Photosynthetic induction and sunfleck responses of three shade-tolerant temperate saplings

Novel understory tree species composition has arisen throughout forests of the eastern U.S. as various, often human-caused, pressures have led to regeneration mismatch between trees in the forest canopy and understory.  Physiological attributes supporting the relative success of these novel understory tree communities are poorly characterized.  We sought to examine photosynthetic induction and sunfleck responses in three shade-tolerant understory tree species now common in the U.S.’s Mid-Atlantic region.  Focal species included the non-native Acer platanoides, as well as the natives Acer rubrum and Fagus grandifolia.  Three experiments were performed on three individuals of each species at three independent study sites in the broadleaf forests of southeastern Pennsylvania, USA.  The experiments were: (1) a photosynthetic induction experiment where uninduced leaves were first exposed to low light, followed by saturating light and measured until photosynthesis and stomatal conductance stabilized, (2) 30 second sunfleck experiments in which uninduced leaves were exposed to low light followed by a series of five 30 second saturating sunflecks, with two minutes of low light provided between each sunfleck, (3) 60 second sunfleck experiments in which uninduced leaves were exposed to low light followed by a series of five 60 second saturating sunflecks, with two minutes of low light provided between each sunfleck.  Findings show fairly similar and rapid photosynthetic induction responses among species, consistent with expectations for shade-tolerant tree species.  However, we observed a divergence in responses to applied sunflecks, where the two Acer species became induced more quickly than F. grandifolia and exhibited higher cumulative carbon assimilation when compared to conditions of constant saturating light.  This suggests the Acer species are well positioned to persist in the variable light environments of the forest understory.

Methods
A full description of the research methods is available in the associated published manuscript.  Three understory tree species, Acer platanoides (Norway maple), Acer rubrum (red maple), and Fagus grandifolia (American beech) were the focus of this study.  Three saplings per species at three southeastern Pennsylvania broadleaf forest study sites were measured.  Experiments were performed on different individual leaves per tree: (1) The first experiment was designed to measure the process of photosynthetic induction when leaves were uninduced (i.e., covered in light-blocking nylon bags), then exposed to low light (10 µmol m-2 s-1) followed by saturating light (1000 µmol m-2 s-1).  (2) Thirty second sunfleck experiments were also performed, in which uninduced leaves were exposed to low light followed by a series of five 30 second saturating sunflecks, with two minutes of low light provided between each sunfleck.  (3) Finally, a 60 second sunfleck experiments was carried out, in which uninduced leaves were exposed to low light followed by a series of five 60 second saturating sunflecks, with two minutes of low light provided between each sunfleck.  Nine different attached leaves per species, spread evenly across three study sites were measured in each experiment.  In each experiment, leaves were measured with the LI-6800 infrared gas analyzer (LI-COR, Lincoln, NE), and measurements of photosynthesis, as well as stomatal conductance were recorded.  All leaves were collected following completion of measurements, scanned to determine fresh leaf area, dried to determine leaf dry mass, and analyzed to determine percent leaf nitrogen content.  In addition to leaf-level measurements, hemispherical photographs were taken adjacent to each sapling measured to determine global site factor (GSF) and percent canopy cover experienced by each study tree.