The Importance of Gap Processes in the Development and Maintenance of Oak Savannas and Dry Forests

Abstract

1 We examined the dynamics of a Quercus-dominated dry forest-savanna complex in the Ozark Highlands of the central United States, focusing on regeneration patterns within gaps. The study area includes one of very few temperate Quercus ecosystems not degraded by fire suppression, logging or excessive grazing. 2 We compared patterns of tree seedling establishment, tree height growth and postfire survival in gaps between savanna, found only on south and west aspects in our study area, and dry forest, found only on north and east aspects, to determine which factors contributed to maintaining structural differences. 3 Canopy gaps constituted 42% of savanna area compared with 17% for dry forest. Mean canopy gap size in savanna was 316 m2 compared with 185 m2 for dry forest, but gap sizes were highly variable and the difference was not significant. Remnants of canopy trees were found in 37 of 38 gaps, indicating that gaps in both savanna and dry forest can usually support trees. 4 Savanna and dry forest gaps were generally favourable for seedlings and growth of small trees. Seedlings of most woody species were widespread in the understoreys of both savanna and dry forest but were more abundant in gaps than under closed canopies. Species had different height growth patterns depending on site, with growth rates ordered as follows: Quercus velutina in savanna > Q. velutina in forest > Q. stellata in forest > Q. stellata in savanna. Site differences were significant for Q. stellata but not for Q. velutina. 5 Tree survival in two surface fires was significantly lower in gaps in savanna compared with gaps in dry forest. Survival within individual gaps was inversely related to grass cover. In savanna, survival of small trees (2 5-7 cm d.b.h.) ranged from >80% for gaps with 90% grass cover. 6 Canopy gaps in the Ozarks are created primarily by windfall and drought-related dieback, but fire influences the timing and rate of gap infilling. Understanding the interactions among these disturbances may help clarify the dynamics of many temperate woodlands once dominated by surface fires.