lobata Nyman Alnus microphylla Arv.-Touv. Alnus obtusifolia Mert. ex Regel Alnus oxyacantha Lavalle Alnus subrotunda Desf. Alnus vilmoriana Lebas Alnus washingtonia Wetzel Hybrids The following hybrids have been described: Alnus × elliptica Req.—Italy. (A. cordata × A. glutinosa) Alnus × fallacina Callier—Ohio, New York State, Vermont, New Hampshire, Maine. (A. incana subsp. rugosa × A. serrulata) Alnus × hanedae Suyinata—Japan. (A. firma × A. sieboldiana) Alnus × hosoii Mizush.—Japan. (A. maximowiczii × A. pendula) Alnus × mayrii Callier—Russian Far East, Japan. (A. hirsuta × A. japonica) Alnus × peculiaris Hiyama—Kyūshū Island in Japan. (A. firma × A. pendula) Alnus × pubescens Tausch.—Northern and central Europe. (A. glutinosa × A. incana) Alnus × suginoi Sugim.—Japan. The status of the following hybrids is unresolved: Alnus × aschersoniana Callier Alnus × koehnei Callier Alnus × ljungeri Murai Alnus × purpusii Callier Alnus × silesiaca Fiek Alnus × spaethii Callier (A. japonica × A. subcordata) Fossils †Alnus heterodonta (Newberry) Meyer & Manchester 1987 – Oligocene fossil, Oregon Phylogeny The oldest fossil pollen that can be identified as Alnus is from northern Bohemia, dating to the late Paleocene, around 58 million years ago. Etymology The common name alder evolved from the Old English word alor, which in turn is derived from Proto-Germanic root aliso. The generic name Alnus is the equivalent Latin name, from whence French aulne and Spanish Alamo (Spanish term for "poplar"). Ecology Alders are commonly found near streams, rivers, and wetlands. Sometimes where the prevalence of alders is particularly prominent these are called alder carrs. In the Pacific Northwest of North America, the white alder (Alnus rhombifolia) unlike other northwest alders, has an affinity for warm, dry climates, where it grows along watercourses, such as along the lower Columbia River east of the Cascades and the Snake River, including Hells Canyon. Alder leaves and sometimes catkins are used as food by numerous butterflies and moths. A. glutinosa and A. viridis are classed as environmental weeds in New Zealand. Alder leaves and especially the roots are important to the ecosystem because they enrich the soil with nitrogen and other nutrients. Nitrogen fixation and succession of woodland species Alder is particularly noted for its important symbiotic relationship with Frankia alni, an actinomycete, filamentous, nitrogen-fixing bacterium. This bacterium is found in root nodules, which may be as large as a human fist, with many small lobes, and light brown in colour. The bacterium absorbs nitrogen