SUPPORTING INFORMATION: Noss et al., “HOW A GLOBAL BIODIVERSITY HOTSPOT, THE NORTH
AMERICAN COASTAL PLAIN, WENT UNRECOGNIZED”
S3: Assessing Vegetation Modification in the Coastal Plain
Methods
To determine the proportion of the Coastal Plain that has been modified from its natural condition, we
included the portion that has been converted to urban or agricultural land uses, as well as the portion
where the fire regime or vegetation composition or structure has been highly altered compared with
reference conditions. For the calculation of conversion rates as well as vegetation composition and
structure alteration, our approach approximately follows Swaty et al. (2011), but we add the important
component of fire regime alteration because fire plays an important role in most ecosystems in the
Coastal Plain (Christensen 2000).
We used data and models from LANDFIRE (Rollins 2009) to determine the rates of conversion and
alteration. The LANDFIRE program mapped Biophysical Settings (BpS) across the conterminous US, and
363 of these were mapped in the Coastal Plain (LANDFIRE 2013a). A BpS is an estimated dominant
vegetation type on the landscape prior to European settlement. Most BpSs correspond to an ecological
system, as defined by NatureServe (Comer et al. 2003), within a LANDFIRE mapping zone. Some BpSs,
especially in wetland or riparian areas, correspond to a group of ecological systems. For each BpS,
LANDFIRE developed a state-and-transition model that represents vegetation dynamics prior to
European settlement, including succession as well as disturbances such as wildfires (Blankenship et al.
2012, LANDFIRE 2013b). In these state-and-transition models, vegetation states are labeled with
information about the successional stage (early, mid- or late succession) and canopy structure (typically
open or closed canopy). Any single location in the landscape is expected to transition among vegetation
states through time, and the models can be used to estimate, for a given BpS, the proportion of
vegetation in each state class. We used these modeled pre-European settlement conditions, along with
BpS spatial data, as reference conditions in our analysis. For current conditions, we used recent (2010)
mapped successional and structural state class (s-class) data from LANDFIRE (2013c). The s-class spatial
data set labels pixels as belonging to one of the vegetation states captured in the state-and-transition
models, or alternatively, labels them as agriculture, urban land, “uncharacteristic native” vegetation or
“uncharacteristic exotic” vegetation.
We calculated rates of conversion, vegetation alteration, and fire regime alteration for each BpS
(hereafter, “ecosystem”) in the Coastal Plain. As the rate of conversion, we used the proportion of pixels
mapped in each BpS that were classified agriculture and urban in the s-class data. We assumed that the
pixels in the s-class data labeled as “uncharacteristic” would still have some native vegetation
remaining, and thus did not include them as converted, but rather incorporated them in the portion of
the landscape on which we evaluated vegetation alteration (see below).
Across the portion of each ecosystem that was not converted, we assessed vegetation and fire regime
alteration. To determine the rate of vegetation structure alteration for each ecosystem, we compared
the distribution of state classes in reference conditions to current conditions using a landscape
approach. For each of the ecosystems mapped in the Coastal Plain, we summarized the current
proportion of vegetation in each s-class within an EPA Level IV ecoregion (EPA 2013). We compared
those proportions with LANDFIRE reference conditions using a dissimilarity index approach detailed
elsewhere (Barrett et al. 2010, Swaty et al. 2011). The index values range from 0 to 1, with values near 1
indicating a high level of alteration in vegetation structure compared to reference conditions.
While the measure of vegetation structure alteration captures differences in successional stage canopy
structure, it does not necessarily capture alteration in understory vegetation composition and diversity
due to changes in the fire regime. In the Coastal Plain, fire suppression is a leading threat, especially for
ecosystems in the Coastal Plain that naturally experience frequent fires (Mitchell et al. 2014). Therefore,
for those ecosystems, we incorporated a measure of fire regime alteration. To measure fire regime
alteration, we combined LANDFIRE reference conditions with recent data we collected from state
agencies on wildfire occurrences in the Southeast. Data on prescribed fires for most of the region were
not available. The temporal extents, and temporal and spatial resolutions of the wildfire data sets varied
among states, but all were suitable for summarizing the average annual area burned in seven states (AL,
FL, GA, MS, NC, SC, VA) for the period 2001-2010. We also used LANDFIRE state-and-transition models
to simulate wildfire dynamics assuming no fire suppression beginning with the 2001 landscape, through
2010, and calculated the annual average burned area. In order to compare the actual area burned
(according to recent wildfire data) and the simulated area burned, we standardized these by the total
area represented. We then calculated the ratio of the actual to simulated average annual areas burned.
This ratio indicates the proportion of the pre-European settlement fire regime that exists in the current
landscape. One minus this ratio indicates the degree of fire suppression; it can also be interpreted as the
proportion of the landscape with an altered fire regime in any given year.
We used this fire suppression measure as the fire regime alteration index. We applied this index only to
ecosystems in the Coastal Plain that were labeled by LANDFIRE as having a frequent fire regime. These
systems are likely to have missed at least one fire return interval during the period of 10 years, leading
to degradation of understory vegetation composition and diversity. Therefore, we have confidence that
our wildfire data indicate alteration of those systems. We note that this index of fire regime alteration is
likely conservative across the Coastal Plain because fire suppression has been common in the region for
several decades.
The fire regime and vegetation alteration indices comprised separate components of an overall
ecosystem alteration index. We defined overall alteration (A) based on these components. A is the
proportion of each ecosystem where:
𝐴𝑉 + 𝐴𝐹
> 0.66
2
Here, Av is the vegetation alteration index, and AF is the fire regime alteration index (if applicable). For
ecosystems where AF was not applied, AF = AV. The proportion of each ecosystem where the average of
the fire regime and vegetation alteration indices was greater than 0.66 (following Swaty et al. 2011) was
considered highly altered, and was our alteration index.
We added this alteration measure to the proportion converted to get the index of modification (M):
𝑀=𝐶+ 𝐴
where C is the proportion of land converted to urban or agriculture. We calculated M for each
ecosystem (Costanza et al. in preparation), and summarized results for groups of ecosystems comprising
three major habitats in the Coastal Plain: Forests; Savannas/Woodlands; Grasslands, Marshes and
Glades (see Table S3.1 for a list of ecosystems and their classification to major habitat types). We then
tallied the totals across the entire Coastal Plain.
Results
According to our analysis, across the Coastal Plain, a total of 85.5% of the landscape has been modified.
Converted land uses occur on 43.4% of the landscape, and highly altered natural ecosystems occur on an
additional 42.2%. The metric of fire regime alteration that we applied to ecosystems with high fire
frequencies was 0.873, meaning that overall in the Coastal Plain, recent wildfire records indicated that
only 12.7% of ecosystems with naturally high fire frequencies have experienced fire regimes that mimic
simulated historic frequencies. The overall portion of the Coastal Plain that had a highly altered
composition of vegetation successional and structural states was 23.3%.
When summarized across three broad habitat types, forested habitats had a lower rate of modification
than the other two types (Savannas/Woodlands; Grasslands, Marshes and Glades) (Table 2 in main text),
because the rate of vegetation and fire regime alteration is substantially lower for Forests than the
other two. All three habitat types had similar rates of conversion to urban and agricultural uses, but the
Grasslands, Marshes, and Glades habitat had a slightly higher rate than the other two.
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