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.