Positive plant spatial association with Eriogonum ovalifolium in primary succession on cinder cones: seed-trapping nurse plants

Cinder cones in Craters of the Moon National Monument in southcentral Idaho, USA were formed over 2200 yr B.P., but are still covered by large, relatively barren areas containing a sparse assemblage of plants. The spatial associations among 6 plant species on these sparsely vegetated areas were examined. All species were positively associated with the canopy region of Eriogonum ovalifolium var. depressum, an initial colonizer. All species were negatively associated with Pinus flexilis, a later-successional species. We examined soil seed densities and N and P levels under canopies of the 6 species to develop testable explanations for positive plant associations with E. ovalifolium. Soil seed densities of Phacelia hastata and Lewisia rediviva were higher under canopies of E. ovalifolium than under 4 of the 5 other species and bare areas. Soil total N under E. ovalifolium canopies was also higher than under 4 of the 5 other species and bare areas. Soil seed densities and N levels were similar under E. ovalifolium and Eriogonum umbellatum, both of which had dense, prostrate canopies. Soil available P levels under E. ovalifolium were not significantly different than under other species or bare ground. The positive associations of all species with E. ovalifolium is likely the result of seed-trapping by its prostrate canopy and more favorable establishment conditions under its canopy. The relative importance of these potential mechanisms can be tested with field experiments.