The Niobrara National Scenic River, in north-central Nebraska, is known for encompassing a wide mix of species at or beyond their typical geographic ranges. Plants representative of eastern and western forests and several prairie types are maintained by localized microclimates that change abruptly with differences in topography, soils, slope, and moisture (Kaul et al. 1988). For example, paper birch trees (Betula papyrifera Marsh) can be found on north-facing slopes on the south bank of the river and in small protected canyons on both the north and south sides of the river (fig. 1, above). Paper birch is typically an important component of boreal forests, but here in the middle of the Great Plains, remnant populations remind us that the area once supported a very different mix of vegetation under different climatic regimes. Birch populations are believed to have persisted in the Niobrara River Valley since the end of the Wisconsin glaciation, when conditions supported boreal species (Wright 1970).
Paper birch rarely occurs naturally where average July temperatures exceed 21°C (70°F; USDA 1965). The average July temperature in Valentine, Nebraska, 4 km (2.5 mi) from the nearest birch trees, is 23°C (73°F; National Weather Service 2008). However, relict populations can persist for thousands of years outside of their typical range in sites with favorable microclimates (Stebbins and Major 1965). Presumably, microclimates in birch sites have maintained birch populations for roughly 10,000 years. However, resource managers have observed many dead or dying birch trees in recent years (fig. 2); in some sites, nearly all trees have died. Why are they dying now after such a long presence in the Niobrara Valley?