CHANGES IN SOIL SOLUTION CHEMISTRY OF ANDISOLS FOLLOWING INVASION BY BRACKEN FERN

Disturbed areas within the Grand Fir Mosaic (GFM) ecosystem of norther n Idaho show little to no natural conifer regeneration. Clear-cut site s are invaded quickly by bracken fern successional communities and see m to be in an arrested state of secondary succession. This study compa red the soil solution composition of Andisols supporting bracken fern successional communities with undisturbed forest to determine the effe cts of shifts in vegetation communities. Treatment areas included undi sturbed forest, a 30-year-old bracken fern glade (clear-cut in 1965), and a natural bracken fern glade estimated to be centuries old. The na tural bracken fern glade was divided into subplots, one of which has b een 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 t he other sites throughout the sampling period, and the 30-year-old gla de was the only site to periodically register below pH 5, The natural bracken fern glade that had been weeded was more similar to the undist urbed forest, suggesting that bracken fern biomass does have an acidif ying effect on soil solution. The highest Al concentration recorded wa s 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 t o be positively related, with r values of 0.533 and 0.824 for 1994 and 1995, respectively. These results indicate that bracken fern does hav e an acidifying effect on soil solution. Aluminum concentrations were lower than reported toxic levels for other conifer species and were co rrelated with DOG. These two findings suggest that Al toxicity may not be a major factor contributing to the lack of conifer regeneration wi thin the GFM.