Plasticity of leaf absorbance in some broadleaf tree seedlings

In spring prior to budbreak, 15 seedlings, from each of five broadleaf tree species that had been exposed to high light regime during the previous summer, were placed under a dense forest canopy while 15 others were left in the open. Leaf absorbance in the 400-1,100 nm waveband was measured for each seedling (two leaves per seedling) following full leaf development (early June) and prior to leaf senescence (late July). The same leaves were measured at both times. In early June, the highly shade tolerant sugar maple (Acer saccharum) already showed significant differences in leaf absorbance of its shaded and sunlit leaves. In this species, light conditions at time of budbreak seem to have greater effects on leaf physiology than light conditions prevailing during formation of leaf primordia. Leaves of mid-tolerant bur oak (Quercus macrocarpa), white ash (Fraxinus americana), and yellow birch (Betula alleghaniensis) responded less quickly to shade, with absorbance differences appearing later in summer. As for the intolerant butternut (Juglans cinerea), the small differences in absorbance between shaded and sunlit leaves in late July indicated a lower response to shade in its seedling leaves. These results indicated that spectral analysis might be a more appropriate tool to differentiate true sun from shade leaves than the morphological parameters commonly used.