Research Featured on Past Journal Covers
Fire statistics and fire-scar chronologies show a close relationship between tropical Pacific sea surface temperatures and fire activity in the southwestern U.S. Based on data from TAO moorings in the equatorial Pacific, a developing El Niño (La Niña) can be used to predict a wet (dry) spring and a mild (severe) fire season as much as 9 months in advance. Fire managers can either prepare months in anticipation of a bad fire season, or start scheduling prescribed burns well ahead of a mild one (Swetnam and Betancourt 1990).
Two characteristics of plant gas exchange and water relations that are quantifiable by measurement in modern and ancient plant leaves are stomatal densities and stable isotope variations. These parameters are sensitive to changes in climate as well as atmospheric CO2 levels, such as during the last deglaciation (200 to 280 ppmv). Limber pine needles abundantly preserved in packrat middens from the western U.S. show a 17 percent decrease in stomatal densities during deglaciation (15,000 to 12,000 years ago. Although this increased water use efficiency (WUE: ratio of carbon gain to water lost), the savings were not enough to offset increasing aridity, so Holocene deserts quickly replaced Pleistocene woodlands. At some threshold in effective moisture, climate trumps direct CO2 effects on vegetation dynamics (Van de Water, Leavitt and Betancourt 1994).
Temperature profoundly influences the physiology and life history characteristics of organisms, particularly in terms of body size. Because so many critical parameters scale with body mass, long-term temperature fluctuations can have dramatic impacts. We examined the response of a small mammalian herbivore, the bushy-tailed woodrat, to temperature changes from the full glacial to the present at five sites on the Colorado Plateau. Our investigations focused on the relationship between temperature, plant composition and abundance, and woodrat size estimated from fecal pellet diameter. Our work suggests that woodrat size is a precise paleothermometer, yielding information about temperature variation over relatively short-term
One of the unique opportunities provided by fossil rodent middens in the Americas is the ability to directly relate rodents and their habitats through space and time, with partial resolution at the molecular level. Ancient DNA analysis, for example, can provide a more precise taxonomic determination of the midden agent than would be possible from fossil bones. Such analyses could also generate a time-lapse view of molecular diversification associated with datable range shifts, and a set of empirical tests for the analytical methodology and conceptual framework that population geneticists use to infer population history. A case in point is the recent determination that Phyllotis limatus, a leaf-eared mouse, was the principal agent in accumulation of an 11,700 yr old midden from the Atacama Desert in northern Chile. The modern and ancient sequences reported in this paper reinforce the proposal that P. limatus was derived very recently from a western lineage of P. xanthopygus rupestris. The midden agent apparently was part of the ancestral P. limatus populations that extended at least 100 km further south than today. This expansion happened during a pluvial period of increased summer rainfall, when steppe grasses and other vegetation expanded as much as 900 m downslope into what is now absolute desert (Kuch, Rohland, Betancourt, Latorre, Steppan, Poinar 2002).
Advancements in ancient DNA analyses now permit comparative molecular and morphological studies of extinct animal dung commonly preserved in caves of semi-arid regions. These new techniques are showcased using large herbivore dung preserved in a late glacial rodent midden from a limestone cave in southwestern Argentina. Phylogenetic analyses of the mitochondrial DNA show that the dung originated from a small ground sloth species not yet represented by skeletal material, and not closely related to any of the four previously sequenced extinct and extant sloth species. Analyses of pollen and plant cuticles, as well as chloroplast DNA, show that this ground sloth browsed many of the same herb, grass, and shrub genera common at the site today, and that its habitat was treeless Patagonian scrub-steppe (Hofreiter, Betancourt, Markgraf, Sbriller and McDonald 2003).
In arid lands, wetlands form wherever the water table intersects the ground surface, resulting in the formation of seeps, flowing springs, wet meadows, and marshes that contrast with the surrounding hillslopes. Desert wetlands produce characteristic deposits, including diatomite, organic mats and tufas, which can be mapped and dated to reconstruct past fluctuations in the heights of local water tables. Paleowetland deposits enjoy several advantages over other types of paleohydrologic records. They often contain abundant plant matter that can be readily radiocarbon dated, free of 14C reservoir or hard-water effects common in other aquatic-based carbon systems such as lakes. Moreover, periodic drops in the water table lead to extensive erosion and exposure of paleowetland deposits, allowing many sections to be described and dated without recourse to coring. In the central Atacama Desert of northern Chile, paleowetland deposits date from >15.4-9.0 cal kyr B.P., some located in areas that today do not support modern wetlands, and ~8.2-3.2 cal kyrs B.P., located between 6-11 m above modern stream levels. The late glacial-early Holocene episode of ground-water level rise corresponds to a time of higher lake levels and vegetation invasions in what is now absolute desert. The mid-Holocene paleowetland deposits suggest slightly wetter conditions than today, in agreement with the rodent midden record for the area but in disagreement with poorly-dated lake level histories from the Chilean Altiplano. At stake in the timing of these wet periods is the importance of seasonal insolation variations over the Bolivian Altiplano vs. tropical Pacific SST gradients as the key forcing in millennial-scale variability of monsoonal precipitation at the tail end of the tropical rainfall belt (Rech, Quade and
So-called 'annual' banding is sometimes evident in speleothems in which the number of bands approximates the time interval between successive U-series dates. The apparent annual resolution of banded speleothem records can be tested in various ways. For example, variations in band thickness from a late Holocene stalagmite in Carlsbad Cavern, New Mexico were compared statistically against three independent tree-ring chronologies from the same region. There was no agreement. In the southwestern USA, both stalagmite and tree ring growth should reflect annual variations in effective moisture and soil recharge. Although there may be various explanations for the discordance, this limited exercise suggests that banded stalagmites should be held to same rigorous standards in chronology building and climatic inference as annually resolved tree rings, corals, and ice cores (Betancourt, Grissino-Mayer, Salzer and Swetnam 2002).
