0

2

4

6

8

10

12

14

16

West

1960-1986

West

1987-2013

East

1960-1986

East

1987-2013

Percent of Months

Moderate

Severe

Extreme

Figure 2. Comparison of Palmer Drought Severity Index 

classes among eastern and western U.S. forested land 

for the period 1960–2013.

Map Description

Droughts are common in virtually all U.S. forests, but their frequency and intensity vary within forest ecosystems 

(Hanson and Weltzin 2000). Accounting for the long-term influence of droughts within a region is difficult due to 

variations in the spatial extent and intensities over a period. Therefore, we created a cumulative drought severity 

index (CDSI) (Fig. 1) for the United States using weighted monthly frequencies of Palmer Drought Severity Index 

values reported at climate divisions for the periods 1960 to 1986 and 1987 to 2013 (National Climate Data Center 

2014, Palmer 1965). The occurrences of ‘severe’ conditions (index value of -3.0 to -3.9) receive a weight of 2 and 

‘extreme’ (index value of ≤ -4.0) receive a weight of 3 to reflect the increased magnitude of these events. 

Forest Service Northern 

Research Station

Research Map

NRS-7

Publication Date 

December 2014

Spatio-temporal Drought Trends by Forest Type in the Conterminous United States, 1960-2013

Matthew P. Peters, Louis R. Iverson, and Stephen N. Matthews

1960-1986 1987-2013

Forest cover types defined by the U.S. Forest Service include 21 distinct groups that were used to mask nonforested 

land and summarize weighted frequencies of drought occurring during each 324-month period. Additionally, forest 

cover types were combined within eastern and western United States and drought frequencies among climate 

divisions were summarized for the two periods (Figure 2).

Drought severity has increased throughout the West, Southeast, and Lake States (see map at right). The Northeast 

and Mid-Atlantic States have not experienced such severe droughts in the latter period. The mapped areas show 

that the frequencies of drought conditions in forests have significantly increased in the West compared to the East.

Drought frequencies for forested land of the conterminous United States during the period 1960-2013 were 

adjusted for the number months per climate division. For each forest type, drought conditions were summarized 

as the percentage of months during the 324-month period (27 years) among climate divisions that contained the 

forested land (Table 1). Data used to create these maps can be obtained by contacting the author: matthewpeters@

fs.fed.us.

Data Sources

Palmer Drought Severity Index values were obtained from the National Climatic Data Center (2014) and used 

to calculate the CDSI. The forest cover types layer was provided by the U.S. Forest Service and U.S. Geological 

Survey (2000) and used to mask nonforested land.

Suggested Citation

Peters, Matthew P.; Iverson, Louis R.; Matthews, Stephen N. 2014. Spatio-temporal trends of drought by forest 

type in the conterminous United States, 1960-2013. Res. Map NRS-7. Newtown Square, PA: U.S. Department 

of Agriculture, Forest Service, Northern Research Station. [scale 1:12,000,000].

Literature Cited

Hanson, P.J.; Weltzin, J.F. 2000. Drought disturbance from climate change: response of United States forests.

Science of the Total Environment. 262(3): 205-220.

National Climatic Data Center. 2014. North American drought monitor indices. Asheville, NC: U.S. Department 

of Commernce, National Oceanic and Atmospheric Administration, National Climate Data Center. Available at 

www.ncdc.noaa.gov/temp-and-precip/drought/nadm/indices.php? (Accessed July 22, 2014).

Palmer, W.C. 1965. Meteorological drought. Washington, DC: U.S. Department of Commerce. 58 p.

U.S. Forest Service and U.S. Geological Survey. 2000. Forest cover types. Washington, DC: U.S. Department 

of Agriculture. Available at www.fia.fs.fed.us/library/maps/ (Accessed July 22, 2014).

The USDA is an equal opportunity provider and employer.

United States Department of Agriculture

Data Sources:

National Climatic Data Center. 2014.

North American Drought Monitor Indices

http://www.ncdc.noaa.gov/temp-and-precip/drought/nadm/indices.php?

U.S. Forest Service and U.S. Geological Survey. 2000. Forest Cover Types. 

http://www.fia.fs.fed.us/library/ maps/

Cumulative Drought 

Severity Index

< 50

51 - 100

101 - 150

151 - 200

201 - 250

> 250

U.S. Lakes

1960-1986 1987-2013

U.S. area Percent of Record Percent of Record

Forest Type (km2

) MD1 SD1 ED1 CDSI MD SD ED CDSI

Aspen-birch 106,263 8.6 4.0 4.3 29.5 7.5 4.3 2.7 24.2

Elm-ash-cottonwood 21,481 8.5 4.2 3.4 27.1 8.7 5.0 3.5 29.2

Loblolly-shortleaf pine 235,600 10.0 3.9 1.0 20.8 11.5 6.0 2.5 31.0

Longleaf-slash pine 81,888 10.7 3.7 0.7 20.2 11.8 6.1 2.7 32.1

Maple-beech-birch 204,826 8.6 3.9 2.9 25.1 8.7 4.5 2.3 24.6

Oak-gum-cypress 119,125 10.2 4.0 1.0 21.2 11.4 5.9 2.5 30.7

Oak-hickory 501,134 9.3 4.0 2.2 23.9 9.6 5.0 2.5 27.1

Oak-pine 192,825 9.5 3.8 1.4 21.3 10.1 4.9 1.9 25.6

Spruce-fir 58,803 8.7 4.2 3.2 26.7 7.7 3.9 2.2 22.1

White-red-jack pine 71,113 8.9 4.1 3.4 27.3 8.2 4.3 1.9 22.5

Chaparral 52,182 10.8 4.5 2.3 26.7 14.4 12.2 9.7 67.9

Douglas-fir 153,728 9.4 4.5 3.0 27.4 13.2 10.2 9.6 62.4

Fir-spruce 76,688 9.7 4.2 2.5 25.6 13.2 10.3 9.5 62.3

Hemlock-Sitka spruce 17,596 10.9 2.0 1.0 18.2 13.0 5.8 1.6 29.4

Larch 11,439 7.9 4.1 4.0 28.1 15.6 12.5 8.3 65.5

Lodgepole pine 117,396 9.6 4.7 3.7 30.1 12.8 10.2 9.9 62.9

Pinyon-juniper 224,678 11.1 4.8 2.0 26.7 13.7 11.0 8.3 60.6

Ponderosa pine 221,476 10.0 4.5 3.1 28.3 13.0 10.2 8.7 59.5

Redwood 5,998 11.1 3.0 1.5 21.6 16.7 6.7 1.3 34.0

Western hardwoods 65,993 9.9 4.3 2.8 26.9 13.6 10.6 8.9 61.5

Western white pine 9,048 7.9 3.5 3.7 26.0 15.1 12.3 10.7 71.8

Eastern Forest 1,593,058 9.3 4.0 2.3 24.2 9.5 5.0 2.5 27.0

Western Forest 956,222 9.8 4.0 2.7 25.9 14.0 10.2 7.9 58.1

MD= moderate drought (-2.0 to -2.9); SD=severe drought (-3.0 to -3.9); ED=extreme drought (≤ -4)

Table 1. Percent of drought occurrence among major forest types of the conterminous United States and relative cumulative drought severity index (CDSI) scores. 

Droughts reported at climate divisions containing each forest type were used to aggregate values and calculate percentages of the 324-month period.

Scale: 1:12,000,000

Projection: Albers Equal Area Conic

Central Meridian: 96° W

Latitude of Origin: 23° N

Datum: NAD83

0 150 300 450 600

Miles

0 250 500 750 1,000

Kilometers

Figure 1. Cumulative drought severity index values, which are a weighted summation of monthly drought occurrences, are mapped among climate divisions of the 

conterminous United States for the periods 1960 to1986 and 1987 to 2013. Forest cover types defined by the U.S. Forest Service were used to mask nonforested land 

depicting the impact of droughts within forests (large map and inset right).

1960-1986 1987-2013

Data Sources:

National Climatic Data Center. 2014.

North American Drought Monitor Indices

http://www.ncdc.noaa.gov/temp-and-precip/drought/nadm/indices.php?

CDSI

< 50

51 - 100

101 - 150

151 - 1200

201 - 250

> 250

U.S. Lakes
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Director, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence 

Avenue, SW, Washington, DC 20250-9410, or call (202) 720-5964 (voice or TDD). USDA is an 

equal opportunity provider and employer.

Spatio-temporal Drought Trends by 

Forest Type in the Conterminous 

United States, 1960-2013

Matthew P. Peters, Louis R. Iverson, and Stephen N. Matthews

Map Description

Droughts are common in virtually all U.S. forests, but their frequency and intensity vary within forest ecosystems (Hanson and 

Weltzin 2000). Accounting for the long-term influence of droughts within a region is difficult due to variations in the spatial extent 

and intensities over a period. Therefore, we created a cumulative drought severity index (CDSI) (Fig. 1) for the United States 

using weighted monthly frequencies of Palmer Drought Severity Index values reported at climate divisions for the periods 1960 

to 1986 and 1987 to 2013 (National Climate Data Center 2014, Palmer 1965). The occurrences of ‘severe’ conditions (index 

value of -3.0 to -3.9) receive a weight of 2 and ‘extreme’ (index value of ≤ -4.0) receive a weight of 3 to reflect the increased 

magnitude of these events. 

The Authors

Matthew P. Peters is a forestry technician and Louis R. Iverson is a research landscape ecologist with the Northern Research 

Station in Delaware, OH. Stephen N. Matthews is a research assistant professor in the School of Environment and Natural 

Resources at Ohio State University and is affiliated with U.S. Forest Service at the Northern Research Station. 

Forest Service Northern 

Research Station

Research Map

NRS-7

Publication Date 

December 2014

United States Department of Agriculture

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Manuscript received for publication

August 2014

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December 2014

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