Chemical Properties and Ecology of Andic and Spodic Forest Soils in the Northern Rocky Mountains.

Chemical Properties and Ecology of Andic and Spodic Forest soils in the Northern Rocky Mountains

 

Max Ross

Abstract

 

The Selkirk Mountains of Northern Idaho consists of mountainous terrain dominated by various forest-type habitats. The presence of Andisols, Spodosols, and Spodic Intergrade soils has been established in this region. Due to their differing morphological characteristics, it is suspected that these soils exhibit distinct chemical properties and support unique forest vegetation communities. Chemical analysis includes the quantification of both organically and inorganically associated Fe and Al, using the Sodium Pyrophosphate and Acid-Oxalate Dissolution methods. Exchangeable Al is analyzed using the KCl extraction method, and soil pH is measured using the 1:1 soil:H₂O method. Spodosol E horizons in the study area exhibit exchangeable Al values as high as 3.4 cmol(+)/kg and pH values as low as 3.2. Andisol surface mineral horizons have pH values as low as 4.4. Spodosol Bhs horizons in the study area have ratios of sodium pyrophosphate extractable Fe and Al to ammonium oxalate extractable Fe and Al of up to 0.56 and 0.45, indicating a relatively high proportion of the secondary Fe and Al in the subsoil is organically complexed. Comparatively, the ratio of sodium pyrophosphate to ammonium oxalate extractable Fe and Al in subsurface horizons of Andisols ranges up to 0.21 and 0.27, respectively. Forest vegetation is analyzed using the proportional basal area coverage of mature tree species present at each site, in addition to the presence of regenerating trees and understory species. Forest habitat type is also identified at each site. The most common habitat type for both the Andisol and Spodosol sites is Western Hemlock/Queencup Beadlilly, while the most common among the Spodic Intergrade sites is Western Red Cedar/Queencup Beadlilly. Although habitat types are similar among the soil types, proportional basal area coverage varies along a moisture gradient. Spodosols generally support a higher proportion of species indicative of wetter conditions, such as Western Hemlock and Western Red Cedar, while Andisols support a higher proportion of species indicative of drier environments, such as Grand Fir, Douglas Fir, and Subalpine Fir.