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Chemical and Mineralogical Conversion of Andisols following Invasion by Bracken Fern

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside CA 92521

P. A. McDaniel and A. L. Falen

Soil Science Division, Univ. of Idaho, Moscow, ID 83844-2339

D. E. Ferguson

USDA Forest Service, Intermountain Res. Stn., Moscow, ID 83843

*Corresponding author (jmaynard{at}ucracl.ucr.edu).

ABSTRACT

Andisols support 200 000 ha of mid-elevation grand fir (Abies grandis [Dougl. ex D.Don] Lindl.) forests in the Pacific Northwest region that are characterized by little or no natural conifer regeneration following removal of the forest canopy. Previous work suggests that the properties of these Andisols have been altered as a result of the establishment of successional communities dominated by bracken fern (Pteridium aquilinum [L.] Kuhn) in deforested areas. In this study, we compared soil properties in a 30-yr-old bracken fern site (clear-cut in 1965), a natural bracken fern site that is estimated to be centuries old, and an adjacent undisturbed forest in the Clearwater National Forest of northern Idaho. Results indicate that changes in chemical properties have accompanied establishment of successional communities. Mean weighted pH within the ash cap of the 30-yr-old bracken fern site (4.6) is significantly lower than that of the undisturbed forest (5.2). Mean values for Al saturation range from 27% in the undisturbed forest to 52% in the 30-yr-old bracken fern site; organic C is also lower in the undisturbed forest (37 g/kg) than in the 30-yr-old bracken fern site (54 g/kg). The dominant secondary mineralogical component of soils of the undisturbed forest is inorganic, short-range-order Al-Fe minerals, while metal-humus complexes are dominant in the bracken-fern-influenced soils. Data indicate that bracken fern successional communities are responsible for a shift from allophanic to nonallophanic properties in these soils, probably due to increased levels of soil organic C associated with bracken fern and a subsequent increase in formation of Al-humus complexes. Furthermore, such a mineralogical shift may contribute to the observed problems with conifer regeneration.

Received for publication March 21, 1996.

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