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INTRODUCTION:
This page describes research conducted by the
USGS, Biological Resources Division, Leetown Science
Center on potential biodiversity losses to stream
communities of Delaware Water Gap National Recreation
Area (DEWA) due to hemlock woolly adelgid (Adelges
tsugae). This project was concluded in 1998. The
final report to the park service is available in PDF
format (requires Acrobat Reader) by following the links at
left.
THE RESOURCE:
Eastern hemlock (Tsuga canadensis) is a stately
conifer tree that provides much of the shade surrounding
the cascading streams of the Delaware Water Gap National
Recreation Area (DEWA), such as the Raymondskill Creek,
Dingmans Creek, Van Campens Brook, and Buttermilk Falls.
When allowed to grow to maturity, eastern hemlock forms
majestic "climax" forests that provide unique
habitats for mammals, birds, fish, and aquatic species,
and also provide special recreation areas for people.
THE THREAT:
In 1989, a tiny but lethal pest of eastern hemlock, the
hemlock woolly adelgid (Adelges tsuga) was found within
DEWA (see figure below). This exotic insect has recently
infested and destroyed hemlock forests in Virginia,
Maryland, Pennsylvania, New Jersey and Connecticut.
THE QUESTION:
At the request of the National Park Service, researchers
at the Biological Resources Division of the US Geological
Survey conducted a study to determine the potential
consequences to aquatic invertebrates and fish from the
loss of hemlock forests at DEWA. Specifically, we wanted
to answer the question: Do hemlock forests enhance the
aquatic biodiversity (numbers and kinds of species
present) at DEWA? Would the loss of hemlock forests
reduce the aquatic biodiversity at DEWA?
DESIGN GOALS:
To answer these questions, we needed to compare the
numbers and kinds of species found in hemlock forest
streams with those found in hardwood forest streams. Of
course, we could not sample all the hemlock and hardwood
streams in the park; instead, we had to select a
relatively small number of representative sites. We also
needed to compare hemlock and hardwood streams that were
as similar as possible except for their forest type.
RESEARCH APPROACH:
What criteria can be used to judge the similarity of
numerous forested areas and streams? Terrain is one of
the most important determinants of animal habitat (other
than forest type), and hence an important criterion of
the similarity of different places. Terrain includes land
characteristics such as elevation, slope, and shape (hill,
valley, or flat), which strongly influence local
microclimates by affecting the amount of sunlight and the
moisture and temperature of an area. Stream size (width
and depth) is an important criterion of similarity of
different streams.
We used computer-based
Geographic Information Systems (GIS) and statistical
analyses of DEWA databases of forest type, terrain, and
stream size to help select appropriate sites to sample.
These analyses enabled us to identify three typical
terrain types for hemlock forests in DEWA: (1) flat
benches, (2) deep ravines, and (3) steep mid-slopes. We
used the computer to match each hemlock forest site to a
hardwood forest site having similar terrain and stream
size. By using this procedure, we were able to sample
representative hemlock stands in DEWA while effectively
removing the influences of terrain and stream size on
stream biodiversity and ecology.
FIELD SAMPLING:
After checking the computer matched sites, we sampled 14
hemlock and hardwood site pairs (28 sites total) in the
spring and summer of 1997. We sampled stream
invertebrates (insects) and fish , and measured stream
water quality habitat diversity (riffles, pools, etc.).
Automatic temperature recorders were left in streams, and
recorded temperatures hourly for an entire year.
RESULTS:
We found that: Hemlock dominated watersheds supported
more aquatic invertebrates (insect) species than streams
draining hardwood forests. Fifteen aquatic insect species
were strongly associated with hemlock, and three species
were found only in hemlock streams. Brook trout (Salvelinus
fontinalis) were two and a half times as likely to occur
in hemlock streams than in hardwood streams, and were
twice as abundant in hemlock streams.
In addition, the trophic composition or food chain of
hemlock streams was different than that of hardwood
streams, suggesting that stream ecosystem may be
disrupted by changes in forest type. We suspect that
hemlock may modify the stream environment by creating a
larger variety of stream habitats and more stable
temperature and hydrologic regimes.
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Hemlock Decline Research in Delaware Water Gap National Recreation Area
Introduction
Study Sites
GIS Analysis
Final Report
In partnership with:
See also:
Hemlock Decline Research in Shenandoah National Park
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