Summary of Climatic Variability and Ecosystem Response at the Jornada LTER Site

Inferred climate variability at the Jornada LTER site, based on several types of evidence, is presented in figure 17.7. This graph is a working hypothesis of the biocli-matic changes during the last 20,000 years. The graph plots aridity, C4 grasslands, C3 woodlands, and C3 shrublands across 9 bioclimatic time intervals. Because vegetation is laterally diverse today, it was probably laterally diverse in the prehistoric past. Basin floor vegetation is commonly different than piedmont slope vegetation which, in turn, is different from mountain vegetation (Dick-Peddie 1993). Therefore, to simplify the inferences, the curve in figure 17.7 is limited to the piedmont slopes (bajadas) at the LTER site.

The first and earliest interval represented by figure 17.7 is the last full glacial pe-

Figure 17.6 Dissolution pipes in a petrocalcic horizon and carbonate filaments of Holocene age in the upper part of the soil profile. (Modified from Gile and Grossman 1979)

riod (about 20,000-18,000 years b.p.). Before this period, less paleoclimatic information is available than after this period for the Jornada region. Notable exceptions include faunal remains in caves in southern New Mexico (Harris 1987). Based on these studies, late Pleistocene climates before the last glacial maximum were relatively mild, cool, and moist compared with today's climate, yet not as cool and moist as the climate during the last full glacial. Still farther back in time, the previous interglacial period (O-isotope stage 5e, about 120,000-130,000 years b.p.) was probably a warm, dry period like the Holocene (Hawley 1993). During the Quaternary as a whole, all of the multiple interglacial periods, each about 100,000 years apart, probably had warm, dry climates during which erosion and sedimentation were active. Glacial periods, in contrast, were each probably periods of landscape stability and soil development.

Interval 1 (20,000 to 18,000 years b.p.) in figure 17.7 represents the last glacial maximum. During this time, the climate was wetter and cooler than today at the Jornada LTER site. The evidence that climate was wetter is based on highstands that occurred at Lake Estancia about 19,700 14C years b.p. (Allen and Anderson 1993), at Lake King about 19,100 14C years b.p. (Wilkins and Currey 1997), and at Lake San Agustin about 22,000 to 19,000 years ago (Forester 1987). Radiocarbon ages of carbonate and organic matter in pipes through petrocalcic horizons suggest deeper wetting in soil profiles (Gile et al. 1981). The climate was cooler based on alpine glaciers that occurred at Sierra Blanca during the last full glacial interval (marine-O-isotope stage 2; Hawley 1993). Elevations of rock glaciers in the mountains suggest temperatures 7 to 8°C cooler than today (Blagbrough 1994). These temperatures roughly correspond to temperature estimates of 5 to 7°C cooler than today based on groundwater in northwest New Mexico (Phillips et al. 1986; Stute et al. 1995). Packrat middens record pinyon-juniper-oak vegetation along rock escarpments that are now desert grassland and shrubland (Van Devender 1990). Packrat middens also suggest greater rainfall than today, rather than cold, dry conditions suggested by Galloway (1970, 1983) and Brackenridge (1978). Fossil pollen indi-

increasing aridity

Figure 17.7 Inferred bioclimatic changes on piedmont slopes (bajadas) at the Jornada LTER site (see text for explanation).

increasing aridity

Figure 17.7 Inferred bioclimatic changes on piedmont slopes (bajadas) at the Jornada LTER site (see text for explanation).

cates pinyon at lower elevations (Hall 1997). Carbon isotopes of pedogenic carbonate in both piedmont slope and basin floor soils suggest some C3 woodland at this time (figure 17.5).

Interval 2 (18,000 to 13,000 years b.p.) in figure 17.7 represents a period having a slight increase in aridity followed by a return to more humid conditions based on the observation that lakes desiccated but returned in the latter part of this interval. At Lake Estancia a highstand is recorded at about 13,700 14C years b.p. (Allen and Anderson 1993). At Lake Cochise, a highstand is recorded at about 13,700 and 13,400 14C years b.p. (Waters 1989). Packrat middens indicate that pinyon-juniper-oak vegetation was still present on rock escarpments (Van Devender 1990). Values of S13C record peak C4 grassland at four sites during this period (figure 17.5).

Interval 3 (13,000 to 10,000 years b.p.) represents a period of increased aridity based on lake desiccation. In addition, both packrat and pollen records indicate the loss of pinyon (Van Devender 1990, Hall 1997). Pollen records also suggest the establishment of drier Chenopodiaceae-Asteraceae shrub grassland. Formation of the Isaacks' Ranch geomorphic surface on piedmont slopes during most of this interval suggests landscape instability, erosion, and sedimentation (Hawley 1975, Gile 1987). Compared to the more extensive Organ surface, however, the Isaacks' Ranch surface suggests that this interval had less aridity than the middle Holocene.

Interval 4 (10,000 to 7,500 years b.p.) represents a return to more effective moisture based on a highstand at Lake Cochise at about 9,000 14C years b.p. (Waters 1989). Packrat middens suggest that winter rainfall was greater than today (Van Devender 1990).

Interval 5 (7,500 to 5,000 years b.p.) represents a major period of aridity. Deflation and dune formation occurred at lake sites (Hawley 1993). Packrat middens record the disappearance of oaks along rock escarpments. Packrat middens also suggest that this was a time when winter rainfall was replaced by biseasonal rainfall dominated by the summer monsoon (Van Devender 1990). Fossil pollen indicates aridity based on the high amount of Cheno-Am pollen (Freeman 1972; Hall 1985; Monger et al. 1998). A major loss of C4 grassland and an increase in C3 shrubs is suggested by S13C values on both piedmont slopes and basin floors (figure 17.5). A major period of aridity is also based on the deposition of Organ I sediments on piedmont slopes (Gile 1975) and Fillmore sediments along the Rio Grande (Gile et al. 1981). Radiocarbon dates of carbonate in the upper zones of soil profiles suggest shallow wetting fronts (Monger et al. 1998). This period corresponds, roughly, to Antevs' hot, dry Altithermal (Antevs 1955) and was a period of major arroyo cutting in southeast Arizona (Waters and Haynes 2001).

Interval 6 (5,000 to 3,000 years b.p.) represents a decrease in aridity based on the return of two small lakes at Lake Cochise after the middle Holocene (Waters 1989). Fossil pollen indicates an increase of grass and a more mesic climate than the middle Holocene (Freeman 1972). Packrat middens, however, suggest that this was a time when the modern climate was established, with fewer winter freezes, mon-soonal summer rainfall, and increased droughts, as well as the appearance of cre-osotebush (Van Devender 1990). On the other hand, landscape stability is suggested by soil development in Organ I sediments during this period (Gile et al. 1981). El Niño events became stronger and more frequent based on an increased frequency of arroyo cutting in southeast Arizona (Waters and Haynes 2001).

Interval 7 (3,000 to 1,100 years b.p.) represents another interval of aridity, albeit less profound than middle Holocene aridity. Evidence for aridity in this interval is based on the deposition of gravelly Organ II sediments that had occurred by 2,200 14C years b.p. (Gile et al. 1981). Pollen also indicates increased aridity, but less than the aridity of middle Holocene (Freeman 1972). Toward the end of this period, aridity waned as indicated by soil formation in Organ II sediments (Gile et al. 1981).

Interval 8 (1,100 years b.p. to a.d. 1850) represents a still later period of aridity based on erosion and deposition of Organ III sediments (Gile et al. 1981). But this period of erosion and sedimentation was of lower magnitude than even Organ II sediments, which were minor compared to Organ I sediments of middle Holocene age. Toward the end of this period, grasses were much more abundant than today, based on anecdotal evidence (Gardner 1951).

Interval 9 (since a.d. 1850) represents the historical record of bioclimatic change, characterized by the progressive increase of shrublands and the progressive decrease of grasslands based on land survey records of the late-1850s and vegetation surveys after 1915 (Buffington and Herbel 1965). Soil-geomorphic studies also record a major period of wind erosion and formation of mesquite coppice dunes in response to the loss of grasslands (Gile 1966). Unlike bioclimatic changes, this change has been affected by humans in ways ranging from direct land use to increased atmospheric CO2 from fossil fuel emissions.

Acknowledgments Funding for various aspects of fossil pollen, carbon isotopes, and radiocarbon analyses was provided by the Jornada LTER program supported by the National Science Foundation, DEB-0080412 and DEB-94111971. Other support was provided by the USDA Jornada Experimental Range, the New Mexico State University Agricultural Experiment Station, and Fort Bliss Military Reservation. Discussions with David R. Cole on terrestrial isotopic records and with Leland H. Gile and John W. Hawley on landscape evolution are greatly appreciated. Grateful acknowledgment is made to Sue Ann Monger, Rebecca Kraimer, Marco Inzunza, and Barbara Nolen for their help with the manuscript.

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