Contemporary plant communities often contain clues to historic flora. Unusual habitats with microclimates that differ from the surrounding landscape can support populations of locally rare species far from the core of their common range. When these disjunct populations occur far south of their core range in sites that are typically cooler than surrounding habitat, they are often considered to be Pleistocene relicts. That is, the species is presumed to have been locally more abundant during the Pleistocene, but warming climates eliminated populations from all but the most climatically favorable sites as the species migrated moved north. Little is known about the abiotic conditions that maintain Pleistocene relict species in their refugia.

In southern Missouri, populations of three Pleistocene relict plant species (Campanula rotundifolia, Zigadenus elegans, and Galium boreale) occur on north-facing bluffs along the Jacks Fork River in the Ozark National Scenic Riverways. These populations are presumed to have persisted for thousands of years in what is assumed to be a very localized cool, moist microclimate atypical of the surrounding habitat. The Missouri Department of Conservation and the National Park Service are charged with the protection of these particular populations, and they are concerned that stochastic habitat changes, especially extreme weather years, could result in extirpation of some or all of the populations. They are therefore interested in characterizing microclimate variables that may be important to the persistence of these plant populations.

In this study, I measured microclimate variables in three sites supporting Pleistocene relict plant populations using Hobo^® data loggers to continually record air temperature, humidity, and light intensity from May 1999 through July 2002. Microclimate data from the study sites were compared to climate data from a nearby automated weather station. Sites that harbor relict plants are not necessarily more cool or more humid than the surrounding area during the growing season, but rather they are buffered from wide variation in daily temperature. Protection from wide swings in temperature provides a physiologically less stressful habitat for these plants and may explain their ability to persist in an otherwise apparently unsuitable habitat. The ability of the site to provide sufficient buffering from temperature variation in extreme years or under climate change scenarios is likely important for the continued persistence of these rare communities of Pleistocene relict plants. Data from May 1999 through July 2002 will be presented.