PARENT SESSION
Contributed Oral Session 24: Disturbance Ecology: Management; Response to Fire
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 516 D, Level 5, Palais des congrès de Montréal

Groundlayer vegetation gradients across oak woodland canopy gaps.

Pavlovic, Noel¹, Grundel, Ralph¹, ¹ U.S. Geological Survey, Porter, IN, USA

ABSTRACT- Oak savannas have been converted by fire suppression to woodland and forest through the processes of tree infiltration and canopy closure. Canopy gaps may persist where there were large openings or where fire and wind have removed trees. With increasing canopy closure, canopy gaps are zones where the former savanna matrix vegetation would remain. We examined how the frequency of groundlayer species responded to small scale canopy cover gradients in woodland gaps in relation to gap-size (small=16-36 m², medium=100-169 m², and large=>289 m²), gap edge azimuth (N, S, E, and W), and position (gap, edge, and canopy). Gap-sites consist of gaps plus surrounding canopy. Gap-sites were separated by location and spread across a pH-mineral composition gradient and a topographic moisture productivity gradient. Similarity between positions in gap-sites increased as the difference in PAR decreased, explaining twice the variation in percent dissimilarity compared to individual soil variables: Mg, A horizon depth, and litter cover. Four species declined, and goats rue (Tephrosia virginiana) and legumes increased from canopy to gap while C4 grasses peaked at the gap edge, independent of gap-size. Indiangrass (Sorghastrum nutans) frequency was three times greater in gap positions in large gap-sites than elsewhere. The vegetation in medium gap-sites was more heterogeneous than within small and large gap-sites. Edges were more similar to gaps than to canopy. The groundlayer varied around the gap edge, but only a few species contributed significantly to these compositional changes. Woodland groundlayer vegetation can be characterized as a mosaic of forbland vegetation that varies across the position and edge side gradients, but with islands of grassland vegetation wherever canopy gaps exceed 170 m². Vegetation patterns in proximity to gaps are primarily driven by light gradients rather than soil gradients with the latter more dominant at larger landscape scales.

Key words: oak woodland, canopy gaps, herbaceous gradients, light gradients