LANDSCAPE CHANGE IN THE SOUTHWEST: Historical changes in selected ecosystems of the southwestern United States

Transcription

1 LANDSCAPE CHANGE IN THE SOUTHWEST: Historical changes in selected ecosystems of the southwestern United States Robert H. Webb U.S. Geological Survey 520 N. Park Avenue Tucson, AZ Raymond M. Turner Retired, U.S. Geological Survey

2 Mojave Desert Scrub Saguaros Riparian Vegetation THE DESERT LABORATORY REPEAT PHOTO COLLECTION Four collections, a total of 5,925 matches 3,058 in Arizona (1,524 in Grand Canyon) 2,096 on major rivers 910 in Utah 544 in California and Nevada Desert Grassland

3 WHY HAVE SOUTHWESTERN LANDSCAPES CHANGED HISTORICALLY? Climate variability, particularly persistent dry and wet periods, have recurred historically (and prehistorically). Land-use practices, particularly livestock grazing, have had considerable landscape effects. The interaction of plant life-history strategies and these effects drives biological (and some geomorphic) changes. Some geomorphic changes drive biological changes.

4 SOUTHERN ARIZONA HISTORIC CLIMATIC VARIABILITY EARLY 20 TH CENTURY DROUGHT EARLY 20 TH CENTURY WET PERIOD MID-CENTURY DROUGHT LATE 20 TH CENTURY WET PERIOD EARLY 21 ST CENTURY DROUGHT

5 INFORMATION ON HISTORICAL CHANGE IN RIPARIAN VEGETATION Observations of riparian vegetation begins with the Spanish explorations of Historical observations become more numerous with the scientific explorations beginning in the 1840s. Photography of begins in Mapped vegetation begins around Aerial photography begins in Satellite-platform remote sensing begins in 1974.

6 STUDY University of Arizona Press, 2007 AREA

7 REACHES THAT LOST RIPARIAN VEGETATION This area was extensively grazed historically. Santa Cruz River at Martinez Hill (1912)

8 Santa Cruz River at Martinez Hill (1981)

9 Santa Cruz River at Martinez Hill (2002)

10 SANTA CRUZ RIVER AT TUCSON

11 REACHES THAT GAINED RIPARIAN VEGETATION San Pedro River, 1890 Livestock grazing was heavy at this time, but note abundant perennial grasses

12 San Pedro River, 1962 Livestock grazing was heavy at this time before establishment of the San Pedro Riparian National Conservation Area

13 San Pedro River, 1994 Livestock have been removed from this reach

14 SAN PEDRO RIVER AT PALOMINAS

15 LANDSAT CHANGE DETECTION: San Pedro River (Kepner et al., 1999)

16 GILA RIVER AT CALVA (1935). The channel is wide from repeated floods in the 1930s. Open cottonwood galleries appear at left and center; willow is in right foreground.

17 GILA RIVER AT CALVA (1964). At the start of the Gila Phreatophyte Project, the reach was clogged with tamarisk, a non-native species.

18 GILA RIVER AT CALVA (1973). An herbicide was applied and mechanical treatments used to kill and remove tamarisk. Meanwhile, the channel is narrowing.

19 GILA RIVER AT CALVA (1984). By the 1980s, despite repeated flooding, tamarisk had begun to regrow in quantities in this reach.

20 GILA RIVER AT CALVA (2000). Tamarisk has regained at least its 1964 quantities in front of this camera station. No native species are visible, although mesquite is to the right.

21 Tamarisk Mesquite Cottonwoods

22 CHANGE IN RIPARIAN VEGETATION Large Reservoirs losses

23 WHY HAVE DECREASES OCCURRED? (How to Kill Riparian Trees Without Chainsaws) Excessively pump ground water: if ground-water use lowers water tables 10 m depth or more, most native riparian vegetation dies. Drop ground-water levels too quickly for roots to follow. Divert most or all surface water and pump ground water excessively. Provide no flood control, install bank protection, and and pump ground water excessively. Build a dam and impound a reservoir.

24 WHY HAVE INCREASES OCCURRED? Two wet episodes characterize 20 th century climate: the early 20 th century wet period (~ ) and the late 20 th century wet period ( ). Both periods had increased winter rainfall and floods beneficial to woody riparian vegetation. Temperatures (particularly winter and nighttime) have increased in the region since the 1960s, leading to a longer growth period, particularly for riparian vegetation.

25 ARROYOS AND FLOODS Modern arroyos downcut between about 1880 and 1909 in most of the Southwest at a time of high livestock grazing. Other Holocene arroyos formed about AD and before AD when no livestock were on the landscape. Most geomorphologists attribute arroyo downcutting to the occurrence of large floods associated with increases in regional climatic variability, not livestock presence Livestock and other land use (e.g., roads) undoubtedly increased erosion locally during historical downcutting

26 NET EFFECT OF ARROYOS Kanab Creek (1871) Kanab Creek (2000)

27 Escalante River near Escalante, Utah 1932 This area was 1985 extensively grazed historically This area was heavily grazed at the time of this photograph.

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29 CHANGES IN THE SONORAN DESERT Repeat photography documents large increases in mesquite and other woody C3 vegetation in what formerly was C4 grasslands. The changes represent an increase of as much as 5 to 10 times the above-ground biomass on upland sites. University of Arizona Press, 2003

30 Woody Plants in Desert Grassland 1901 Fire frequency ~ 5-10 years, grazing moderate-heavy 1995 Fires suppressed, grazing levels unchanged?

31 1887 Earthquake Fault, Sonora, Mexico 2000

32 Mesquite Increase in Desert Grassland (1890). Santa Cruz River just north of Mexico (1996). Santa Cruz River just north of Mexico

33 (1903). Coyote Mountain

34 (1999). Coyote Mountain

35 Grazing was heavy until 1958, when it was 1935 eliminated 1960 CHANGES IN THE SONORAN DESERT

36 But Drought is Killing Paloverdes (saguaro nurse plants), In permanent plots in Arizona and Sonora, saguaros are increasing in some and decreasing in others There is no clear relation between livestock grazing and saguaro demography Saguaro establishment requires nurse plants and favorable climatic conditions; grazing, drought, and severe freezes can reduce populations directly or influence nurse plants

37 MacDougal Pass, Sonora 1907 Grazing increased, eliminates big galleta grass, saguaros increase (n=255)

38 Saguaro Demographics In 9 permanent plots in Arizona and Sonora, saguaros are increasing in some and decreasing in others There is no clear relation between livestock grazing and saguaro demography Saguaro establishment requires nurse plants and favorable climatic conditions; grazing, drought, and severe freezes can reduce populations directly or influence nurse plants

39 Biomass Change SPECIES 1ST PHOTO MEDIAN 2ND PHOTO MEDIAN 3RD PHOTO MEDIAN % CHANGE 1ST- 2ND Bursage Chihuahuan whitethorn Cardón Creosotebush Foothill paloverde Jumping cholla Mesquite Oaks Ocotillo ND- 3RD One-seed juniper Pricklypear Saguaro

40 VEGETATION CHANGES IN THE MOJAVE DESERT Dr. Janice C. Beatley established 68 permanent vegetation plots on the Nevada Test Site in 1962 Perennial vegetation was measured in 1963, 1975, and (all), 2003, 2005 (partial) USGS, 2003

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42 PLOT 3, ROCK VALLEY, MOJAVE DESERT

43 42 YEARS OF CHANGE IN PLOT 3 From , the total number of shrubs on Plot 3 increased by 24%. Larrea increased in height and cover by 43% and 168%. Ambrosia increased in in height and cover by 17% and 166%. Grayia decreased in cover by -80% ( drought). From , Ambrosia decreased in cover by -35% ( drought). Larrea remained about the same. Total cover changed +38% and -23% from and , respectively.

44 42 YEARS OF CHANGE IN COVER (ALL UNDISTURBED PLOTS) Creosote Sagebrush, pinyonjuniper Chenopods

45 Plot 50 (1964) This area may never have been grazed (no nearby water sources). This assemblages is a chenopod shrubland.

46 Plot 50 (2001) This assemblages is now dominated by perennial grasses.

47 PERENNIAL GRASSES ON THE NEVADA TEST SITE Perennial species (Achnatherum speciosum and A. hymenoides) had massive pulse establishment in the early 1980s (start of late 20 th century wet period). The drought of caused widespread mortality of perennial grasses established less than 10 years earlier. Wet periods ( , ) caused significant reestablishment. The early 21 st century drought caused mortality again. No grazing occurs here these dynamics in perennial grass populations is climatically driven.

48 SUMMARY Landscapes (and ecosystems) should be expected to change. Sometimes landscapes (and ecosystems) change for reasons other than land use. Heavy livestock grazing causes significant ecosystem damages. However, the mere presence of livestock does not drive change on most landscapes. Climate never has been constant. Climate variability should be expected.

49 WHY ARE CERTAIN SPECIES OF WOODY PLANTS INCREASING IN BIOMASS? Increased growing season (increased winter temperatures), combined with increased winter precipitation, is the most plausible reason. Increased carbon dioxide could be a factor favoring C3 versus C4 plants. However, photosynthetic pathway does not fully explain the response of many species. For riparian vegetation, flow regulation gives and takes away hydrologic stability versus recruitment potential. Sustained drought could reverse these gains; drought combined with ground-water pumping could increase amount of riparian mortality.