Three-dimensional light structure of an upland Quercus stand post-tornado disturbance

Abstract

Light is the most common limiting factor in forest plant communities, influencing species composition, stand structure, and stand productivity in closed canopy stands. Stand vertical light structure is relatively simple under a closed canopy because most light is captured by overstory trees. However, wind disturbance events create canopy openings from local to landscape scales that increase understory light intensity and vertical light structural complexity. We studied the effects of an EF-1 tornado on horizontal and vertical (i.e. three-dimensional) light structure within a Quercus stand to determine how light structure changed with increasing disturbance severity. We used a two-tiered method to collect photosynthetic photon flux density at 4.67 m and 1.37 m above the forest floor to construct three-dimensional light structure across a canopy disturbance severity gradient to see if light intensity varied with increasing tornado damage. Results indicate that increased canopy disturbance closer to the tornado track increased light penetration and light structure heterogeneity at lower forest strata. Increased light intensity correlated with increased sapling density that was more randomly distributed across the plot and had shifted light capture higher in the stand structure. Light penetration through the overstory was most strongly correlated with decreased stem density in the two most important tree species (based on relative dominance and relative density) in the stand, Quercus alba L. (r =-0.31) and Ostrya virginiana (Mill.) K. Koch (r =-0.27, p \ 0.01), and indicated that understory light penetration was most affected by these two species. As managers are increasingly interested in patterning silvicultural entries on natural disturbances, they must understand residual stand and light structures that occur after natural disturbance events. By providing spatial light data that quantifies light structure post-disturbance, managers can use these results to improve planning required for long-term management. The study also provides comparisons with anthropogenic disturbances to the midstory that may offer useful comparisons to natural analogs for future silvicultural consideration.

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