NE Resilience Report - Conservation Gateway

We hypothesized that sites with a large variety of landforms and long elevation gradients will retain more
species throughout a changing climate by offering ample microclimates and thus more options for
rearrangement. However, we found that in areas with very little topographic diversity, we needed a finerscale
indicator of subtle micro topographic features, to distinguish between otherwise similar landscapes.
We chose wetland density as a surrogate for micro-topography in flat landscapes after experimenting with
several rugosity measures. Our final measure of landscape complexity was based on landform variety,
elevation range and, in flats, wetland density. Below we describe how we measured each of these
landscape elements.
Landform Variety: To be explicit about the number of microclimatic settings created by an area’s surface
features we created a landform model that delineated local environments with distinct combinations of
moisture, radiant energy, deposition, and erosion. The model, based on Ruhe and Walker’s (1968) fivepart
hillslope model of soil formation, and Conacher and Darymple’s (1977) nine-unit land surface model,
categorizes various combinations of slope, land position, aspect, and moisture accumulation (Figure 3.1
and 3.2). The methods to develop the model were based on Fels and Matson (1997) and are described in
Anderson (1999) and in Appendix II. The major divisions are based on relative land position and slope
(Figure 3.3) with side slopes further subdivided by aspect, and flats further subdivided by flow
accumulation. The landform model can distinguish an unlimited number of landform units, but we used a
simple 11 unit model that captures the major differences in settings and combines some landform types
that typically occur as pairs (e.g. cliff/steep slope, cove/slope bottom) so they did not skew the results.
The types include the following (Figure 3.1-3.3):
Cliff/steep slope
Summit/ridgetop
NE sideslope
SE sideslope
Cove/slope bottom,
Low hill
Low hilltop flat
Valley/toeslope
Dry flat
Wet flat
Water/lake/river
To calculate the landform variety metric we tabulated the number of landforms within a 100-acre circle
around every 30-meter cell in the region using a focal variety analysis on the 11 landform types. Scores
for each cell ranged from 1 to 11 (Map 3.1, Figure 3.4 a. & b.) with a mean of 6.05 and a standard
deviation of 1.85.
With respect to climate change, our assumption was that separate landform settings will retain their
distinct processes despite a changing climate. For example, a hot dry eroding upper slope will continue to
offer a climatic environment different from a cool moist accumulating toe slope.
16 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