Effects of Bracken Fern Invasions on Harvested Site Soils in Pacific Northwest (USA) Coniferous Forests

Bracken ferns are known to dominate early successional vegetative assemblages in disturbed coniferous forests of the Pacific Northwest often resulting in a suppression of conifer seedling establishment. This study was designed to explore mechanisms by which this suppression might occur. Bracken fern induced changes in forest soils were studied at 10 separate sites in the Oregon Cascade Mountains in or near the H. J. Andrews Experimental Forest. All sites contained uncut mature forests and adjacent harvested stands that had been clear-cut 15–20 years prior to this study. Each harvested unit contained areas dominated by bracken fern. Of the 10 harvested units studied, six also had areas dominated by woody shrubs. Samples were collected both in spring/early summer when soils were relatively cool and moist and in summer/early fall when soils were warmer and drier. In spring/early summer, fern plots were significantly cooler than mature forest or nearby shrub areas. Fern soils were also significantly lower in K and NO3-N, and had lower denitrification enzyme activity (DEA) rates. In summer/early fall, NH4-N, NO3-N, DEA, and phosphatase activity were all significantly lower in fern plots. Of all variables, mineral N pools showed the strongest consistent trends differentiating fern from conifer soils. Our data suggest that bracken fern may have a competitive advantage by sequestering N to the point that reduced soil mineral N pools limit growth of competing vegetation.