Gap dynamics in mature, mesic quercus stands on the Cumberland Plateau, Alabama

Abstract

Gap scale disturbances are important processes in forest stand development in the southern Appalachian Highlands. Canopy gaps within secondary forest throughout the southern Appalachian Highlands have been documented as critical mechanisms in canopy tree replacement and stand regeneration. I quantified gap characteristics, gap formation and closure mechanisms, and intra-gap tree and sapling distribution patterns for 60 canopy gaps in secondary mesic, Quercus stands on the Cumberland Plateau in north Alabama. Snag-formed gaps were the most common. We documented the influence gap formation mechanisms had on gap size, which ultimately contributed to gap closure. The projected closure mechanism was significantly related to the area of the gap whereby smaller gaps usually closed via lateral crown expansion and larger gaps typically closed by subcanopy recruitment. Based on the results, I hypothesized that gaps exceeding 200 m2 had higher probabilities of closing via subcanopy recruitment rather than lateral crown expansion. Several gaps projected to close by subcanopy recruitment were doing so through Quercus capture. However, Quercus capture of gaps was restricted to upper slope position with low understory competition from shade tolerant species and adequate light levels based on the ratio of gap diameter to peripheral canopy height. Liriodendron tulipifera was projected to capture seven gaps, all of which were smaller than the hypothesized minimum gap area for capture by the species. The majority of gaps were projected to close via lateral crown expansion. Based on the composition of saplings and trees in gap environments, I project the forest to transition from a Quercus dominated system to one with much stronger Fagus grandifolia and Acer saccharum components. My study fills a void in the literature on the role of canopy gaps in secondary, mesic Quercus stands that established just prior to 1900 for the southern Appalachian Highlands region.