U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3046

August 2008

Detecting Evidence of Climate Change in the Forests of the Eastern United 

States

Introduction

Changes in land use or disturbances such 

as defoliation by insects, disease, or fire all affect 

the composition and amount of tree canopy in a 

forest. These changes are easy to detect. Noticing 

and understanding the complex ways that global or 

regional-scale climate change combines with these 

disturbances to affect forest growth patterns and 

succession is difficult. This is particularly true for 

regions where changes in climate are not the most 

extreme, such as the mid-latitude forests of the Eastern United States. If land and water resources are to 

be managed responsibly, it is important to know how 

well the impacts of climate change on these forests 

can be measured in order to provide the best information possible to respond to any future changes. The 

goal of this study is to test whether climate-induced 

changes in forests in the Eastern United States can be 

detected and characterized using satellite imagery.

A Focus on Landscape Phenology

Phenology is the study of the influence of climate change on the timing of biological events, such 

as annual plant flowering and seasonal bird migration. These events are partially driven by changes in 

temperature and precipitation; therefore, phenology 

studies how these events may reflect changes in 

climate. To study climate-change relations over broad 

areas (at landscape scale), the timing and amount of 

annual tree leaf emergence, maximum foliage, and 

leaf fall for forested areas are of interest.

Study Area

The forests of the Shenandoah National Park 

in the Blue Ridge province of Virginia (fig. 1) have 

been protected from major land-use change for many 

decades. In addition, the National Park Service (NPS) 

has recorded the dates and often the location of disturbances such as fires and insect infestations. These 

factors make the Shenandoah National Park an excellent “laboratory” in which to conduct an experiment.

Approach

The relation between seasonal forest change 

and weather is being tracked and analyzed by comparing precise field observations to regional patterns 

shown in long-term satellite imagery. In consultation with NPS authorities, meteorological stations 

(fig. 2) have been placed in the park at locations 

that were selected after considering forest-cover 

type, the slope of the ground, the direction in which 

the ground is facing (aspect), and whether there is 

adequate distance from a trail or historic feature. 

Most of these stations must be visited periodically 

for maintenance or to download data; however, two 

stations provide continuous data retrieval through 

automated communications with weather satellites; 

their data are available in real time via the Web at 

http://erg.usgs.gov. Canopy conditions are recorded 

periodically using onsite photography (fig. 3) or 

other handheld instruments. Field observations, 

which began in 2006, are helping us understand 

how short-term weather events affect vegetation and 

how satellite measurements over small areas help us 

understand the phenology of a local area.

Ground-data collection is full of challenges 

(fig. 4) and can be both difficult and expensive to 

continue over a long period of time. Time-sequenced 

satellite imagery provides an opportunity to observe 

phenological changes over large areas. Because the 

changes might be very subtle, frequent recording of 

satellite images over very long periods of time will be 

required for the analysis of landscape phenology.

To date, a database of government-produced, 

moderate-resolution imagery has been assembled 

for the Shenandoah National Park and surrounding 

areas. The Landsat and the Advanced Very High 

Resolution Radiometer (AVHRR) satellite series 

provide a data record spanning multiple decades 

(fig. 5). Spatial computer modeling of the ground 

and satellite data also allows us to examine the 

influence that elevation, slope, and terrain aspect 

have on forest growth, composition, and phenology.

Analysis

This long record of satellite data can be 

manipulated to show the relative amount and condi78°50' 78°40' 78°30' 78°20' 78°10'

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Area of Shenandoah 

National Park

Weather Station

Pittsburgh

Harrisburg

Baltimore

Washington, D.C.

Richmond

Raleigh

Norfolk Atlantic Ocean

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W. Va.

Va.

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Md.

Figure 1. The Shenandoah National Park includes 

part of the Appalachian Trail along the Blue Ridge of 

the Eastern United States.

Figure 2. As part of the ground-data collection 

effort, six weather stations are deployed in the 

Shenandoah National Park to provide weather data 

from a sampling of different forest and terrain types.

Figure 3. Periodic photographs taken from beneath 

the canopy using a skyward-facing fish-eye lens can 

be used to measure when leaves come out and how 

much tree cover exists at specific locations. 

Figure 4. Maintaining weather stations in the park 

is difficult for a number of reasons. For example, 

black bears seem to consider them opportunities for 

play and exercise.
tion of green vegetation through time. Using records 

from the park and other types of satellite data 

analyses, the satellite vegetation-condition database 

is being processed to identify areas changed by 

fire, disease, insects, and other disturbances. These 

areas will be eliminated from further analysis. 

The remaining areas are those where changes in 

phenology most likely would be due to changes 

in temperature, light, and water availability, all of 

which are factors affected by changes in climate. 

Using statistical approaches similar to those applied 

in stock market analysis (Reed and others, 1994) 

allows for the derivation of multiple phenologic 

measurements (fig. 6) for the forested areas that 

have been relatively unaffected by disturbances over 

the past several decades. Whether the growing season is starting sooner or lasting longer than in the 

recent past, as has been observed in some northern 

latitude forests (Myneni and others, 1997; Zhou 

and others, 2001), is of particular interest. Climate 

change may also alter the timing of the peak of the 

growing season or the total amount of growth in 

a given season. Trends in these characteristics are 

being investigated.

You Can Help!

In collaboration with the National Science 

Foundation, the U.S. Geological Survey 

has established the USA National Phenology Network (USA-NPN) to coordinate 

the collection and analysis of phenologic 

data on plants and animals from throughout 

the Nation (Betancourt and others, 2005). 

Become involved by recording when plants 

first leaf out and bloom each year. Citizen 

participation is encouraged. More information on the USA-NPN and the citizen science effort can be found at www.usanpn.org.

A B C D

May 2002 September 2002

Figure 5. Examples of Advanced Very High Resolution Radiometer (AVHRR) (A and C) and Landsat Thematic Mapper (TM) (B and D) satellite imagery for the study area 

in late May and early September 2002. Areas with more abundant and vigorous green vegetation material are shown as dark green, while areas of less green vegetation 

material are shown in yellow to white colors. Analysis of AVHRR imagery through time provides information on landscape phenology. Landsat data show disturbances 

and small changes that must be accounted for when calculating phenology.

200

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198919911993

19951997

1999

2001

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Number of growing days

Resized scale of Y-axis

Year

1989

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Figure 6. An example of phenologic metrics derived for a single square kilometer area imaged by the 

Advanced Very High Resolution Radiometer (AVHRR).

References Cited

Betancourt, J.L., Schwartz, M.D., Breshears, D.D., 

Cayan, D.R., Dettinger, M.D., Inouye, D.W., 

Post, E., and Reed, B.C., 2005, Implementing a 

U.S. National Phenology Network: Eos, Transactions American Geophysical Union, v. 86, 

no. 51, p. 539–541.

Myneni, R.B., Keeling, C.D., Tucker, C.J., Asrar, 

G., and Nemani, R.R., 1997, Increased plant 

growth in the northern high latitudes from 

1981-1991: Nature, v. 386, p. 698–702.

Reed, B.C., Brown, J.F., VanderZee, D., Loveland, 

T.R., Merchant, J.W., and Ohlen, D.O., 1994, 

Measuring phenological variability from satellite 

imagery: Journal of Vegetation Science, v. 5, 

p. 703–714.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, 

D., Shabanov, N.V., and Myneni, R.B., 2001, 

Variations in northern vegetation activity inferred 

from satellite data of vegetation index during 

1981 to 1999: Journal of Geophysical Research, 

v. 106, no. D17, p. 20,069–20,083.

For more information, contact:

John W. Jones

Phone: (703) 648-5543

Fax: (703) 648-4603

E-mail: jwjones@usgs.gov

By John W. Jones and Jesse D. Osborne