Responses of floodplain forest species to spatially condensed gradients: a test of the flood–shade tolerance tradeoff hypothesis
L. L. Battaglia1 and R. R. Sharitz2
1Department
of Plant Biology, Southern Illinois University, Mail Code 6509, Carbondale,
IL 62901, USA
2Savannah
River Ecology Laboratory, University of Georgia, Drawer E, Aiken, SC
29802, USA
Abstract:
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
SREL Reprint #3105
Battaglia,
L. L. and R. R. Sharitz. 2006. Responses of floodplain forest species
to spatially condensed gradients: a test of the flood-shade tolerance
tradeoff hypothesis. Oecologia 147(2006): 108-118.
