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Fremont cottonwood (Populus fremontii S. Watson) is the most important cottonwood species of the southwestern United States. It is usually found in riparian areas of desert riverine systems. Often it is the only tree species in such environments. Lack of Fremont cottonwood seedling recruitment is of concern in many areas. This is especially an issue in river systems infested with the exotic saltceder (Tamarix ramosissima Ledeb.). The proposed suppression of saltceder with a biological control agent, raises the question of the spontaneous recruitment of Fremont cottonwood seedlings if competition is reduced from exotic woody species. Several studies have stressed that geomorphologic-hydrologic conditions in riparian habitats control safesites for Fremont and other cottonwood species seed germination and seedling establishment. Our purpose was to investigate the physiological amplitude for Fremont cottonwood seeds to germinate under a wide range of constant or alternating temperatures. Immediately after dispersal the seeds of Fremont cottonwood are highly germinable. In each of the 3 years that seeds were collected multiple temperature regimes supported 100% germination. Optimum germination averaged over 90%. At moderate to high warm period temperatures, most germination that will occur does so during the first week after imbibition of moisture. Temperature regimes that supported optimum germination at least once ranged from 0/5degrees C to 25/40degrees C. The regimes that always supported optimum germination were in 2 distinct group: 2/25 and 2/30degrees C; and a wider dispersed group with 15, 25, or 25degrees C cold period temperatures and 25, 30, or 35degrees C warm period temperatures. There was one outlier at 10/15degrees C. Fremont cottonwood seeds are highly and rapidly germinable at a wide range of temperatures.