The ecology and biogeography of the Ceanothus-Frankia symbiosis in California

Nitrogen fixing symbioses perform important ecological functions and provide fertile ground for the study of many basic and applied ecological questions. This dissertation examines several aspects of the fire ecology and biogeography of the Ceanothus-Frankia symbiosis in California.; As part of the first comprehensive vegetation survey of the Teakettle Experimental Forest, an old-growth mixed-conifer forest in the southern Sierra Nevada, C. cordulatus was found to be the most common and dominant understory plant. Because of the importance of fire in maintaining the dominance of Ceanothus in this old-growth forest, I performed a controlled burn experiment in the field to determine the effects of fire on Ceanothus and the symbiotic N-fixing bacteria Frankia. Before burning, soil beneath Ceanothus was significantly enriched in total and inorganic N relative to two other common understory patch types. After burning, Ceanothus patches generally remained significantly enriched in soil N. Because subsurface soil heating was of limited magnitude and duration and postfire resprouting of Ceanothus was generally vigorous, I concluded that Frankia is an important biological legacy surviving fire.; To determine the effects on tree growth of the soil N enrichment associated with Ceanothus patches, I compared multiple growth parameters of coniferous tree saplings growing in the same three patch types utilized in the burn experiment. Growth rates and foliar N content of trees growing in Ceanothus patches were slightly greater than in other patch types, but not significantly so. Although Ceanothus patches represent resource islands of available N, they did not impart a significant growth advantage to coniferous saplings in this ecosystem. Light and moisture appear to be more limiting for tree growth than soil N.; Lastly, I applied statistical inference and experimental manipulation to determine what factors explained the molecular diversity of Frankia symbiotic with Ceanothus in the study area and throughout California. Frankia strain identity was most closely related to host species identity. This result was confirmed by DNA sequences of Frankia strains from nodules formed on C. cordulatus in the greenhouse in Seattle which were nearly identical to Frankia collected from the same host in California despite very different growing environments.