Land Cover Trends Project

Sierra Nevada Ecoregion Summary

By Christopher E. Soulard 1

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Refer to caption

Figure 1. Sierra Nevada and surrounding ecoregions. Information shown includes land-use/land-cover data from the 1992 National Land Cover Dataset and the 36 randomly selected 100 km² sample blocks used to create estimates of change for the entire ecoregion (Vogelmann and others 2001).

 

Ecoregion Description

 

The Sierra Nevada ecoregion covers approximately 53,200 square kilometers (km²) with the majority of area (98 percent) in California and a small portion (2 percent) in Nevada (fig. 1). The Sierra Nevada ecoregion is gener­ally orientated from north to south and is in essence defined by the Sierra Nevada physiographic province, which separates the California Central Valley to the west from the Great Basin to the east. The Sierra Nevada is a granitic batholith, much of which is exposed at higher elevations, with a gradual western slope and a generally steep eastern escarpment.

 

The climate of the Sierra Nevada ecoregion is primarily Mediterranean, char­acterized by cool, wet winters and long, dry summers. Most areas of elevation above 2,100 meters have a Boreal climate, and the highest elevations, typically above 3,600 meters, have an Alpine climate. Precipitation increases with elevation from west to east as storm systems moving from the west are subject to orographic uplift causing rain and snowfall. As most precipitation from storm systems falls on the western slope of the Sierra Nevada mountains, a strong rainshadow results and limits precipitation on the steep eastern slope. This climatic gradient plays a sig­nificant role in determining type and distribution of ecologi­cal communities. In order to provide water resources for the growing populations in arid, low-elevation areas of California and Nevada, numerous reservoirs on the western and eastern slopes of the Sierra Nevada ecoregion collect runoff from the winter snow pack.

 

Prior to the 20th century, resource use within the Sierra Nevada ecoregion was largely unregulated. However, laws and administrative policies such as the Wilderness Act of 1964, National Environmental Policy Act of 1969 (NEPA), and National Forest Management Act of 1976 (NFMA) provided a mechanism to manage national forests. Furthermore, other environmental laws, annual appropriations legislation, and administrative policies relating to fire and fuels guide resource use and likely have significant environmental effects in the Sierra Nevada region (Ruth 1996). Today, public lands make up 84.5 percent (444,677 km²) of the ecoregion with the majority (68.3 percent) managed by the U.S. Forest Service (USFS) as National Forests.

 

Despite resource regulation, California’s growing urban population greatly increased the demand for wood, water, hydroelectricity, and recreational opportunities from the Sierra Nevada ecoregion. Timber harvesting surged and continued until the economic recession in the early 1980s. Water, which is considered the region’s most valuable resource, is controlled in nearly every major river basin in the region and managed to provide municipal water supplies and hydroelectric power (SNEP Science Team and Special Consultants, 1996). Major highways and ski resorts were constructed in the 1950s and 1960s to meet the demand for year-round recreation (SNEP Science Team and Special Consultants, 1996). Over the last several decades, the demand for natural resources within the Sierra Nevada ecoregion has altered ecological communities in the region by changing land-use/land-cover (LU/LC) patterns.

 

In terms of nonmechanical LU/LC change components, frequent fires of low to moderate intensity have historically been and continue to be an integral driver of change within the region’s ecological communities. Fires created a cycle of disturbance and succession that floral and faunal communities have adapted to and often require to propagate and thrive (Skinner and Chang, 1996). By the late twentieth century the regional fire regime had greatly changed primarily due to logging during the settlement period of the 1950s and 1960s and effective fire suppression activities mandated by State and Federal policy since the 1920s. As a result, fires were less frequent and more severe (Skinner and Chang 1996). Forest density increased and contributed to higher tree mortality due to greater intertree competition, insect attack, disease, and storm damage (Oliver and others 1996). These conditions led to an increased supply of fuels which, in turn, resulted in an increased fire hazard, including the likelihood of high-severity fire. (Manley and others 2000). A shift to a warmer and moister climate may have also contributed to this altered fire regime by reducing winter severity and providing a longer growing season (McKelvey and others, 1996; Stine, 1996).

 

Contemporary Land Cover Change (1973 to 2000)

 

The overall areal extent, or “footprint,” of LU/LC change from 1973-2000 was 4.9 percent (2,591 km²), which means that 4.9 percent of the Sierra Nevada ecoregion underwent LU/LC change dur­ing at least one of the four multiyear periods that make up the entire 27-year study period. Of this 4.9 percent footprint, 3.2 percent of the ecoregion changed during only one period, 1.5 percent changed during two periods, and 0.3 percent change during three periods (table 1). This footprint of change in the Sierra Nevada was low-moderate when compared to other completed ecoregions in the western United States (fig. 2).

 

The estimated average annual rate of LU/LC change is cal­culated by normalizing each period’s gross change by the num­ber of years in that period. Normalizing gross change by year effectively allows comparison of the amount of change in each period when periods are of varying duration (6, 7, or 8 years). It is important to note that the resulting rates-of-change, although presented as per-year rates, are only an estimate and should be viewed as a description of the period and not of the individual years within the period. The estimated average annual rate of LU/LC change for the entire 27-year study period from 1973-2000 was 0.3 percent/year, which means that on average 0.3 percent (or roughly 144 km²) of the Sierra Nevada ecoregion changed each year. However, the annual rate of change has not been constant during the 27-year study period as shown by the estimated average annual rates for the four periods. From 1973-1980 and from 1980-1986, change occurred at identical rates of 0.1 percent/year. The annual rate of change increased to 0.3 percent/year from 1986-1992 and continued to increase to 0.5 percent/year from 1992-2000 (table 2).

 

Our results show that in 2000 the Sierra Nevada ecoregion was dominated by forest cover (70.1 percent) with grassland/shrubland (20.4 percent), barren (2.7 percent), nonmechanically disturbed (2.4 percent), wetland (2.2 percent), and water (1.1 percent) making up the remainder of land cover (table 3). Developed, mining, agriculture, snow/ice, and mechanically disturbed LU/LC types each made up less than one percent of the region (table 3). Land-use/land-cover classes that underwent the greatest net change (that is, total area gained minus total area lost) in relation to total ecoregion area since 1973 were forest (3.5 percent decrease), grass/shru­bland (1.1 percent increase), and nonmechanically disturbed (2.3 percent increase). Although the developed and agricul­ture classes each makeup less than 1 percent of the Sierra Nevada ecoregion, the developed class underwent the greatest increase in area (16.6 percent) and agriculture underwent the greatest decrease in area (5.5 percent) in relation to their respective total class area not considering the transitional nonmechanically disturbed class.

 

The net change values as a percentage of ecoregion area at the beginning (1973) and end (2000) dates of the study period in table 3 show little variability and may seem to indicate stabil­ity. Net change values, however, often mask LU/LC dynamics. For example, a class may gain 100 km² and at the same time lose 100 km² which would yield a net change of 0 km². Reporting the net change value of 0 km² misses much of the story of landscape change. However, analysis of gross change (that is, area gained and lost) by individual LU/LC classes by period shows classes have fluctuated throughout the 27-year study period to a greater degree than net change values may indicate. Figure 3 shows that the forest, grass/shrubland, mechanically disturbed, and nonmechanically disturbed classes were the most dynamic from 1973-2000. The transitional char­acteristic of the mechanically disturbed class is also illustrated as area gained (809 km²) and nearly equals area lost (753 km²) from 1973-2000. Land-cover change was clearly at its peak during the period from 1992-2000 when gains and losses were generally greatest for the four most dynamic classes.

 

All individual LU/LC conversions between classes were ranked by summing the total area changed during each of the four periods. Each conversion documents land changing from one class to another (for example, forest to developed) and shows the direction of change. Table 4 shows each individual conver­sion ranked from greatest to least area converted. The most common individual conversions describe the disturbance of forested lands by mechanical (that is, clear-cuts) and nonme­chanical (that is, fire) means. Overall, the most common conversion was that of 1,404 km² of forest to the nonmechanically dis­turbed class which accounted for 37.3 percent of all conver­sions (fig. 4). The second most common conversion was that of 784 km² forest to the mechanically disturbed class accounting for 20.8 percent of all changes (fig. 5). Conversion of mechanically and nonmechanically disturbed lands to the grass/shrubland class (753 km² and 307 km², respectively) were the two next most common conversions and represented the process of vegetation regeneration after clear-cutting or fire (fig. 6). Similarly, conversion of grass/shrubland to forest (303 km²) represented the final stage of the regeneration cycle. A much less common but noteworthy conversion was that of water to mechanically disturbed (26 km²), which accounted for 0.7 percent of all individual conversions (fig. 7). This conversion indicates surface-level fluctuations of reservoirs in the ecoregion.

 

More insight can be provided by aggregating the conversions listed in table 4 to identify how a single to or from class was affected. Between 1973 and 2000, 1,540 km² of vegetation (forest, grass/shrubland, and wetland) area was converted to the nonmechanically disturbed class. Fire caused all of these con­versions. Regeneration after disturbance was captured as the conversion of nonmechanically disturbed lands to vegetation classes (forest and grass/shrubland) and conver­sion of mechanically disturbed lands to vegetation classes (forest and grass/shrubland) for aggregated totals of 317 km² and 787 km², respectively.

 

            In conclusion, the LU/LC change patterns measured in the Sierra Nevada ecoregion between 1973 and 2000 are consistent with the literature. Much of the clear-cutting and reservoir water-level change in the region has been driven by the demand for wood, water, hydroelectricity, and recreational opportunities associated with California’s growing urban population. As for fires, much of the severe contemporary fires in the Sierra Nevada ecoregion are likely the result of a fuel build up caused by fire suppression activities mandated by State and Federal policy since the 1920s.

 

The preceding summary provides selected details from:

 

Raumann, Christian G., and Soulard, Christopher E., 2007, Land-cover trends of the Sierra Nevada Ecoregion, 1973-2000:  U.S. Geological Survey Scientific Investigations Report 2007-5011 [http://pubs.usgs.gov/sir/2007/5011/].

 

 

References

 

Manley, P.N., Fites-Kaufman, J.A., Barbour, M.G., Schlesinger, M.D., and Rizzo, D.M., 2000, Biological integrity, in Murphy, D.D., and Knopp, C.M., eds., Lake Tahoe watershed assessment: U.S. Forest Service Pacific Southwest Research Station, Albany, Calif., Gen. Tech. Rep. PSW-GTR-175, v. 1, c. 5, p. 403-598.

 

McKelvey, K.S., Skinner, C.N., Chang, C., Erman, D.C., Husari, S.J., Parsons, D.J., van Wagtendonk, J.W., and Weatherspoon, C.P., 1996, An overview of fire in the Sierra Nevada, in Sierra Nevada Ecosystem Project final report to Congress, vol. II, Assessments and scientific basis for management options: Davis, University of California, Centers for Water and Wildlands Research, v. 2, c. 37, p. 1033-1040.

 

Oliver, W.W., Ferrell, G.T., and Tappeiner, J.C., 1996, Density management of Sierra Forests in Sierra Nevada Ecosystem Project final report to Congress, vol. III, Assessments, commissioned reports, and background information: Davis, University of California, Centers for Water and Wildlands Research, v. 3, c. 11, p. 217-276.

 

Ruth, L., 1996, Conservation and controversy—national forest management, 1960-95, in Sierra Nevada Ecosystem Project final report to Congress, vol. II, Assessments and scientific basis for management options: Davis, University of California, Centers for Water and Wildlands Research, v. 2, c. 7, p. 145-162.

 

Skinner, C.N., and Chang, C., 1996, Fire regimes, past and present, in Sierra Nevada Ecosystem Project final report to Congress, vol. II, Assessments and scientific basis for management options: Davis, University of California, Centers for Water and Wildlands Research, v. 2, c. 38, p. 1041-1069.

 

SNEP Science Team and Special Consultants, 1996, People and resource use, in Sierra Nevada Ecosystem Project final report to Congress, vol. I, Assessment summaries and management strategies: Davis, University of California, Centers for Water and Wildlands Research, v. 1, c. 2, p. 17-45.

 

Stine, S., 1996, Climate, 1650-1850, in Sierra Nevada Ecosystem Project final report to Congress, vol. II, Assessments and scientific basis for management options: Davis, University of California, Centers for Water and Wildlands Research, v. 2, c. 2, p. 25-30.

 

Vogelmann, J.E., Howard, S.M., Yang, L., Larson, C.R., Wylie, B.K., and van Driel, N., 2001, Completion of the 1990s National Land Cover Data Set for the conterminous United States from Landsat Thematic Mapper data and ancillary data sources: Photogrammetic Engineering and Remote Sensing, v. 67, p. 650-662.

 

 

Figures and Tables

 

Sierra Nevada and surrounding ecoregions

 

Figure 1. Sierra Nevada and surrounding ecoregions. Information shown includes land-use/land-cover data from the 1992 National Land Cover Dataset and the 36 randomly selected 100 km² sample blocks used to create estimates of change for the entire ecoregion (Vogelmann and others 2001).

 

Table 1. Footprint of Change in the Sierra Nevada ecoregion. Estimated percentage of ecoregion that experienced change within the study period. 

 

 

 

 

 

 

 

 

 

 

 

85% CONFIDENCE INTERVAL

 

 

 

 

FOOTPRINT of CHANGE

(1973-2000)

% of ECOREGION

 +/- (%)

LOWER

UPPER

STANDARD ERROR

RELATIVE ERROR

ECOREGION AREA (km²)

 +/- (km²)

ALL CHANGE

4.9%

2.5%

2.4%

7.5%

1.7%

34.9%

2632.4

1354.2

1 Change

3.2%

2.5%

0.7%

5.7%

1.7%

53.5%

1698.6

1338.3

2 Changes

1.5%

0.5%

1.0%

2.0%

0.3%

22.3%

796.6

261.4

3 Changes

0.3%

0.3%

0.0%

0.5%

0.2%

77.9%

133.1

152.5

4 Changes

0.0%

0.0%

0.0%

0.0%

0.0%

90.3%

4.1

5.5

 

 

 

 

 

 

 

 

 

 

See caption

 

Figure 2. Comparison between the Sierra Nevada ecoregion (outlined in red) and the other completed ecoregions in the western United States. Between 1973 and 2000, the Sierra Nevada ecoregion had a low to moderate footprint of change relative to other ecoregions in the West.

 

 

Table 2. Overall change in the Sierra Nevada ecoregion. Overall change estimates by interval, 85% confidence interval, standard error, relative error, and normalized average annual change.

 

 

 

 

 

 

 

 

% stratum

 

85% Confidence Interval

 

 

 

 

Change Estimate

+/-

Lower Bound

Upper Bound

Standard Error

Relative Error

Average Annual %

1973 to 1980

0.9%

0.5%

0.4%

1.3%

0.3%

36.0%

0.1%

1980 to 1986

0.7%

0.4%

0.4%

1.1%

0.2%

33.2%

0.1%

1986 to 1992

1.6%

0.5%

1.1%

2.1%

0.4%

21.6%

0.3%

1992 to 2000

3.9%

2.5%

1.3%

6.4%

1.7%

44.3%

0.5%

 

 

 

 

 

 

 

 

km²

 

85% Confidence Interval

 

 

 

 

Change Estimate

+/-

Lower Bound

Upper Bound

Standard Error

Relative Error

Average Annual

1973 to 1980

452.4

239.9

212.5

692.2

0.3%

36.0%

64.6

1980 to 1986

398.7

194.8

203.9

593.5

0.2%

33.2%

66.4

1986 to 1992

864.5

275.0

589.5

1139.5

0.4%

21.6%

144.1

1992 to 2000

2051.3

1338.9

712.4

3390.1

1.7%

44.3%

256.4

 

 

Table 3. Land-cover trends in the Sierra Nevada ecoregion. Percentages and amounts of each land cover class for each of the five mapped dates and associated margins of error.

 

 

 

 

 

 

 

 

 

 

 

 

 

1973

1980

1986

1992

2000

 

 

Estimate

85% CI

Estimate

85% CI

Estimate

85% CI

Estimate

85% CI

Estimate

85% CI

Change

Water

1.15%

0.53%

1.13%

0.53%

1.13%

0.53%

1.11%

0.53%

1.10%

0.53%

0.0%

Developed

0.24%

0.25%

0.24%

0.25%

0.24%

0.25%

0.24%

0.25%

0.28%

0.28%

0.0%

Mech. Disturbed*

0.36%

0.26%

0.12%

0.07%

0.29%

0.16%

0.77%

0.29%

0.40%

0.19%

0.0%

Mining

0.14%

0.18%

0.14%

0.18%

0.14%

0.18%

0.14%

0.18%

0.14%

0.18%

0.0%

Barren

2.71%

1.49%

2.71%

1.49%

2.71%

1.49%

2.71%

1.49%

2.71%

1.49%

0.0%

Forest*

73.53%

4.63%

73.21%

4.61%

73.10%

4.59%

72.53%

4.46%

70.07%

4.63%

-3.5%

Grass/ Shrub*

19.21%

4.01%

19.72%

3.91%

19.88%

3.88%

19.75%

3.91%

20.36%

3.82%

1.1%

Agriculture

0.30%

0.41%

0.30%

0.41%

0.30%

0.41%

0.30%

0.41%

0.28%

0.39%

0.0%

Wetland

2.20%

1.24%

2.20%

1.24%

2.20%

1.24%

2.20%

1.24%

2.20%

1.24%

0.0%

N.M. Disturbed

0.16%

0.20%

0.21%

0.28%

0.00%

0.00%

0.23%

0.23%

2.45%

2.51%

2.3%

Snow/Ice

0.02%

0.02%

0.02%

0.02%

0.02%

0.02%

0.02%

0.02%

0.02%

0.02%

0.0%

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1973

1980

1986

1992

2000

 

 

(km²)

85% CI

(km²)

85% CI

(km²)

85% CI

(km²)

85% CI

(km²)

85% CI

Change

Water

609.5

282.0

603.1

282.0

603.1

282.0

590.1

282.0

584.0

282.0

-25.4

Developed

126.1

133.0

126.1

133.0

126.1

133.0

128.3

133.0

147.0

149.0

21.0

Mech. Disturbed*

190.4

138.3

64.5

37.2

152.0

85.1

409.0

154.3

213.6

101.1

23.2

Mining

73.2

95.8

73.2

95.8

73.2

95.8

73.2

95.8

73.2

95.8

0.0

Barren

1440.2

792.7

1440.2

792.7

1440.2

792.7

1440.2

792.7

1440.2

792.7

0.0

Forest*

39117.5

2463.2

38948.1

2452.5

38890.1

2441.9

38586.1

2372.7

37278.1

2463.2

-1839.4

Grass/ Shrub*

10218.1

2133.3

10491.5

2080.1

10573.9

2064.2

10508.0

2080.1

10829.0

2032.2

610.9

Agriculture

159.4

218.1

159.4

218.1

159.4

218.1

159.4

218.1

151.2

207.5

-8.2

Wetland

1171.8

659.7

1170.7

659.7

1171.8

659.7

1171.8

659.7

1171.5

659.7

-0.3

N.M. Disturbed

84.0

106.4

113.3

149.0

0.5

0.0

124.1

122.4

1302.3

1335.3

1218.2

Snow/Ice

9.9

10.6

9.9

10.6

9.9

10.6

9.9

10.6

9.9

10.6

0.0

 

 

 

 

 

 

 

 

 

 

 

 

Note: Only the Mech. Disturbed, N.M. Disturbed, Forest, and Grassland/Shrubland classes had Wilcoxon pairs (N) ≥10.

 

 

 

* Denotes classes with statistically significant trends and N≥10

 

 

 

 

Refer to caption

Figure 3. Gross change (area gained and lost) by class by period in the Sierra Nevada ecoregion. Class area gained is shown by positive values and class area lost is shown by negative values. No changes were detected to the mining, snow/ice, and barren classes and consequently are not listed here.

 

 

 

 

 

 

Table 4. Common Land Cover Conversions in the Sierra Nevada ecoregion. Top five land cover conversions, margin of error, standard error, and as a percentage of all changes.

 

 

 

 

 

 

 

 

 

 

 

Area changed

Standard Error

85% CI

% of ecoregion

% of all

Period

From class

To class

(km²)

+/- (km²)

changes

1973-1980

M.Disturbed

Grass/Shrub

191.2

97.7

143.9

0.4%

42.3%

 

Forest

N.M.Disturbed

111.7

103.3

152.1

0.2%

24.7%

 

N.M.Disturbed

Grass/Shrub

84.4

74.2

109.2

0.2%

18.7%

 

Forest

M.Disturbed

58.4

26.1

38.4

0.1%

12.9%

 

Water

M.Disturbed

6.4

6.2

9.1

0.0%

1.4%

 

Other classes

Other classes

0.3

n/a

n/a

0.0%

0.1%

 

 

 

452.4

 

 

0.9%

100%

 

 

 

 

 

 

 

 

1980-1986

Forest

M.Disturbed

146.2

60.5

89.0

0.3%

36.7%

 

N.M.Disturbed

Grass/Shrub

109.9

103.3

152.1

0.2%

27.6%

 

Grass/Shrub

Forest

81.4

52.8

77.7

0.2%

20.4%

 

M.Disturbed

Grass/Shrub

54.2

25.4

37.4

0.1%

13.6%

 

M.Disturbed

Forest

4.2

4.1

6.0

0.0%

1.1%

 

Other classes

Other classes

2.8

n/a

n/a

0.0%

0.7%

 

 

 

398.7

 

 

0.7%

100%

 

 

 

 

 

 

 

 

1986-1992

Forest

M.Disturbed

391.1

104.7

154.1

0.7%

45.2%

 

Grass/Shrub

Forest

190.2

116.5

171.5

0.4%

22.0%

 

M.Disturbed

Grass/Shrub

146.2

60.5

89.0

0.3%

16.9%

 

Forest

N.M.Disturbed

102.0

65.5

96.4

0.2%

11.8%

 

Grass/Shrub

N.M.Disturbed

22.6

21.8

32.1

0.0%

2.6%

 

Other classes

Other classes

12.3

n/a

n/a

0.0%

1.4%

 

 

 

864.5

 

 

1.6%

100%

 

 

 

 

 

 

 

 

1992-2000

Forest

N.M.Disturbed

1190.1

835.3

1229.6

2.2%

58.0%

 

M.Disturbed

Grass/Shrub

361.4

92.0

135.5

0.7%

17.6%

 

Forest

M.Disturbed

187.8

70.8

104.3

0.4%

9.2%

 

N.M.Disturbed

Grass/Shrub

112.0

81.0

119.2

0.2%

5.5%

 

Grass/Shrub

N.M.Disturbed

111.7

78.6

115.6

0.2%

5.4%

 

Other classes

Other classes

88.2

n/a

n/a

0.2%

4.3%

 

 

 

2051.3

 

 

3.9%

100%

Overall:

 

 

 

 

 

 

 

1973-2000

Forest

N.M.Disturbed

1404.3

845.1

1243.9

2.6%

37.3%

 

Forest

M.Disturbed

783.5

202.9

298.7

1.5%

20.8%

 

M.Disturbed

Grass/Shrub

753.0

219.4

323.0

1.4%

20.0%

 

N.M.Disturbed

Grass/Shrub

306.7

145.2

213.7

0.6%

8.1%

 

Grass/Shrub

Forest

303.0

132.5

195.0

0.6%

8.0%

 

Grass/Shrub

N.M.Disturbed

135.3

86.4

127.2

0.3%

3.6%

 

M.Disturbed

Forest

33.9

32.6

47.9

0.1%

0.9%

 

Water

M.Disturbed

25.6

20.7

30.5

0.0%

0.7%

 

Agriculture

Developed

10.4

10.0

14.7

0.0%

0.3%

 

N.M.Disturbed

Forest

9.8

7.2

10.6

0.0%

0.3%

 

Other classes

Other classes

1.3

n/a

n/a

0.0%

0.0%

 

 

 

3766.8

 

 

7.1%

100%

 

Refer to caption

 

Figure 4. September 2004 appearance of an area (background slope) undergoing regeneration following the Manter Fire at the southern end of the Sierra Nevada ecoregion in Sequoia National Forest, Tulare County, CA. The Manter Fire ignited on 22 Jul 2000 and burned about 300 km². Land-cover types shown are forest, grass/shrubland, and wetland.

 

 

 

Refer to caption

 

Figure 5. Recently clearcut area in Plumas National Forest, Plumas County, CA. Land-cover types shown are forest and mechanically disturbed.

 

Refer to caption

 

Figure 6. Forest regeneration after seeding, Plumas National Forest. Land-cover types shown are forest and grass/shrubland.

 

 

Refer to caption

 

Figure 7. Courtright Reservoir in Sierra National Forest, Fresno County, CA exhibited lowered surface levels by late summer (Sep 2004). Land-cover types shown are forest, natural barren, and mechanically disturbed due to reservoir drawdown.

 

[1] U.S. Geological Survey, Western Geographic Science Center, Menlo Park, CA 94025

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