White oak (Quercus alba) leaf morphology and shoot structure responses to silvicultural burning treatments

Background/Question/Methods
The lobing of leaves, and aggregation of leaves within shoots, collectively impact the function of these units as well as the microclimate of the canopies that they create. Leaf lobing, a plastic leaf trait shown to respond to sun:shade developmental conditions, has been shown to influence heat dissipation and to correlate with leaf hydraulic function.  The aggregation, or clumping, of foliage within shoots and canopies affects microclimatic gradients (light, temperature) as well as leaf physiology (e.g., photosynthesis, due to self-shading).  Foliage clumping has been shown to influence optical-based estimates of leaf area that rely on assumptions of random leaf distributions. Shoot-level foliar clumping has been well-characterized for conifer canopies but has been rarely characterized in broadleaved canopies. The purpose of this study was to investigate the plasticity of leaf lobing and shoot clumping of white oak (Quercus alba) in its response to increasing frequency of forest burning. To what degree will increasing burn frequency alter foliar morphology and aggregation within canopies?

This study was a component of a larger study assessing the ecosystem impacts of frequent burning (4x), infrequent (2x) and no burns (control) applied from 1995 to 2002 (Hutchinson et.al. 2005). Shoot samples were collected from the top, middle and bottom of canopies 15 felled white oak trees in the summer of 2005 (McEwan et al. 2007). Leaf lobing index, leaf mass per unit area (LMA) were determined on leaves and the shoot silhouette area to projected leaf area ratio (SPAR) was determined on shoots. Leaf lobing index was calculated as the ratio of area of the leaf area to the leaf envelope shape; an unlobed leaf has an index of 1.

Results/Conclusions
Frequent burning significantly affected leaf lobing at the top of, but not within, tree canopies. Mean leaf lobing index from the upper canopies in the frequent burn plots was significantly different from upper canopy leaves in the infrequent burn and control plots (0.58 +/- 0.02 s.e. versus 0.73 and 0.69 +/- 0.01 s.e., respectively). LMA was twice as great at the top of canopies than within them, but no significant treatment differences were detected. Treatment and within-canopy differences in shoot clumping results will be presented. Improving our understanding of canopy architecture and plasticity in a widely distributed tree such as white oak will be a significant contribution to future efforts to remotely measure canopy leaf area and to model canopy function in Eastern deciduous forests.