Forest habitat, not leaf phenotype, predicts late-season folivory of Quercus alba saplings

1. Natural sunlight gradients occur on multiple scales in space and time. However, the direct (via habitat) and indirect effects (via plant quality) of light environment are often confounded, obscuring the relative importance of each in influencing herbivore responses. 2. Potted saplings of Quercus alba (white oak, Fagaceae) were grown in either full sun or full shade from budburst to leaf hardening to manipulate leaf phenotype (creating sun or shade leaves), then placed in either sunny light gaps or adjacent shaded forest understorey habitats. This two-way factorial design isolated the effects of sunlight level during leaf expansion from light environment late in the growing season on leaf phenology, leaf traits associated with host plant quality for herbivores, herbivore density and folivory. 3. Sunlight level during leaf expansion and hardening had strong and persistent effects on Q. alba leaf phenology and phenotype. Shade saplings had later budburst (c. 4 days), and fewer but larger leaves, resulting in greater total leaf area compared with sun leaf saplings. Shade leaves had higher water content, specific leaf area and nitrogen content, and lower toughness, carbon content, C/N ratios and concentrations of hydrolysable and condensed tannins than sun leaves. 4. Despite the apparent higher quality of shade leaves, forest habitat better predicted damage by folivores than leaf type, suggesting that the direct effects of light environment predominate for herbivory. Potted saplings of both leaf types placed in the shaded understorey suffered almost two times more folivory, on average, than saplings in sunny light gaps, despite more than three times higher mid-season herbivore density on sun leaf saplings relative to shade leaf saplings. 5. Taken together, these results suggest that both leaf phenotype and forest habitat, two factors frequently confounded in nature, have significant but distinct effects on leaf quality and herbivory. These findings have implications for plant—herbivore interactions following disturbances such as treefalls, when shade leaves may be present in sunny habitats, and may help explain patterns of herbivory in understorey plants with early leaf flushing phenology relative to the canopy, when sun leaves are present in the shaded understorey.