Patterns of competition and insolation along a canopy disturbance severity gradient: implications for the Quercus bottleneck

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University of Alabama Libraries

Regeneration failure of Quercus has been widely reported throughout the Central Hardwood Forest of the eastern United States across all but the most xeric site types. Quercus seedlings are often abundant in upland hardwood stands, but typically fail to recruit to larger size classes (i.e. the Quercus bottleneck). The goal of my study was to investigate the competition with small-sized Quercus stems to elucidate what variables may cause resistance or facilitation of Quercus recruitment. I established a 2 ha plot in the Sipsey Wilderness, AL that was subdivided into 5 x 5 m quadrats to encompass the entire extent of damage caused by an EF1 tornado in 2011. The study area encompassed a range of damage (light, moderate, severe), starting from the center of the tornado tract and ending in an undamaged area (control). The canopy disturbance provided a range of light conditions in the understory (0.5–91.0% full sunlight). In each quadrat, the height of all small-sized Quercus stems (≥ 0.5 m in height to >5 cm DBH) were measured and recorded. The nearest neighbor was identified by species and measured for height and distance from the focal Quercus stem. Photosynthetically active radiation was quantified at 1.4 m above the forest floor. Quercus’ competition was largely interspecific and the majority of competitors were shade-tolerant species such as Acer saccharum and Ostrya virginiana. Competitive indices of Quercus’ primary competitors were significantly different (p < 0.01) revealing the relative competitive ability amongst species within the plot. Disturbance neighborhoods had no significant (p > 0.01) effect on the competitive ability of species. Spatial patterns of Quercus stems, their nearest neighbor, and percent full sunlight were analyzed at 5 x 5 m, 10 x 10 m, and 20 x 20 m. Analysis at a fine-scale resolution revealed spatial patterns which would otherwise not be apparent at coarser scales. Creative and flexible silvicultural techniques may need to be implemented at finer scales to optimize the recruitment of Quercus spp.

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