The Andes

The Andes span a wide latitudinal range from 5°N to 55°S, and the modern vegetation types and altitudinal zonation are quite diverse along the length of the cordillera. In the northern Andes of Colombia, Peru, and Bolivia, the upper limit of the Andean forest lies at ca. 3500 m elevation. The tree line decreases in elevation in the southern Andes to ca. 1500 m at latitude 40°S and to 500 m at latitude 55°S. The northern Andes receive moisture from convection related to the easterly trade winds, whereas the southern Andes are under the influence of the westerlies. Because of regional topographic differences and availability of suitable sites, the pollen records from the northern Andes are primarily from high altitudes above tree line in the paramo and sub-paramo, whereas records from the southern Andes are from forest zones at lower elevations. Consequently, the Holocene vegetation changes recorded at northern sites primarily involve proportional shifts between Andean forest and paramo elements and, therefore, record tree-line changes and temperature shifts. At southern sites, however, the pollen data indicate changes between different forest types, and the inferred climate signal is primarily precipitation and seasonality.

To characterize the Holocene climate changes in the Andes, we discuss representative pollen records from Colombia (La Primavera [Melief, 1985], Fúquene [Van Geel and van der Hammen, 1973]), Peru (Laguna Hu-atacocha [Hansen et al., 1984]), Chile (Caunahue [Markgraf, 1991a]), and Argentina (Mallín Aguado [Markgraf and Bianchi, 1999] Harberton [Markgraf, 1991a]).

19.5.1. Early Holocene

At high-elevation paramo sites from Colombia to Peru, the dominants of the early Holocene (11,000-7800 B.P.) are Poaceae, subparamo elements (Asteraceae and Alnus), and Andean forest elements (Melief, 1985; Hansen et al., 1984, 1994). The pollen diagrams from La Primavera (3525 m) in Columbia and Laguna Huata-cocha (4500 m) in Peru illustrate this trend (Fig. 16). Locally, subparamo vegetation replaced the paramo. Compared to the preceding late glacial pollen spectra, which have low species diversity and a low influx of herbaceous taxa, the early Holocene spectra indicate an upslope shift of vegetation zones in response to increased temperatures and moisture.

Subparamo records obtained at elevations below the paramo zone in the northern Andes, for example, Laguna Fúquene, Colombia (2580 m), show a comparable upslope shift of vegetation zones (Fig. 16). Andean forest elements moved upward, especially Quercus in Colombia (van Geel and van der Hammen, 1973) and Urticales, Hedyosmum, and Podocarpaceae in Peru (Hansen et al., 1984, 1994; Hansen and Rodbell, 1995). Sediment records from glacial cirque lakes in the Bolivian Andes suggest increased temperatures in the early Holocene. These sites show greatly increased productivity compared to the preceding late glacial times (Abbott et al., 1997), and receding glaciers in the region suggest greatly decreased precipitation as well as higher temperatures (Abbott et al., 1997). In contrast, the early Holocene was apparently wet and cool in northern Chile and Argentina. A pollen record from the Altiplano in northwestern Argentina shows species-rich herbaceous grassland (Markgraf, 1985; Fernandez et al., 1991), and lake levels were high in northern Chile from 12,800-9500 B.P. (Valero-Garcés et al., 1996; Grosjean et al., 1995; Sylvestre et al., 1999; Bradbury et al., 2000).

Temperatures were apparently higher than they are today throughout the northern Andes in the early Holocene. However, moisture patterns varied latitudi-nally, with drier conditions than today in Colombia, Peru, and Bolivia, but seasonally more uniform wet conditions in the Andes of northern Chile and Argentina. This difference suggests that during the early Holocene the seasonal shift in latitude of the ITCZ was not as great as it is today. A possible explanation for this behavior may be linked to a different insolation pattern in the early Holocene. Seasonally, insolation was greater in June-July-August (southern winter, northern summer) and lower in December-January-February (southern summer, northern winter). Thus, seasonality was greater than it is today north of the equator, but the opposite was true south of the equator. As a consequence, precipitation was higher north of the equator and lower south of the equator (Abbott et al., 1997; Kutzbach et al., 1993). However, the paleoprecip-itation patterns show greater latitudinal differentiation than insolation alone could cause, and other factors may be involved.

In the temperate forest region of the southern Andes, climate patterns were also latitudinally different throughout the Holocene. The sites Caunahue (Markgraf, 1991a) on the west slope of the Andes and Mallín Aguado (Markgraf and Bianchi, 1999) on the east slope of the Andes, both at 40°S, illustrate the middle latitude pattern (Fig. 17). In the early Holocene, forests at this latitude on both sides of the Andes contained a higher

La Primavera

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