Frost pockets on a level sand plain: Does variation in microclimate help maintain persistent vegetation patterns?

MOTZKIN, G., S. C. CICCARELLO AND D. R. FOSTER. (Ha-rva-rd Forest, Harvard University, Petersham, MA 01366). Frost pockets on a level sand plain: Does variation in microclimate help maintain persistent vegetation patterns? J. Torrey Bot. Soc. 129:154-163. 2002-Climatic variation is recognized as a major driver of vegetation patterns across broad geographic regions and areas of steep topography. However, few studies have evaluated the potential influences of local climatic variation an vegetation patterns or dynamics. We investigated the relationships between microclimatic variation, vegetation structure, and leaf phenology on a topographically level sand plain that is characterized by a forest mosaic with abrupt transitions to shrublands dominated by scrub oak (Quercus ilicifolia). The timing of bud break, leaf phenology, and height of scrub oak are strongly related to variation in temperature across the study site. Moderate maximum and minimum temperatures occur beneath forested canopies, whereas nearby shrublands experience frequent late-spring frosts and shorter frost-free growing seasons. resulting in chronic dieback of developing leaves, slow growth rates, and reduced stem height. Our results indicate that extreme radiational cooling in open areas on level, xeric sites may result in the development of 'frost pockets' similar to those that occur in topographical depressions and may contribute to slow establishment of a forest canopy after disturbance. Such frost-prone areas may have become more common as a result of historical cutting and burning and may contribute to the persistence of scrub oak stands that support several rare species and are high priorities for conservation.