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The Role of Lupine in Succession on Mount St. Helens: Facilitation or Inhibition?

Morris, William F., Wood, David M.
Ecology 1989 v.70 no.3 pp. 697-703
Anaphalis, Epilobium angustifolium, Lupinus, aboveground biomass, flowering, greenhouses, landscapes, models, mulches, mulching, nitrogen fixation, pioneer species, planting, primary succession, probability, seedlings, survival rate, Washington (state)
The barren landscape created on the north side of Mount St. Helens (Washington State, USA) by the 1980 eruption provides an excellent setting in which to examine the role of pioneer species in facilitating or inhibiting subsequent invaders in primary succession. We investigated the influence of Lupinus lepidus, a nitrogen—fixing pioneer species, on two invading species, Anaphalis margaritacea and Epilobium angustifolium. Seedlings of both invaders were initiated in the greenhouse and transplanted to four field treatments: (1) control (plots devoid of lupine), (2) live (plots with vigorous lupine), (3) mulch (lupines herbicided but left in place), and (4) no—mulch (lupines herbicided and dead aboveground biomass removed). Patches of L. lepidus exerted both facilitative and inhibitory effects on the other species. First season survivorship of seedlings planted into lupine patches was generally lower than that of seedlings planted into barren control plots. However, for both A. margaritacea and E. angustifolium, surviving seedlings within lupine patches grew larger than did controls. In addition, A. margaritacea seedlings had a much higher probability of flowering when planted within lupine patches. Comparisons among treatments indicated that both substrate alteration and the mulching effect of lupine litter mediated the effects of lupin patches on transplant performance. Our results show that both facilitation and inhibition occurred, but at different stages in the life cycle of invading species. Consequently, a complete demographic model may be needed in order to assess the net effect of a pioneer on its successors.