The Physiological Benefit of Leaf Flutter in Poplars

DOCTORATE DISSERTATION: Poplars (Populus spp.) have, in general, a flattened non-rigid petiole oriented perpendicular to the blade causing the leaf to flutter in even light winds. This study tested three hypotheses concerning the adaptive significance of this trait. Leaf flutter may enhance whole canopy carbon gain by a. creating a more dynamic light environment in the understory, b. maintaining more favorable leaf temperatures or c. increasing the CO2 flux of the leaf. Leaf flutter caused high frequency variations (from 3 to 5 Hz) of light in poplar (Populus tremuloides Michx. And P. fremontii Wats.) canopies. Flutter decreased the light interception of leaves at the top of the canopy, allowing increased light penetration into the understory. Leaf flutter created a canopy light environment that was more dynamic temporally and more evenly distributed spatially. An increase in the dynamics of the understory light environment would increase the utilization efficiency of transient light. Post-illumination CO2 fixation contributed proportionally more to the carbon gain of leaves during short frequent light flecks than longer less frequent ones. Leaves of these poplars become photosynthetically induced quickly and activation of Rubisco was the primary limitation for induction. Fluttering leaves at the top of the canopy were 2 to 4 degrees C. cooler than constrained leaves. However, lower canopy leaves showed no such reductions in temperature. Temperature differences of upper canopy leaves can be attributed to an increase in convective heat exchange or a decrease in light interception or both. However, this decrease in upper canopy leaf temperatures would increase whole canopy carbon gain only slightly and then only during the warmest ambient temperatures. Flutter can increase the CO2 flux of poplar leaves. However, neither an increase in boundary-layer conductance’s or bulk flow can account for significant increases in intercellular CO2 concentrations or photosynthetic rates for these poplars. It is the conclusion of this study that the primary adaptive significance of leaf flutter appears to be the enhancement of canopy light environments and any enhancement of leaf temperatures or CO2 flux should be considered secondary benefits.