Nitrogen fixation by Cercocarpus ledifolius (Rosaceae) in pioneer habitats.

Soil properties of pioneer Pinus flexilis stands with similar topography and climate were investigated. Soils supporting this tree in association with Cercocarpus ledifolius were found to have higher percentages of total nitrogen than soils beneath similar stands lacking Cercocarpus.An excavated Cercocarpus ledifolius shrub in a Pinus flexilis stand in the San Bernardino Mountains of California was found to be nodulated and these nodules were found to be capable of fixing nitrogen. Other known nitrogen-fixing shrubs are frequent associates of Pinus flexilis in extreme sites.Implications of the phylogenetic relationship of some nitrogen-fixing species are briefly discussed, as well as their frequent occurrence in pioneer or extreme habitats.