Previous work in southeastern US floodplains led to the hypothesis that a tradeoff between flood and shade tolerance underlies
species-specific responses to flooding and light, which drive forest regeneration. In systems where community turnover can
occur with small-scale environmental changes, testing this hypothesis requires recognizing that turnover of species along
the two gradients can be large relative to the spatial distances involved. We test the tradeoff hypothesis in an old-growth
bottomland hardwood forest by (1) comparing shade and moisture profiles of woody juveniles versus random points and (2) using
individual-based sampling of woody juveniles to model probability of occurrence in response to distance-to-water table and
canopy openness gradients. We found that juveniles of all species combined occupied a similar range of distance-to-water table
compared to measurements taken at random points, but average canopy openness above seedlings was significantly higher than
at random points. On average, shade-tolerant species, with the exception of
Acer rubrum, were found in shaded areas that were also drier, whereas less shade-tolerant taxa, plus
A. rubrum, were found in wetter, more open areas, suggesting a tradeoff between flood and shade tolerance. Predictive models of species
occurrence, which incorporate the availability of canopy and microtopographic conditions, indicated that three taxa (
Fraxinus pennsylvanica,
Quercus spp., and
Ulmus americana) had patterns consistent with a flood–shade tolerance tradeoff. In contrast,
Asimina triloba,
Celtis laevigata, and
Liquidambar styraciflua had positive responses when the joint stresses of flooding and shade were diminished.
A. rubrum appeared to be the most tolerant to both stresses. Our work not only lends support to the flood–shade tradeoff hypothesis
but also indicates that a more general model is needed that includes a “flood–shade release” component. We also suggest that
responses to small-scale gradients should not be overlooked, particularly in systems where the gradients are ecologically
steep.
Keywords Bottomland hardwood forest - Disturbance - Flooding - Gradient - Light - Response curves
Communicated by Louis Pitelka