Nitrogen-fixers Alnus and Lupinus influence soil characteristics but not colonization by later successional species in primary succession on Mount St. Helens

Changes to the primary successional environment caused by colonizing plants that present symbiotic associations with nitrogen-fixing bacteria were investigated at two areas on Mount St. Helens. One area was occupied by alder (Alnus viridis) thickets and old lupine (Lupinus lepidus) patches and the other area by young lupine patches and pumice barrens. Alder thicket soils had higher levels for a few soil nutrients and had greater cover by other pioneer species as compared to old lupine patches. Many soil nutrients, including nitrogen and soil organic matter, were below detection limits in old lupine patches but not in alder thicket soils. Young lupine patch soils were generally not different from barren site soils but had greater cover by other pioneer species. Below detection nitrogen and soil organic matter levels also occurred in many barren soil samples but not in young lupine patch soils. Barren soils were moister than were the other sites. The apparent increase in soil fertility has not led to invasion by later successional species, perhaps due to dry conditions or to other inhibitory factors. Seedbanks, composed of early successional species, appear to be developing in these areas.