TheGrandFirMosaic�(GFM)�ecosystemisfoundonash-capsoilsinsomemid-elevationforests� ofnorthernIdahoandnortheasternOregon.�HarvestingonGFMsitesresultsinsuccessionalplant� communitiesthataredominatedbybrackenfern� (Pteridium aquilinum)� andwesternconeflower� (Rudbeckia occidentalis),�andhavelargepopulationsofpocketgophers�(Thomomys talpoides).�Succession� totreesandshrubsisveryslowondisturbedGFMsites.�Fourfactorscontributetotheprotractedstages� ofbrackenfernplantcommunitiesandtheslowrateofsuccessiontowoodyplants:�(1)�competition� amongplantsforsiteresources,�(2)�allelopathyfrombrackenfernandwesternconeflower,�(3)�pocket� gopheractivity,�and�(4)�anonallophanicsoilformingprocess.�Nonallophanicsoilsoccurunderthe� brackenfernsuccessionalplantcommunities�—�theyaredominatedbyAl-humuscomplexes,�have� stronglyacidpH,�havehighKCl-extractableAl,�andmaycauseAltoxicitytoplants.�Allophanicsoils� occurunderforestedconditions�—�theyaredominatedbyallophaneandimogolite,�haveweaklyto� moderatelyacidpH,�andhavelowAlavailability.�Allophanicandnonallophanicsoilsexistside-by- sideintheGFM,�withmineralogybeingdependentuponthedominantvegetation.�Brackenfernand� westernconeflower,�withbelow-groundcarboninputsfromtheirwell-developedrootsystems,�provide� amechanismthatpromotestheshiftfromallophanictononallophanicsoils.�Recommendationsfor� reforestingGFMsitesareprovided.

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 DOG. These two findings suggest that Al toxicity may not be a major factor contributing to the lack of conifer regeneration within the GFM.

Andisols support approximate to 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.