A. L. Falen

Expression of andic and spodic properties in tephra-influenced soils of northern Idaho, USA

Abstract Tephra from Mount Mazama (Crater Lake) has strongly influenced genesis and morphology of forest soils in the Selkirk Mountains of northern Idaho. Characteristics associated with Andisols and their intergrades are common in soils throughout the area. In addition, tephra-influenced soils of the higher elevations exhibit properties of Spodosols as well. This study was initiated to examine development and relative expression of these properties along an elevational transect with its associated bioclimatic gradient. This paper presents data from three soils that encompass the range in expression of andic and spodic properties found along this gradient. Cooler, moister soils of the higher elevations have formed under subalpine fir forest and are strongly influenced by volcanic ash as indicated by color. NaF-pH, glass content, and quantities of oxalate-extractable Fe, Al, and Si. Additionally, podzolization processes have created E-Bhs horizon sequences in these soils within the last 6800 years. The clay fraction of the very strongly acid E horizon is dominated by a well-crystallized smectite mineral. Formation of an E horizon has effectively erased andic characteristics such as accumulation of short-range order minerals (SROMs), high NaF-pH, and high P retention. Soils of the middle elevations have formed under western hemlock forest and also have significant ash influence. These soils have properties transitional to those of the higher-elevation Spodosols and do not exhibit the overprint of podzolization processes. Eluvial horizons are absent although there has been significant accumulation of metal-organic complexes and inorganic SROMs in the upper horizons of these Andisols. Soils of the lower elevations have developed under relatively warmer and drier conditions associated with the lower end of the western hemlock forest zone and have comparatively less volcanic ash influence. The ash has been extensively mixed with coarse-textured glacial drift, and, as a result, these lower elevation soils lack the morphological or chemical properties associated with Spodosol development but do exhibit weak andic characteristics.

Changes in soil solution chemistry of Andisols following invasion by bracken fern

Disturbed areas within the Grand Fir Mosaic (GFM) ecosystem of northern Idaho show little to no natural conifer regeneration. Clear-cut sites are invaded quickly by bracken fern successional communities and seem to be in an arrested state of secondary succession. This study compared the soil solution composition of Andisols supporting bracken fern successional communities with undisturbed forest to determine the effects of shifts in vegetation communities. Treatment areas included undisturbed forest, a 30-year-old bracken fern glade (clear-cut in 1965), and a natural bracken fern glade estimated to be centuries old. The natural bracken fern glade was divided into subplots, one of which has been weeded 2 to 3 times each growing season since 1988. Soil solution was collected in porous ceramic cup lysimeters at 12- and 25-cm depths. Samples were collected from May to July in 1994 and 1995. Solutions were analyzed for pH, Al, and dissolved organic carbon (DOC). The soil solution pH in the 30-year-old glade was consistently lower than in the other sites throughout the sampling period, and the 30-year-old glade was the only site to periodically register below pH 5. The natural bracken fern glade that had been weeded was more similar to the undisturbed forest, suggesting that bracken fern biomass does have an acidifying effect on soil solution. The highest Al concentration recorded was 1.6 mg/L in the 30-year-old glade, whereas in the undisturbed forest Al was often below the detection limit. Aluminum and DOC were found to be positively related, with r values of 0.533 and 0.824 for 1994 and 1995, respectively. These results indicate that bracken fern does have an acidifying effect on soil solution. Aluminum concentrations were lower than reported toxic levels for other conifer species and were correlated with DOC. These two findings suggest that Al toxicity may not be a major factor contributing to the lack of conifer regeneration within the GFM.