The heterogenous structure of eastern deciduous forest canopies creates an understory light continuum ranging from deep shade to extended periods of direct solar irradiance below large canopy gaps. We designed an experiment to explore how shagbark hickory (Carya ovata (Mill.) K. Koch; Juglandaceae) seedlings respond to changes in both the total amount of available light and the durations of experimental sun patches. Seedlings were grown in a common garden under shadehouses where they experienced one of two light levels (high vs. low light), one of two sun patch durations (short vs. long duration) and 100% light (control). Over the growing season, seedlings responded morphologically to the amount of available light by changes in mean stem height, weight, basal diameter, specific leaf mass, chlorophyll content, taproot length, primary and secondary root weight and root ash weight. Chlorophyll content and stem height were negatively related to the amount of available light whereas the other morphological characters were positively related to increased amounts of light. Changes in the duration of experimental sun patches significantly affected stem biomass, basal diameter, secondary root dry weight and root ash weight independent of total light amount. Though the effects of sun patch duration were not as pronounced as those found for amount of light, our results demonstrate the importance of including periods of direct light in experimental designs involving responses of seedlings to varying light regimes. The ecological responses of many hardwoods, especially those of midshade tolerance, to the middle portion of the gap-understory continuum remain poorly studied.
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Vol. 141 • No. 1