NE Resilience Report - Conservation Gateway

The landform model describes major difference in local climatic settings, but it is theoretically possible to
detect smaller gradations in topography, or to distinguish between settings that have the same landform
diversity, but longer or shorter elevation gradients. We experimented with a variety of ways to measure
these nuances and settled on the two described below after comparing the results at known sites and
talking with practitioners about the results.
Elevation Range: Species distributions may increase or decrease in elevation in concert with climate
changes, particularly in hilly and mountainous landscape where the effects of elevation are magnified by
slope. In flat landscapes, small elevation changes may have a dramatic effect on hydrologic processes
such as flooding. To measure local elevation range we created an elevation range index by compiling a
30-meter digital elevation model for the region (USGS 2002) and using a focal range analysis to tabulate
the range in elevation within a 100-acre circle around each cell. Scores for each cell ranged from 1 to 795
meters (Map 3.2, Figure 3.4 c) with a mean of 59.4 m and a standard deviation of 54.3. The data were
highly skewed towards zero and were log transformed for further analysis (mean 3.64 and standard
deviation of 1.08).
Wetland Density: A large part of this region is flat and wet, the result of past glaciations. Moreover,
climate models disagree on whether the region will get wetter or drier, or both. In these flat areas,
landform variety is low, elevation change is minimal, and wetlands are extensive. Visual examination of
the landform variety and elevation range maps described above suggested that this information alone did
not always provide enough separation between sites, with respect to the long term resilience of extensive
wetland areas. Further, modeled measures of moisture accumulations had the highest rates of error in
extremely flat landscapes. After experimentation with local rugosity measures, we determined that
directly measuring wetland density provided the best available gauge of small and micro-scale
topographic diversity and patterns of freshwater accumulation. We assumed that areas with high density
of wetlands had higher topographic variation, and therefore offered more options to species, and that
small isolated wetlands were more vulnerable to shrinkage and disappearance than wetlands embedded in
a landscape crowded with other wetlands. Thus, our hypothesis was that wetland dependent species and
communities would be more resilient in a landscape where there was a higher density of wetland features
corresponding to more opportunities for suitable habitat nearby.
20 Resilient Sites for Terrestrial Conservation in the Northeast and Mid-Atlantic Region
The Nature Conservancy • Eastern Conservation Science • Eastern Division • 99 Bedford St • Boston, MA 02111