were superior at reproducing potential vegetation under current climate compared to Holdridge, although there was significant variability among these models.  The Holdridge implementations generally showed similar or greater climate sensitivity with respect to spatial redistribution of vegetation compared to the mechanistic models run both with and without doubled CO2 levels; however, the sensitivity of the Holdridge model depended on the implementation.  Reduced sensitivity of the mechanistic models arises from direct (physiological) CO2 effects and other compensating feedbacks not captured by the Holdridge model.  The greater degree of physical realism in the mechanistic models makes them the model class of choice for climate impact assessment.  However, under circumstances of limited data availability, computation resources, and access to mechanistic models and model expertise, simple correlational models such as Holdridge may be the only method that can be applied.  The paper makes some recommendations on the use of the Holdridge model for impact assessment if it is the only available model.

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