"Climate, Microclimate and the Decline of Relict Paper Birch Trees in the Niobrara Valley, Nebraska" Esther Stroh, Columbia Environmental Research Center, US Geological Survey The Niobrara River Valley supports scattered stands of paper birch (Betula papyrifera) on steep north-facing slopes along the river and in north-facing spring-fed canyons. These stands occur approximately 500 km southwest of the present day paper birch range and have been present in the Niobrara Valley since the end of the Wisconsin glaciation. Recent (15 - 20 years) dieback of canopy-sized trees has caused concern for the populations’ continued persistence. In this study, we continuously monitored microclimate in ten paper birch stands since June 2005; we also assessed birch health and environmental conditions in these and other Niobrara sites. Local microclimates in the birch sites differ from the surrounding climate as measured by a nearby weather station located in the Niobrara Valley. Initial analyses show differences in microclimate among birch stands according to river location. Birch stands in the downstream portion of the study area also have a higher proportion of dead trees than do upper river sites. Increased frequency of spring thaws and May freezes in the Niobrara Valley from 1978-2006 compared to 1949-1977 may partially explain Niobrara birch dieback and higher proportion of dead trees in lower river sites.