paleoenvironmental changes at the northern limit of the subantarctic nothofagus forest, lat 37°s,...

QUATERNARY RESEARCH 28, 119- 129 (1987)

Paleoenvironmental Changes at the Northern Limit of the Subantarctic Nothofagus Forest, Lat 37’S, Argentina

VERA MARKGRAF

INSTAAR, University of Colorado, Boulder, Colorado 80309

Received May 15, 1987

Paleoenvironmental changes dating back to before 10,000 yr BP. at the northernmost occurrences of Nothofagus forests in Argentina at about latitude 37”s permit the reconstruction of past changes in the intensity of the winter rains, related to the southern westerlies that appear to deter- mine the forest boundary. The paleoenvironmental interpretation is based on changes in the proportions of different Nothofagus species and changes in the ratio between forest and steppe taxa. The relatively most diverse and dense Nothofagus forest developed only during the last 4500 yr, prior to human impact during the last 300 yr. Before the middle Holocene, climatic conditions must have been different from the modern ones, with less overall precipitation, judging from the overall lower amount of tree pollen and the reduction to primarily Nothofagus pumilio. An interval dated to older than 10,000 yr B.P. is characterized by co-occurrence of Prumnopitys andina, Nothofagus pumilio, and shrub-steppe taxa. Prumnopitys andina is known today only from scattered upper montane forest sites in Chile between 36” and 43”s lat and its ecological requirements are essentially unknown. The taxa combination, however, suggests that late-glacial climate must have been drier, and probably cooler than today. This implies that the winter rains and, consequently, the seasonal shift of the westerly circulation was reduced during the late Pleistocene and did not reach modern levels before 8500 yr B.P. o 1987 University of Washington.

INTRODUCTION

On the Argentine side of the Andes, Nothofagus antarctica and N. pumilio represent the northernmost occurrences of the subantarctic Nothofagus forests of southern South America. The known northernmost stand of these trees is at the site “El Concon” in northwestern Neu- quen Province at 36”35’S lat (Fernandez, 1976). Because of rugged topography and seasonal moisture stress, these stands grow in widely separated islands and are re- stricted to mesic habitats such as south- facing slopes, shores of lakes and streams, and bogs. Further south, and with de- creasing moisture stress, other elements of the Nothofagus forest appear. In addition to Nothofagus antarctica and N. pumilio, Nothofagus obliqua is found at the La- gunas de Epulauquen (36YO’S). Austro- cedrus chilensis occurs on west-facing slopes of the Cordillera de1 Viento (37”04’S; Boninsegna and Holmes, 1978a;

LaMarche et al., 1979); Araucaria araucana and Nothofagus dombeyi occur south of 37”45’S lat (Tortorelli, 1956; Boninsegna and Holmes, 197gb).

On the Chilean side of the Andes, the northernmost trees with subantarctic forest affinity occur further north than on the Ar- gentine side. Austrocedrus chilensis is found to 32% lat, Nothofagus dombeyi to 34”4O’S lat, Nothofagus antarctica to 35”s lat, and Araucaria araucana to 37”2O’S lat (Donoso, 1981). Thus, the latitudinal spread of the several species is quite different on both sides of the Andes, which seems to reflect the different northernmost expansion of the winter-rain climates, which are linked to the circulation of the westerlies.

Even though logging has destroyed much of the original forest cover, in Argentina these forest islands merge into a continuous forest belt at about 38’S lat. At 41”s lat the forest shows a clear elevational zo- nation with a lower forest zone, from the

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120 VERA MARKGRAF

steppe-forest limit to about 1000 m elevation, characterized by Nothofagus dombeyi, Austrocedrus chilensis, and many smaller trees in the families of Proteaceae and Myrtaceae. Between 1000 m and upper timberline (ca. 1500 m), Nothofagus pumilio dominates the forest. A krummholz zone near upper timberline is composed of Nothofagus pumilio and N. antarctica. Above 1600 m is treeless Andean vegetation

The lower timberline, the steppe-forest ecotone, as well as the northernmost forest occurrences in Chile and Argentina, relate to the isohyet of about 600 mm, a seasonal moisture deficit, and mean monthly temperatures that remain above freezing. Near the transition to the Nothofagus pumilio zone, mean annual precipitation exceeds 2000 mm and below-freezing temperatures are recorded during several winter months (June, July, and August). Upper timberline is more likely related to low summer temperatures, expressed in number of days with temperature below 10°C (Tranquillini, 1979).

Given the geographic distribution of these forest taxa, the northernmost outliers have been considered relics, remnants of a formerly more continuous and wider forest distribution (Auer, 1958). Such greater forest expansion was thought to be related to climatic conditions of glacial ages. Whereas on Chilean side of the Andes the relic character of these northernmost forest islands seems probable, on the Argentine side the northern forest limit more likely is related to the climatic gradient and mountain structure. North of 36”2O’S lat the Andes are substantially higher and an increase in the number of mountain ranges further augments the precipitation gradient (Fernandez, 1976). This suggests that on the Argentine side the forest could not have expanded northward; however, under different climatic conditions, the forest could have expanded its area substantially, especially to the east.

A study of past vegetational changes in

this northernmost forest region can provide a test of this hypothesis. A sediment core was taken in the Lagunas de Epulauquen valley, one of the northernmost sites with Nothofagus pumilio, N. obliqua, and N. antarctica, and was analyzed for pollen and diatoms. In addition to providing an environmental chronology for the forest ecotone, this record also permits specula- tion about former shifts of the westerlies that today seem to be related to this forest boundary.

Shifts of winter precipitation circulation previously suggested for full- and late-glacial periods included both an equatorward shift (Heusser, 1983) and a poleward shift (Markgraf, 1983). Climate modeling on the basis of Milankovitch insolation changes through time indicates that a poleward shift is more likely because the temperature gradient toward Antarctica is steeper during glacial ages than today (V. Markgraf, unpublished data; J. Kutzbach, unpublished data).

MODERN ENVIRONMENT AND METHODS

The Lagunas de Epulauquen and Vaca Lauquen are located in open, formerly gla- ciated valleys near the Chilean/Argentine border at 36”5O’S lat, 71”OS’W long, about 60 km northwest of the town of Andacollo in northwestern Neuquen Province (Fig. 1). The lakes are dammed by a sequence of moraines, the lowest of them at about 1500 m elevation. The highest surrounding peaks reach about 2500 m. Bunchgrass steppe, with bushes of Baccharis (Compo- sitae), Ephedra, and Azorella (Umbelli- ferae), is the dominant local vegetation in the area, except for groves of Nothofagus antarctica along the north shore of the La- guna Epulauquen. Only the steeper slopes that surround the lakes are covered by a more extensive forest belt with tall trees of Nothofagus pumilio and N. obliqua up to about 1700 m. Above 1700 m krummholz vegetation forms the transition to the treeless Andean vegetation.

ARGENTINE FOREST HISTORY

FIG. 1. (a) Map showing the location of the study region in northwestern Neuquen Province, central Argentina. (b) Location of Lagunas de Epulauquen in northwestern Neuquen Province with 600- and 400-mm isohyets, based on the “Climatologic Atlas of South America” (J. A. J. Hoffman, Ed.), UNESCO, 1975. (c) Location of the pollen record site in the Lagunas de Epulauquen valley with Norhofagus forest distribution based on the interpretation of airphotos, 1:50,000 scale.

122 VERA MARKGRAF

Climate near the lagunas, interpolated from data in the OMM/WMO/UNESCO Climatic Atlas (1975), is characterized by about 600 mm mean annual precipitation, two-thirds of which falls in winter (May through August). Mean annual temperature is about 4”C, with 5 months of temperature at or below freezing (May through Sep- tember) and 3 summer months at 6°C (De- cember through February). A climadiagram constructed for the Paso Epulauquen at 2000 m elevation (Fig. 2) suggests possible moisture stress during summer (January through March). Because of considerable interannual variability of precipitation (Boninsegna and Holmes, 1978b), this seasonal stress could be highly variable.

Apparently the equatorward limit of the Nothofagus occurrences is related to the narrowing of a zone where (1) winter precipitation is sufficient to balance the summer moisture stress and (2) temperatures in summer remain sufficiently warm to permit tree growth. Thus, the north- ermost Nothofagus forests represent a unique environmental setting where the al- titudinal separation between the lower and upper tree line is, at best, 200 m. Consid- ering the resolution of pollen analysis, this difference is probably indistinguishable, and we are faced with the paradox that changes in forest pollen proportions could be due either to shifts in the upper or lower

‘J’ S N’ ‘J‘ M M

FIG. 2. Climadiagram for Paso Epulauquen (2000 m alt) according to Walter and Lieth (1967), based on the mean monthly temperature and precipitation data from the “Climatologic Atlas of South America” (J. A. J. Hoffman, Ed.), UNESCO, 1975. Mean annual temperature and precipitation at this site are 4°C and 700 mm, respectively.

tree line, or a combination of both. How- ever, distinction of Nothofagus species gives a clue as to which tree line is shifting.

The 280-cm-long sediment core is from a small, seasonally inundated, boggy meadow, just inside the outermost moraine of the Vaca Lauquen valley at 1450 m. A spring mound with open water provides habitat for the waterplant Myriophyllum. The upper 25 cm of the core are fibrous peat, underlain by alternating layers of dark (peaty) and light (more inorganic) gyttja. Below 276 cm depth, down to the base at 280 cm, is gray clay. Diffuse and distinct layers of black or grayish lapilli are intercalated at various depths.

Samples for pollen analysis were taken volumetrically (1.8 ml vol) and treated according to standard techniques with hydro- fluoric acid to remove silicates, and an ace- tolysis mixture was used to reduce the organic matter (Faegri and Iversen, 1975). Fern spore tablets were added to the samples as tracer before treatment to cal- culate pollen concentration and pollen influx.

Three radiocarbon dates suggest a nearly constant sedimentation rate of about 39 yr/cm throughout the sediment section. The dates are 4490 ? 150 (A-4104 at 90-93 cm); 7100 + 340 (A-4103 at 195-198 cm); and 10,070 t 190 (A-3550 at 250-260 cm).

RESULTS

Paleoenvironmental and paleochmatic in- ferences can be made according to proportional and qualitative changes in the pollen spectra, and their comparison with modern pollen spectra and modern plant distribution. Arboreal pollen is represented by Nothofagus pumilio, N. dombeyi, N. antarctica, and N. obliqua-type (including N. alpina), distinguished on the basis of mor- phologic characters illustrated in Markgraf and D’Antoni (1978). Prumnopitys andina (Podocarpus andina) pollen is common in the lower part of the section and occasional grains of Austrocedrus chilensis-type, Araucaria araucana, Schinus, Lomatia,

ARGENTINE FOREST HISTORY 123

Buddleju, Orites (Proteaceae), Myrtaceae, Rhamnaceae, Berberis, and Ephedra occur throughout. Nonarboreal pollen taxa in- clude Gramineae (dominant throughout), Cyperaceae, Compositae (Tubuliflorae and Mutisieae), Umbelliferae (including Azor- e&z), Caryophyllaceae, Acaena (Rosa- ceae), Geraniaceae ( Wendtia), Ranuncula- ceae, Plantago, Adesmia (Leguminosae), Chenopodiaceae, Calyceraceae, Gentiana- ceae, and Polygonaceae. Pollen and spores of waterplants (Myriophyllum), ferns (including Zsoetes), and algae (Pediastrum boryanum-type, Botryococcus, and diatoms) provide additional information on the limnologic changes in the record.

Six paleoenvironmental zones can be distinguished during the ca. 1 l,OOO-yr interval represented by the section (Fig. 3). Gramineae and herbaceous taxa are the dominant components up to 4000 yr ago, when aboreal taxa reached codominance.

The lowermost zone, between 250 and 280 cm, is older than 10,000 yr B.P. and characterized by 20 to 45% arboreal pollen (up to 14% Prumnopitys andina, up to 20% Nothofagus pumilio, 10% N. dombeyi, less than 5% Austrocedrus-type, and Rhamna- ceae pollen). The nonarboreal component consists of 50 to 60% Gramineae, 5% Com- positae, and 8% other herbaceous taxa, among them primarily Azorella, Acaena, and Ephedra, suggesting steppe affinity. Most remarkable in the vegetation is the abundance of Prumnopitys andinu. Scat- tered individuals of Prumnopitys andina grow today on the Chilean side of the Andes from 35”49’S to 43”s lat between 500 and 1000 m elevation, in association with Austrocedrus chilensis, Nothofagus obliqua, and Nothofagus pumilio (Rodriguez et al., 1983). Schmithtisen (1960), on the other hand, reports Prumnopitys an&a from the Antuco region at 35”s lat at the timberline associated with Nothofugus alpina. Few individuals are known from the Argentine side of the Andes, and usually occur as individual trees near the upper timberline (Tortorelli, 1956). The ecology of

this tree is essentially unknown, but on the basis of its distribution, a preference for summer arid environments that could be both warm or cold seems indicated. Cer- tainly the co-occurrence of Prumnopitys andina with Compositae and steppe taxa in the fossil record point in that direction.

Based on a shift from Nothofugus pumilio dominance in the lower portion of this interval to Nothofugus dombeyi codominance in the upper portion, a further subdivision can be proposed. During this abundance shift in Nothofagus, Prumno- pitys andina briefly decreases while Gra- mineae increase, which together suggests a substantial environmental change. Using the modern distribution of both Notho- fugus species, the older, lower portion must have been somewhat colder and/or wetter, while the upper portion must have turned warmer and/or dryer. Another ex- planation for increased Nothofugus dombeyi would relate to its character as a successional species to disturbance (Veblen et al., 1981). Perhaps both climatic and non- climatic parameters were involved at that time.

Pollen influx during this interval is highly variable, shifting from extremely high to extremely low values simultaneously for all taxa. Quite likely these shifts are the result of a highly variable sedimentation rate which far exceeded pollen production changes that could be interpreted in terms of climate.

Low values of Pediastrum boryanum- type, scattered Zsoetes spores, and a low abundance of diatoms (among them Fragi- laria construens var. venter and Cyclotella stelligera) suggest a low-productivity environment. Such an assemblage is quite common for sites near glaciers (Bradbury and Whiteside, 1980; Brugam, 1983) and is found in many late-glacial records during the initial stages of lake evolution (Ha- worth, 1976).

There are no known modern pollen assemblages that closely resemble the pollen assemblages prior to 10,000 yr B.P., mostly

124 VERA MARKGRAF


because of the presence of Prumnopitys andina in the record. Without this taxon, the most similar assemblage is the surface sample from the site itself, suggesting that the environment at that time was a shrub- steppe with scattered trees or forest islands. These islands, however, were somewhat less extensive than the modern ones, and also composed of different Nothofagus taxa, in different proportions than today. In general climatic terms, total precipitation between ca. 11,000 and 10,000 yr B.P. could have been similar to today, but probably summers had a longer period of moisture stress, i.e., they were either warmer or longer than today.

During the following interval, between 250 and 225 cm (ca. 10,000 to 8500 yr B.P.), Prumnopitys andina is reduced to trace occurrences, Nothofugus pumilio to less than 12%, and N. dombeyi to less than 6%. Myrtaceae pollen, however, is quite abundant during this time. Among nonarboreal taxa, Gramineae dominate. Herbaceous taxa increase and are represented by large Umbelliferae grains (cf. Heracleum-type), Ranunculaceae, Valeriana, and Gentia- nella-type. Compositae, on the other hand, decrease somewhat. Pollen influx values during this interval are variable, as in the previous period; however, the different taxa vary independently, suggesting that the differences are due more to pollen production than to sedimentation changes. First, influx for herbaceous taxa reaches a maximum, then Gramineae, and ultimately Nothofugus, attain maximal influx.

Substantial amounts of Myriophyllum, coupled with high numbers of epiphytic diatoms, of Botryococcus, and of Pedias- trum suggest that the lake became more productive and shallow. The abundance of wetland taxa coupled with low proportions of aboreal taxa suggests that higher temperatures then prevailed, resulting in the reduction of forest islands, and especially the disappearance of Prumnopitys andina, but that running water was abundant.

The following interval, between 225 and

155 cm (ca. 8500 to 6000 yr B.P.), is characterized by an increase in Nothofagus pumilio to over 20%. Gramineae still dominate the pollen spectra, but herbaceous taxa are generally lower than before. During this interval, Cyperaceae reach their highest percentage and influx values, and fern spores are abundant. Pollen influx values for Nothofagus pumilio and herbaceous taxa peak briefly at the onset of this interval, while those for Gramineae decrease. After the initial peaks, influx values generally become lower and remain constant at that level for the remainder of the record. At one level, pollen of Caryophyl- laceae increase sharply to 18% (calculated out-of-the-sum). But because only Cypera- ceae show a small decrease at that time, the environmental significance of the Car- yophyllaceae peak is unclear. Generally such short-term percentage changes are considered “accidents” of pollen dispersal or deposition, due to some local phenom- enon.

Climatic conditions during that time must have become optimal for Nothofugus pumilio, which perhaps implies upward expansion of the tree line that can be interpreted as due to higher temperatures. This, in turn, would imply greater aridity at the lower tree line. Support for this interpretation comes from low values of N. obliqua- type and N. dombeyi, and from the algae and Cyperaceae. Based on the abundance of Botryococcus and Cyperaceae (?Scirpus), most of the lake was probably shallow and overgrown.

Between 155 and 100 cm (ca. 6000 to 4500 yr BP), total tree pollen is lower than before, mostly because of the decline in Nothofagus pumilio and N. dombeyi that is not sufficiently counterbalanced by the increase in N. obliqua-type and N. dombeyi (toward the end). Gramineae and herbaceous taxa increase somewhat, and for several levels Caryophyllaceae reach percentages between 22 and 160% (calculated out-of-the-sum). Even though variable, Cy- peraceae during this stage are generally

126 VERA MARKGRAF

lower than before. The Caryophyllaceae abundance during this stage is less easily dismissed as an accident. It is unlikely, however, that its presence can be interpreted in terms of a pioneer or recoloniza- tion phase as suggested from pollen records in the arctic (Bartley and Matthews, 1969; Mode, 1980). On the other hand, presence of a 2-cm-thick diatomite layer in the sediment at the onset of the Caryophyllaceae peak suggests some change in the limnologic conditions that could have provided sudden, extensive habitat increase for the Caryophyllaceae. The diatoms indicate shallow, but draining, water, and the sole presence of Botryococcus and decreased amount of Cyperaceae (?Scirpus) suggest variable water levels. The regional climate probably had become cooler than before, with locally more-abundant moisture. This is also supported by the expansion of Nothofagus obliqua-type, followed by N. dombeyi, with the minimum requirements of about 600 mm mean annual precipitation.

Between 100 and 30 cm (4500 and <IO00 yr B.P.) Nothofagus percentages reach 50%, with over 25% N. pumilio, and about 10% each for N. dombeyi and N. obliqua. During this interval, the proportions of Nothofagus dombeyi suggest that it actu- ally was growing in the region. Today at this latitude it can only be found on the west side of the Andes, while in Argentina it occurs 1” lat farther south. Gramineae and herbaceous taxa decrease. Cyperaceae are high, and sporadically Myriophyllum and Pediastrum become abundant during this interval. This, and the increased amount of fibrous organic material in the sediment, suggests that the lake was more permanent again, with open but shallow water, gradually turning into a bog. At that time, overall climatic conditions must have been similar to the modern regime, with cold, moist winters and moisture stress during several months in the summer. Ap- parently Nothofagus pumilio again expanded its upper elevational range, im-

plying greater warmth than during the previous interval. At the same time, low-elevation forest taxa appear to have thrived equally well, suggesting continuation of ad- equate moisture. Such a condition of higher temperatures without increase in summer moisture stress cannot result from increased winter precipitation alone, but must imply additional summer precipitation that, in this case, is related to easterly circulation.

From 30 cm to the surface, pollen of weeds (Rumex and Plantago lanceolata) suggest the onset of European impact, both by logging and grazing. The increase in pollen influx of Nothofagus is due to the increase of N. antarctica, a successional species after disturbance. Similar to other such records from grassland areas (Mark- graf, 1985a), a plant succession can be rec- ognized that represents the increasing human impact. First, tree pollen, Gra- mineae, and herbaceous taxa decrease. Subsequently, Compositae (mostly because they are represented by shrubs) and other successional disturbance species, in this case Nothofagus antarctica, become abundant, partially in response to competition release and partially to a preference for dis- turbed habitats. Ultimately, even Compo- sitae and Nothofagus antarctica decrease, and primarily annual weeds are left to pro- duce pollen.

In the Vaca Lauquen record the surface sample indicates a return of Nothofagus pumilio and N. obliqua-type, suggesting that the logging activity might have decreased somewhat. Low proportions of Gramineae and the abundance of weeds indicate that grazing pressure is as strong as ever. This interpretation is supported by historical data indicating that with the more recent change in construction techniques and decrease in mining activities, logging in the region has decreased (Fernandez, 1976; C. Baied, unpublished data).

DISCUSSION

The paleoenvironmental history of this


high-elevation site near the northernmost Nothofagus forests show that the extent and distribution of the Nothofagus stands have changed relatively little during the last ca. 11,000 yr; however, proportions of different Nothofagus species changed substantially. The highest abundance and species diversity for Nothofagus during this time is recorded during the last 4500 yr, but prior to human impact during the last ca. 300 yr. Because the record does not ex- tend back into full-glacial times, the possibility still exists that at that time the forest may have expanded further north and east of the central Andes. The late-glacial low abundances of Nothofagus at Vaca Lau- quen, however, do not render much support to this possibility. In addition, the similarity of the full- and late-glacial pollen spectra from the Laguna Tagua Tagua in central Chile (Heusser, 1983) where Prum- nopitys andina is also a major component does not suggest a major difference between full- and late-glacial climates at this latitude.

The changes that did occur during the last ca. 11,000 yr of record refer to changes in proportion and composition of the No- thofugus forest. Thus, the modern codominance of Nothofagus pumilio, N. dombeyi, and N. obliqua in the potential vegetation only dates back to about 4500 yr B.P., whereas prior to that date, N. pumilio was the single dominant species. Because all Nothofagus species are present from the onset of the record, a migrational lag that could explain the proportional changes can be excluded. Instead, climatic regime is the more likely reason, especially shifts in the length of the summer moisture stress, and in the length of months with freezing temperatures in winter. Because the ecological amplitude of Prumnopitys andina is essentially unknown, it is difficult to define the paleoclimate prior to 10,000 yr B.P. The combination of shrub-steppe taxa with Nothofagus pumilio suggests that climates must have been colder than today, and also drier,Because 10,000 and 8500 yr B.P. tem-

peratures must have increased to modern levels, but sufficient open and running water existed locally to support a tall-herb vegetation. Between 8500 and 6000 yr B.P. temperatures may have been even higher than today, resulting in an upward shift of the upper tree limit and an increase in aridity at the lower tree limit. A cooling trend characterized the interval between 6000 and 4500 yr B.P., resulting in a low- ering of the upper tree line and increased runoff near the lower tree line. From 4500 yr B.P. onward temperatures increased pre- sumably to modern levels, but the develop- ment of the lower forest vegetation suggests less-severe summer moisture stress periods than between 8500 and 6000 yr B .P. Despite the problem of distinguishing effects of human impact from possible climatic effects, the lack of Nothofagus dombeyi in the modern forest could indicate that during the last 3000 yr this summer moisture stress has somewhat increased in severity again.

From this record, then, it is unlikely that the northernmost Nothofugus stands are glacial relics of a former, more-extensive forest cover. Instead, they seem to represent a late-Holocene expansion to locally suitable sites where the minimum climatic requirements are fulfilled. One of these climatic requirements is the amount of moisture that is dependent on the seasonal shift of the westerlies. Based on latitudinal shifts of mean monthly precipitation, the modern equatorward shift occurs between May and August only. Couplied with this winter precipitation are temperatures below freezing. Thus, while increased precipitation in winter would favor Notho- fagus species near the lower tree limit, by reducing the length of summer moisture stress (N. dombeyi, N. obliqua-type), it would prevent forest expansion near the upper tree limit (N. pumilio) that is sensitive to temperature. Intervals that document greater abundance of N. pumilio, simultaneous with N. dombeyi and N. obliqua- type, are then probably not the result of

128 VERA MARKGRAF

greater winter precipitation alone, but rather a combination of increased winter and summer precipitation, the latter related to the subtropical high-pressure cell and easterly circulation.

The late-glacial dominance of N. pumilo (and Prumnopitys an&a), suggesting climatic conditions drier than today, and probably somewhat colder than today, could not be caused by intensified westerly circulation. The higher lake levels docu- mented in the Laguna Tagua Tagua record (Heusser, 1983) at 34% lat must be related to increased subtropical moisture instead. This latter precipitation scenario was suggested earlier to explain full- and late-glacial moisture in the desert latitudes of Ar- gentina (Markgraf, 1983), using Pittocks’s (1980) correlation analysis of precipitation and circulation anomaly patterns over southern South America.

Only between 8500 and 6000 yr B.P. did the westerlies expand equatorward, perhaps even farther than today. The modern scenario dates back to about 4500 yr B.P., on the other hand, seems to reflect a combination of the westerlies’ northward expansion and increased influence of easterly, summer precipitation. Such increased summer moisture circulation has earlier been proposed for this interval on the basis of paleoenvironmental records from the desert region of Mendoza Province (Mark- graf, 1983). It was interpreted as the response to a poleward shift of the subtropical high-pressure cell.

In comparing the overall paleoclimatic history of the high-elevation record from Vaca Lauquen with other sites (all low-elevation sites) from the Nothofagus forest region in Argentina (Markgraf, 1983, 1984), a greater similarity emerges with the high- latitude sites from Tierra de1 Fuego than with the mid-latitude sites at 41”s lat. Thus, the onset of postglacial-type moisture conditions (derived from aboreal pollen percentages reaching near-modem levels), occurred at Vaca Lauquen at about 8500 yr BP This is contemporaneous with records

from Tierra de1 Fuego, but not with the mid-latitude records, where this event dates back to 12,000 yr B.P. Vaca Lauquen, as well as Tier-t-a de1 Fuego, shows dry conditions prior to 8500 yr B.P., warm and dry conditions between 10,000 and 8500 yr B.P., and dry but probably colder conditions prior to 10,000 yr B.P. (Markgraf, 1983, 1985b). The mid-Holocene dry and cool interval at Vaca Lauquen occurred between 6000 and 4500 yr B.P., simultaneous with the steppe expansion in Tierra de1 Fuego, interpreted there as increased aridity. At mid-latitudes, however, the relatively driest interval during the Holocene was dated between 8500 and 7000 yr B.P. (Markgraf, 1983, 1984).

Such large-scale paleoclimatic similarity is not a result of chance, but rather must represent a similar position in terms of climatic gradient. In fact, both Tierra de1 Fuego and northwestern Neuquen lie near the margin of the westerlies. Late-glacial aridity in both regions is the result of a contraction of the westerlies south of 41’S lat. The postglacial mode of the position of the westerlies was first felt at 12,000 yr B.P. at 41”s lat (where its influence is strongest today), but could only be felt near its modern margins, north and south, after 8500 yr BP

To test these ideas, records from high- elevation sites from the different latitudes are needed, as well as records from inter- mediate latitudes (41”-50”s).

ACKNOWLEDGMENTS

For support and help in the field, I express my thanks to Ing. J. Fernandez and Dr. J. Mead. Sample prepara- tion was done by C. Wahli, whom I thank very much. Dr. J. P. Bradbury is acknowledged for his help in evaluating the diatom content of some of the samples. The research was supported by the National Science Foundation (Grant ATM-8212836).

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paleoenvironmental changes at the northern limit of the subantarctic nothofagus forest, lat 37°s,...

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