NITROGEN FIXATION: A LARGELY UNEXPLORED DRIVER OF CENOZOIC PLANT EVOLUTION, BIOGEOGRAPHICAL RANGES, AND PALEOECOLOGICAL TRENDS

One biological process of largely unexplored consequence to Cenozoic plant evolution and biogeographic and ecological trends is the rise of symbiotic associations with nitrogen-fixing bacteria. The best known of these associations today are between certain legumes and Rhizobium, however, ecologically important connections also occur in over a dozen families of angiosperms including Rosaceae and Betulaceae, not to mention cycads and heterosporous water ferns. Along with Rhizobium, the bacteria Frankia, Anabaena, Nostoc and others act as symbionts with vascular plants to bring them a direct supply of accessible nitrogen essential for their growth and development in nutrient-poor soils. A wide range of researchers from theorists (e.g., J. Raven) to microbiologists (e.g., Boyd and Peters 2013) have proposed hypotheses suggesting a variety of mechanisms for the ancient origins and radiations of plant-nitrogen-fixing bacteria symbiotic relationships.

Recent studies have recognized the ubiquitous distribution of the water fern Azolla, a plant supported by the nitrogen-fixer Anabaena today, in the Northern Hemisphere high latitudes during the Cretaceous. During the Paleocene-Eocene Thermal Maximum (PETM) period of greenhouse conditions, the ability of legumes and other plants to obtain abundant N contributed to its highly productive ecosystem. It is noteworthy that today, legumes with Rhizobium symbiosis are common in temperate regions but are rarely found in the tropics. In some cases, such as water ferns, it appears that the genus with a mutualistic relationship (Azolla) outcompetes the one lacking such a symbiosis (Salvinia). After the PETM, early Eocene families that become dominant elements of temperate floras diversify in such areas as the Okanogan Highlands of northeastern Washington and central British Columbia, Canada. These include Betulaceae (Alnus), Rosaceae [Cercocarpus, Stonebergia, (a relative of Chamaebatia)], Myricaceae (Comptonia), and Ulmaceae (Ulmus), families that host the nitrogen-fixer Frankia.

Predicting patterns of mutualism of plants with nitrogen-fixing bacteria in both fossil and contemporary ecosystems may provide another filter for understanding the operation of a major driver of ecosystem success both today and in the past.