Sensitivity of a biogeography model to soil properties

This paper presents the changes in vegetation distribution and hydrological balance resulting from a change in soils input data to the biogeography model MAPSS (Neilson, 1995). The model was run for the conterminous United States using three different sets of soil characteristics: (1) all soils were assumed to be sandy loam; (2) soils characteristics came from the Food and Agriculture Organization (FAO) soils map of the world (FAO, 1974–1979) and (3) soil characteristics came from the Natural Resource Conservation Service (NRCS) National Soil Geographic (NATSGO) dataset. Resulting changes in vegetation distribution appear small on a country-wide basis, but large changes in simulated runoff in savannas, shrublands and deserts reflect the importance of using the best available soils dataset. In the state of Oregon, a 16% relative decrease in forest areal extent is accompanied by an 18% relative increase in shrubland when switching from FAO to NATSGO datasets. Conversely, forest cover increases by 24% while shrubland extent decreases by 14% when all Oregon soils are assumed to be sandy loam. MAPSS vegetation distribution projections were compared to Küchlerʹs potential vegetation map (Küchler, 1964). The generalization of all US soils to sandy loam soils decreases the similarity between MAPSS predictions and Küchlerʹs map and is clearly inappropriate. If the similarity between MAPSS projections and Küchlerʹs map does not clearly improve by using NATSGO rather than FAO soils data, NATSGO soil representation is more reliable and thus we recommend using it in the future.