Canopy gap dynamics and development patterns in secondary Quercus stands on the Cumberland Plateau, Alabama, USA

Forest disturbances of various spatial extents and magnitudes shape species composition, structure, and stand development patterns. The disturbance regimes of most complex stage hardwood stands of the deciduous forests of eastern North America are typified by asynchronous and localized disturbance events. The overwhelming majority of gap-scale disturbance studies in hardwood forests of the region have analyzed late-successional stands. As such, there is a paucity of data on gap dynamics in hardwood stands prior to a complex developmental stage. We quantified biophysical characteristics of 60 canopy gaps in secondary Quercus stands on the Cumberland Plateau in Alabama to analyze gap-scale disturbance processes in developing systems. We found most gaps (90%) were caused by the removal of a single tree. Of the three gap formation mechanisms, snag-formed gaps were most common (40%). However, based on the number of uprooted and snapped stems we speculate that wind was also an important disturbance agent in these stands. Gap size and shape patterns were similar to what has been reported in other hardwood forests of the southern Appalachian Highlands. We did not find differences in gap size or shape based on formation mechanisms; a finding that may be related to the number of single-tree gap events. Gaps projected to close via subcanopy recruitment were significantly larger than those projected to close through lateral crown expansion. Most gaps (65%) were projected to close by lateral crown expansion of gap perimeter trees. However, the number of gaps projected to fill by subcanopy recruitment indicated the stands were approaching a transition in their developmental stage. Gap-scale processes modify residual tree architecture and stand structure. Through time these alterations result in progressively larger gaps, eventually reaching a size when most will fill by subcanopy recruitment, thus marking the complex stage of development. Gap capture by Quercus was restricted to relatively xeric sites that did not contain abundant shade-tolerant mesophytes in the understory. However, the majority of gaps contained abundant subcanopy Fagusgrandifolia, Acer saccharum, and Acer rubrum leading us to project that the forest will undergo a drastic composition shift under the current disturbance regime. Liriodendron tulipifera was projected to capture several relatively small gaps illustrating the role of topography on gap closure mechanisms.

► We quantified biophysical characteristics of canopy gaps in secondary Quercus stands.
► Localized canopy disturbance creates conditions needed for complex structure.
► Quercus gap capture was limited to xeric sites without shade-tolerant competition.
► Shade-tolerant taxa were abundant in the understory indicating a composition shift.