CHEMICAL AND MINERALOGICAL CONVERSION OF ANDISOLS FOLLOWING INVASION BY BRACKEN FERN

Andisols support approximate to 200 000 ha of mid-elevation grand fir (Abies grandis [Dougl. ex D.Don] Lindl.) forests in the Pacific Northw est region that are characterized by little or no natural conifer rege neration following removal of the forest canopy. Previous work suggest s 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 stud y, 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 cen turies old, and an adjacent undisturbed forest in the Clearwater Natio nal Forest of northern Idaho. Results indicate that changes in chemica l properties have accompanied establishment of successional communitie s. Mean weighted pH within the ash cap of the 30-yr-old bracken fern s ite (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 lo wer 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-fer n-influenced soils. Data indicate that bracken fern successional commu nities are responsible for a shift from allophanic to nonallophanic pr operties in these soils, probably due to increased levels of soil orga nic C associated with bracken fern and a subsequent increase in format ion of Al-humus complexes. Furthermore, such a mineralogical shift may contribute to the observed problems with conifer regeneration.