Clear Pond Lake Tulane

Pollen Oklahoma
FIGURE 10 Representative pollen diagrams from the Southeast: Clear Pond (Watts et al., 1996) and Lake Tulane (Grimm et al., 1993).

nie (Watts, 1975), and Buck Lake (Watts et al., 1996) in the Florida peninsula are deep lakes with excellent Holocene records. At Buck Lake, which is now over 17 m deep and was dry until after 10,000 B.P., a shallow-water assemblage occurs at the base of the sediment at over 20 m depth (Watts et al., 1996). The dominant pollen types of the early Holocene are Quercus, Poa-ceae, and moderate amounts of Pinus. The vegetation was sclerophyllous oak scrub with scattered Pinus, with either an open understory or with prairie openings, similar to that occurring on xeric dune tops in Florida today. After ca. 5500 B.P., Pinus became much more abundant, prairie herbs declined, and extensive swamps developed, all indicating increased precipitation (Watts, 1980; Watts and Hansen, 1988, 1994; Watts et al., 1996).

Farther north on the Coastal Plain, Pinus increased earlier: between 8500 and 7500 B.P. at Barchampe Lake and Langdale Pond just south of the Georgia border and between 10,000 and 9000 B.P. at Clear Pond (Fig. 10) in northeastern South Carolina (Watts et al., 1996). Thus, the Florida peninsula appears to have been drier in the early Holocene than sites farther north. Fagus was abundant in the late glacial and earliest Holocene at Clear Pond, and its presence indicates very humid conditions on the sandy soils of the Coastal Plain. Fa gus and Carya were also abundant in the earliest Holocene at White Pond (Watts, 1980) on the South Carolina piedmont and at Camel Lake (Watts et al., 1992) in the Florida panhandle. These humid conditions are not evident in the earliest Holocene in the Florida peninsula.

Thirteen species of Pinus occur in eastern North America (Kral, 1993). The three northern pines occur in the Great Lakes region and in boreal regions. The 10 southern pines are widely distributed in the southeastern United States, but they do have different ranges and climatic requirements. Some species occur as far north as New England (e.g., Pinus rigida to southern Maine), whereas P. elliottii and P. clausa are essentially restricted to Florida. P. palustris, P. echinata, and P. taeda are important pines on the Coastal Plain, and these three are probably the main species that expanded there in the early Holocene. P. palustris occurs in central Florida today, but the most important species there are the two pines with the most southerly distributions, P. elliottii and P. clausa.

Pinus spread westward from its early Holocene stronghold on the Coastal Plain of northern Florida to North Carolina. In central and southern Florida, Pinus populations expanded ca. 5500 years ago, although the taxon was present in the early Holocene. P. elliottii and P. clausa, which expanded in central and southern Flori da, today are essentially restricted to Florida and, therefore, are not the species that spread northwards from the Coastal Plain. The southward expansion of pine in Florida may result from different climatic causes from the taxon's westward expansion, which probably was related to migration rather than to expansion of a population already in place. P. echinata and P. taeda have migrated westward rapidly during the last 3000 years, reaching the Tombigbee site in northeastern Mississippi (Whitehead and Sheehan, 1985), Cupola Pond in southeastern Missouri (Smith, 1984), and Ferndale Bog in eastern Oklahoma (Albert and Wyckoff, 1981), all within the last 3000-2000 years. Webb et al. (1987) argued that increasing winter temperatures were responsible for the expansion of the southeastern Pinus forest.


Most of the Holocene pollen records from México are from two main areas: the central highlands or Trans-Mexican Volcanic Belt (TMVB) at 22°-18°N and the northern and southern Yucatán Peninsula (Fig. 11).

In the tropical regions, the trades and the subtropical high-pressure belt are the major influences on climate. Precipitation occurs over most of México during summer when the Intertropical Convergence Zone (ITCZ) is at its northernmost position and the easterly trade winds collect humidity from the warm waters of the Gulf of México and the Caribbean Sea. Precipitation along the Gulf coast is higher than along the Pacific coast. During winter, the ITCZ moves southward, and the central and southern regions in México are dry. Occasionally, winter precipitation occurs related to arctic air outbreaks colliding with humid air from the Gulf of México (see also Chapter 3). Hurricanes or tropical cyclones, which develop in the Gulf of México and Pacific Ocean during summer and early autumn, are another significant source of precipitation.

Topography greatly influences precipitation and temperature patterns; closed basins in the central highlands receive 600-800 mm/year, and the Sierras receive up to 1600 mm/ year. Temperatures are relatively low with annual averages between 12° and 18°C. In the low-lying Yucatán Peninsula, with a maximum elevation of 300 m, precipitation is less than along the Gulf coast. The northern part of the peninsula receives 800

FIGURE 11 México and Central America with sites discussed in the text. BL, Church's Blue Hole; CB, Lake Cobá; CC, Cenote San José Chulchacá; CH, Lake Chalco; CI, Lake Chichancanab; CO, Laguna Cocos; CR, Lago Chirripó; CT, La Chonta; CU, Lake Cuitzeo; GA, Gatún Basin; LA, Lachner Bog; MI, Lake Miragoane; MO, Lago de las Morrenas; NI, La Hoya de San Nicolas de Parangueo; PA, Lake Pátzcuaro; PE, Lago Petén Itzá; PU, Punta Laguna; QU, Lake Quila; QX, Quexil; SA, Lake Sayaucil; SL, Salpeten; SP, Laguna San Pedro; TE, Lake Texcoco; TL, Tlaloqua; TR, La Trinidad; YE, La Yeguada; ZA, Lake Zacapu; ZE, Lake Zempoala.

FIGURE 11 México and Central America with sites discussed in the text. BL, Church's Blue Hole; CB, Lake Cobá; CC, Cenote San José Chulchacá; CH, Lake Chalco; CI, Lake Chichancanab; CO, Laguna Cocos; CR, Lago Chirripó; CT, La Chonta; CU, Lake Cuitzeo; GA, Gatún Basin; LA, Lachner Bog; MI, Lake Miragoane; MO, Lago de las Morrenas; NI, La Hoya de San Nicolas de Parangueo; PA, Lake Pátzcuaro; PE, Lago Petén Itzá; PU, Punta Laguna; QU, Lake Quila; QX, Quexil; SA, Lake Sayaucil; SL, Salpeten; SP, Laguna San Pedro; TE, Lake Texcoco; TL, Tlaloqua; TR, La Trinidad; YE, La Yeguada; ZA, Lake Zacapu; ZE, Lake Zempoala.

Zempoala Lake
FIGURE 12 Representative pollen diagrams from México: Lake Chalco (Lozano-García et al., 1993; Lozano-García and Ortega-Guerrero, 1994; Sosa-Najera et al., 1998), and Lake Zacapu (Lozano-García and Xelhuanzti-López, 1997).

mm/year precipitation, and the south-central region receives up to 1600 mm/year. Temperatures have an annual mean above 22°C.

Major centers of pre-Hispanic culture developed in both areas, the Maya in the Yucatán Peninsula and several groups in central México, and environmental degradation with deforestation and agricultural activities has affected the landscape for the last 3500 years (Met-calfe et al., 1989; Leyden et al., 1998; Curtis et al., 1996).

In the mountainous region of central México, numerous internally drained basins formed related to volcanic activity during the late Neogene and Quaternary, associated with the oblique subduction of the Cocos Plate under southwestern México (Morán-Zenteno, 1994). Several closed basins have deep lakes, such as Lakes Cuitzeo and Pátzcuaro, but in general the lakes are relatively shallow or seasonally dry. Physiographi-cally the area is a volcanic plateau with several volcanic peaks with altitudes up to 5000 m.

The dominant climate in this region is temperate with mild winters and summer rains. Tropical climates occur in the eastern part, and hot, arid climates occur in areas where mountain ranges block the moisture-bringing easterly trade winds. The vegetation in the mountainous areas is primarily mixed Pinus-Quercus forest, although several different species of Pinus also occur in other ecological communities. Repeated volcanic disturbances have created microhabitats conducive to species evolution. Thus México, with half of the world's Pinus species, has the greatest diversity of this taxon on Earth (Styles, 1993). Similarly, the central mountainous region of México has the highest diversity of Quercus in the Western Hemisphere, and approximately one-third of the world's species of Quercus occur in México (Nixon, 1993).

FIGURE 12 (Continued).

20 40 60 20 40 20 percent

FIGURE 12 (Continued). Puebla/Tlaxcala Area

Palynological studies from Tlaloqua Crater Maar at 3100 m elevation (Ohngemach, 1977), together with the regional glacial chronology, provide a climate history of the eastern highlands (Heine, 1988; Straka and Ohngemach, 1989). Conditions in the early Holocene were mesic; alpine meadows occurred from 11,00010,000 B.P. and were replaced by mixed Pinus forest at 9000 B.P. Above a thick pumice layer dated at 2760 B.P., open Pinus forest replaced the mixed-pine forest. High amounts of Abies pollen indicate cooler-moister conditions in the late Holocene from 1290-1440 B.P. Farming in the area began 4000-3000 years ago (Ohngemach, 1977).

Pollen records from Lake Chalco (Fig. 12) in the southern basin of México show a temperate mesic early Holocene with cold winters based on peaks of Quer-cus and the occurrence of Abies from 11,000-8000 B.P.

(Lozano-Garcia et al., 1993; Lozano-Garcia and Ortega-Guerrero, 1994; Sosa-Najera et al., 1998). In central México, at this time, conditions were cold and dry, as indicated by relatively high amounts of Cupressaceae pollen in the records (Watts and Bradbury, 1982; Bradbury, 1997). The subsequent decrease of Abies and Cupressaceae pollen indicates the end of glacial conditions in the basin of México. Pollen records from the early Holocene are interpreted as indicating an increase in moisture, which is also suggested by glacier advances at 12,000-10,500, 10,000, and 8800-7400 B.P. in the high elevations around the basin (Vazquez-Selem and Phillips, 1998). However, low lake levels in the basin of México and halophytic diatom assemblages suggest high evaporation rates from 10,000-6000 B.P. (Bradbury, 1989; Caballero-Miranda and Ortega-Guerrero, 1998). In Lake Texcoco, the shallowest lake in the central basin of México, a hiatus in sedimentation occurred from 13,000-6500 B.P. (Lozano-Garcia and Ortega-Guerrero, 1998). The conflicting evidence for wetter conditions in the mountains and drier conditions in the basin of México in the early Holocene may be due to several factors. Higher summer temperatures may have promoted high evaporation and high E-P (evaporation-precipitation) ratios from the shallow lakes in the basin of México. A difference in seasonality of precipitation may also be involved, with increased winter snowfall promoting glacial advance at high altitudes, coupled with decreased summer precipitation resulting in lower lake levels at lower altitudes. This situation is opposite to that in the southwestern United States, where the low-elevation deserts were wetter and the higher elevation mountains were drier in the early Holocene. At Lake Quila, a high-elevation site (3010 m) 65 km southwest of the basin of México, a humid phase with Alnus and Pinus forests occurred at 10,950 B.P., followed by a Pinus forest expansion that probably indicates cooler conditions from 9000-6400 B.P. (Almeida-Leñero, 1997).

During the middle Holocene, a short period (70006000 B.P.) of drier conditions was established in the southern sector of the basin of México. Lower values of arboreal pollen, especially Abies and Cupressaceae, mark this period, as does an increase in fire frequency as evidenced from the charcoal record (Fig. 12). At Lake Texcoco, lake sedimentation resumed at 7300 B.P., and arboreal pollen declined somewhat (Lozano-García and Ortega-Guerrero, 1998). However, at the high-elevation Lake Quila site, no major change in vegetation occurred in the pollen record from this time (Almeida-Leñero, 1997).

Increases in moisture are evident in late Holocene limnological records from the southern, central, and northeastern regions of the basin of México (Bradbury, 1989; Lozano-García et al., 1993; Caballero-Miranda and Ortega-Guerrero, 1998; Caballero-Miranda et al., 1999). Higher lake levels at all sites indicate wetter conditions, but the vegetation apparently did not respond to this wetter climate. Instead, the pollen data indicate a significant increase in herbaceous taxa related to human activities. A late Holocene pollen record from Lake Zempoala (2800 m) near Lake Quila indicates a temperate, semidry climate with abundant Quercus, Salix, and Alnus at the end of the middle Holocene, and this trend continues until 2000 years ago. After this time, the Abies forests expanded at Lakes Quila and Zempoala, indicating cooler and more humid conditions, with a short dry period at 1500 B.P. The human impact in this high-altitude region is less important than in other parts of central México (Almeida-Leñero, 1997). Western-Central Region

Toward the eastern section of the TMVB, records from three lakes, Pátzcuaro, Cuitzeo, and Zacapu, in the Michoacan area provide a history of Holocene cli mate changes. Tephra layers, changes in sedimentation rates, and magnetic susceptibility data provide evidence for the recent volcanic and tectonic history of these three sites. According to Israde Alcantara (1997), Lake Cuitzeo evolved under intense faulting activity, which controlled sedimentation. Some of these fault systems are currently active, and Holocene lava flows are present along the western margin of Lake Zacapu. This volcanic activity complicates paleoclimate interpretations (Metcalfe, 1997; Lozano-García and Xel-huantzi-López, 1997). Pollen and diatom records from Lake Pátzcuaro indicate the late Pleistocene/Holocene environmental history (Watts and Bradbury, 1982). Lake Cuitzeo is the second largest lake in central México, and recent paleolimnological and palynological data provide more information on late Quaternary environmental changes (Velázquez-Durán, 1998). One of the better dated Holocene sequences from central México is from Lake Zacapu, where multiproxy studies of pollen, diatoms, and sediment stratigraphy provide information on environmental history (Metcalfe, 1992; Xelhuantzi-López, 1994; Arnauld et al., 1997).

The Holocene record of Lake Cuitzeo begins with evidence for a dense forest after a hiatus in sedimentation from 15,000-9000 B.P. Pine forest dominated the early Holocene, and the diatom Cyclotella meneghi-niana, which grows in swamps, indicates low lake levels (Velázquez-Durán, 1998).

The Lake Zacapu record (Fig. 12) shows evidence for intermittent volcanic activity, and the pollen record begins in the later part of the early Holocene. From 90008000 B.P., Pinus forest with Quercus, Alnus, and Poaceae dominated. The marsh was shallow with circumneu-tral pH and abundant macrophytes. A major volcanic and tectonic event sometime during 8000-7000 B.P. compromised the record, and increases in erosion rates and fires around the basin are evident (Xelhuantzi-López, 1994; Arnauld et al., 1997).

At Lake Pátzcuaro, pollen and diatom data show that the early Holocene was moister than the late Pleistocene, based on a decrease in Cupressaceae pollen and an increase in planktonic diatoms (Watts and Bradbury, 1982; Bradbury, 1997).

The record from La Hoya de San Nicolás de Paran-gueo, a caldera lake located between central México and the Chihuahua Desert, shows the development of Pinus forest from 13,000-3400 B.P., with a peak of Chenopodiaceae-Amaranthaceae at the Pleistocene/ Holocene transition (Brown, 1984; Metcalfe et al., 1989). The establishment of grassland at lower elevations and Pinus forest at higher elevations continued during the early and middle Holocene.

At Lake Cuitzeo, mixed Pinus-Quercus forest developed and herbaceous pollen became more abundant in the middle Holocene. The diatom data show a trend to drier conditions, culminating with the deposition of a sand layer (Velázquez-Durán, 1998). At Lake Zacapu, conditions began to stabilize at the earliest part of the middle Holocene. The marsh area became successively smaller, suggesting a trend to less humid climate. Increases in Chenopodiaceae-Amaranthaceae and Poa-ceae, coupled with low values of boreal taxa and diatoms, indicate a change to a semiarid climate. Water levels reached a minimum between 6800 and 4400 B.P. By the end of the middle Holocene, wetter conditions returned, the marsh expanded, and Pinus forests reexpanded. The reduction of Alnus and an increase in Pinus pollen mark the beginning of the middle Holocene at Lake Pátzcuaro; shallow-water littoral diatoms became abundant during this time. The record from Laguna San Pedro, located in the westernmost part of the TMVB, shows moderate mid-Holocene aridity, with a change from mixed Pinus-Quercus parkland to a Quercus parkland occurring at 5000 B.P. (Brown, 1985). Comparable to other central México records, human presence is documented in the pollen diagrams, with Zea pollen in the Lake Pátzcuaro record and intense deforestation at Lake Zacapu, La Hoya de San Nicolás de Parangueo, and Lake Cuitzeo.

19.3.2. Yucatán Peninsula

The Yucatán Peninsula is one of the northernmost tropical areas of the Americas. It lies between 17°50'-21°30' N and 87°00'-91°00' W and includes the states of Quintana Roo, Yucatán, and Campeche in México, as well as Belize and northern Guatemala (Petén lowlands). The peninsula is a flat platform composed mainly of tertiary marine limestone with thin soils, with elevations mostly below 200 m. Climate in the Yucatán Peninsula is tropical subhumid, with a summer wet season. Precipitation has an east-to-west gradient, becoming drier westward. The northwestern region is relatively dry, with 500-1000 mm/year precipitation. Dominant vegetation is low deciduous forest with trees 6-8 m tall and thorn-scrub forest with trees 3-8 m tall. Dominants of the low deciduous forest are Jatropha goumeri, Metopium brownei, Alvaradoa, Bursera simaruba, Maclura tinctoria, Bumelia, Mimosa bahamensis, and Cae-salpinia vesicaria. Dominants in thorn-scrub forest include Acacia, Mimosa bahamensis, Pithecolobium, Leu-caena, Senna, Piscidia piscipula, and Bursera simaruba. Precipitation in the eastern region is >1500 mm/ year, and a semi-evergreen forest dominates. Important trees are Brosimum alicastrum, Manilkara zapota, Vitex gaumeri, Lysiloma latisiliquum, and Swietenia macrophylla (Flores and Carbajal, 1994).

In this karst landscape of the Yucatán Peninsula, the presence of numerous lakes and water-filled sinkholes (cenotes) offers the possibility for paleoecological studies. At sites in the north-central Yucatán Peninsula, sedimentation began after 8000 B.P., when groundwater rose probably because of both rising sea level and increased precipitation (Whitmore et al., 1996). In the Petén lowlands in the southernmost portion of the peninsula, deeper lakes also preserved a late Pleistocene record as well as the Holocene record (Islebe et al., 1996b).

Multiproxy paleoecological studies of several lakes in the Yucatán Peninsula provide information on environmental and climatic change during the Holocene and on human impact related to the rise and collapse of the Maya civilization. Climate proxies include geochemistry, stable isotopes, diatoms, phytoliths, and pollen (Curtis et al., 1996; Islebe et al., 1996b; Hodell et al., 1995; Leyden, 1984, 1987; Leyden et al., 1994, 1996, 1998; Vaughan et al., 1985; Whitmore et al., 1996). We present here palynological data from Lake Cobá and Cenote San José Chulchacá.

Two cores from Cenote San José Chulchacá (Fig. 13), located in the driest part of the Yucatán Peninsula, cover the last 8120 years B.P. (Leyden et al., 1996). Limno-logical, oxygen isotope, and palynological data suggest a dry early Holocene. Cyperaceae pollen is absent, suggesting low water levels. Significant values of pollen of Brosimum, a canopy tree of the medium-high semievergreen forest, suggest an increase in precipitation at 7600 B.P., although mesic forest probably did not grow around the site (Leyden et al., 1996). Isotopic data and the absence of Brosimum indicate another dry episode from 7000-5280 B.P. The return of Brosimum indicates a return to wetter conditions from 5280-3500 B.P. The presence of Poaceae and Trema pollen in the regional vegetation suggests a trend to drier environments with open, dry forests from 3000-1350 B.P. According to the pollen data, the changes in vegetation at Cenote San José Chulchacá indicate that dry forest has existed since the middle Holocene, and only variations between different types of dry communities have occurred.

The paleoecological record at Lake Cobá in the east-central part of the peninsula starts at 8370 B.P. with a wooded swamp represented by Eleocharis, Typha, and Dalbergia from 8000-6600 B.P. (Leyden et al., 1998). Stable isotopes from ostracodes show that the lake was shallow and saline during this time (Whitmore et al., 1996). Significant amounts of Piscidia pollen and other taxa such as Metopium indicate semideciduous dry forest in the region. During the middle Holocene, 66004000 B.P., a more diverse plant community expanded, with Brosimum, Moraceae, and Bursera indicating a dense forest. In the late Holocene, between 2700 and 1250 B.P., herbaceous taxa (Poaceae and Chenopodi-

20' 20 1 20' 20 ' 20 1 20 20 1 20 FIGURE 13 Pollen diagrams from Cenote San José Chulchacá in the Yucatán Peninsula (Leyden et al., 1996).

aceae) dominate the pollen spectra, indicating forest clearance. Zea pollen provides additional evidence of human activities. During this period, pollen and lim-nological records indicate intense environmental dis turbance; forest cover diminished drastically, and Zea pollen is abundant. The diatom data indicate fresher and deeper lake conditions, with the deepest episode at 1300 B.P. From 1250-700 B.P., the Late Classic period,

Zea, Chenopodiaceae-Amaranthaceae, and Caryophyl-laceae pollen decreased. This change may have been associated with the growth of the city of Cobá and the shift of the milpa fields farther away from the city. The Maya collapse, which may have resulted from drier conditions in the peninsula (Curtis et al., 1996; Hodell et al., 1995), is documented in the pollen diagram from Cobá by a decline in Chenopodiaceae-Amaranthaceae, an absence of Zea, and an increase in tree pollen, represented by Brosimum, Trema, Moraceae, and Piscidia.

At Laguna Cocos in northern Belize, a pollen sequence that covers the last 6300 years shows a sequence of changes from predominantly arboreal vegetation to nonarboreal vegetation and a return to arboreal vegetation. The presence of Zea and other herbs related to agricultural activities indicates a strong human influence (Hansen, 1990).

Stable isotopes, diatoms, and geochemical data from Lake Chichancanab and Lake Sayaucil provide additional evidence for climate change. Lake Chichancanab refilled after 8200 B.P., similar to other water bodies in the Yucatán region (Hodell et al., 1995). Conditions continued to become wetter, and lake levels rapidly increased after 8000 B.P. A change to drier conditions and lower lake levels occurred between 1260 and 980 B.P. A record from Lake Sayaucil covers the last 3000 years. After the initial lake-level rise, a trend to drier conditions occurred from 3200-1940 B.P. Subsequently, lake levels rose at the same time as human settlement activity increased, and wet conditions continued until recent times (Whitmore et al., 1996). Paleoclimatic reconstruction based on vegetation history is consistent with the data obtained from other climate proxies. At Punta Laguna, the late Holocene record indicates climatic variability based on 818O data. This record documents a dry period from 1700-800 B.P., contemporaneous with the Maya collapse (Curtis et al., 1996). Petén Lowlands (Guatemala)

The Holocene sediment sequences from the Petén lowlands have dating problems owing to the hard-water effect (Deevey and Stuiver, 1964) and to the scarcity of datable carbon. In the pioneering work of Vaughan et al. (1985), only 5 of 23 dates from several cores were reliable. Vaughan et al. (1985) described six pollen zones, P-1 to P-6, which, because of chronological problems, they correlated with dated archeological events. Other investigators have used this zonation in the Petén Peninsula. The recent work of Islebe et al. (1996b) at Lake Petén Itzá improved the chronology for this tropical region.

According to Leyden et al. (1994), zones P-1 and P-2 from Quexil and Salpeten show a moist early Holocene based on the replacement of Pinus, Quercus, and mesic temperate hardwoods by rain forest taxa such as Moraceae (zone P-1). Higher lake levels also indicate increased precipitation (Leyden, 1987; Brenner, 1994a). The recent work of Islebe et al. (1996b) at Lake Peten Itza, the largest and deepest lake in the region, reconstructed Holocene paleoclimatic changes with more precision. Abundant Moraceae/Urticaceae pollen are the signal of lowland, semi-evergreen forest and mesic conditions. The pollen sequences of Quexil, Salpeten, and Peten Itza suggest that the tropical mesic forest was fully developed after 10,000 B.P. and that the environments in the Peten lowlands at that time were moister than they are today. Inferences from pollen data agree with 818O, magnetic susceptibility, and geochemical climate-proxy data from Salpeten and Quexil. All these data suggest that conditions in the early Holocene were mesic. However, a contradiction exists at Peten Itza, where the presence of Moraceae-Urticaceae pollen suggests humid conditions, whereas high 818O values from shell material suggest increased E-P ratios during this period (Curtis et al., 1998).

Middle Holocene pollen records from Salpeten and Quexil (zone P-2) show the development of more open vegetation with Quercus, Cecropia, Byrsonima, and Melastomataceae. At Peten Itza, a decline in precipitation occurred at 6500 B.P. with a drastic reduction of Moraceae/Urticaceae at the end of zone P-2. At the same time, elements indicative of more open vegetation increased, such as Poaceae, Melastomataceae, and Byrsonima, suggesting drier climates. Increased Pinus and Quercus also indicate more open vegetation and a drier climate (Islebe et al., 1996a). In the upper part of zone P-2 at Peten Itza, correlated to zone P-3 of Vaughan et al. (1985), human impact is documented as associated with the Maya population increase.

All the late Holocene palynological records from the Peten lowlands present signs of human impact. The agricultural activities of the Maya altered the vegetation substantially, and the pollen records from Salpeten and Quexil show more open vegetation with abundant herbaceous taxa for the last 4000 years. The human impact significantly increased erosion in the basins, and a layer of clay (the Maya Clay) with thicknesses of up to 7 m was deposited in some lakes from 3200 to 470 B.P. (Curtis et al., 1998). During this period, a reduction in arboreal cover is evident with concomitant expansions of herbaceous taxa (zones P-3 to P-5). Although this anthropogenic unit is not present in all lakes (e.g., Peten Itza), changes in sediment composition and magnetic susceptibility occur in zone P-3.

After two millennia of strong human disruption to the vegetation, a recovery of tropical forest is evident in the pollen records. In the records from Salpeten, Quex-

il (P-6), and Peten Itza (P-4), tropical trees (Brosimum, Trema, and Moraceae-Urticaceae) increase. Forest regeneration postdates the Maya collapse, which is archeologically dated to a.d. 800-900. The precise time of regeneration is difficult to determine because of problems in radiocarbon dating, but at Peten Itza forest recovery began ca. 1000 years ago (Islebe et al., 1996b).

19.3.3. Central America Costa Rica

A number of high-elevation sites in the Sierra de Ta-lamanca document vegetation history and shifts in tree line: Lachner Bog (2400 m) (Martin, 1964), Lago de la Morrenas (3480 m) in the Chirripo Paramo (Horn, 1993), and La Chonta (2310 m) and La Trinidad Bogs (2700 m) (Hooghiemstra et al., 1992; Islebe et al., 1996a; Islebe and Hooghiemstra, 1997).

After the La Chonta stadial (13,000-12,250 B.P.), a cool episode at the end of the Pleistocene, climate warmed to modern conditions by 11,000 B.P. (Islebe et al., 1996a). At La Chonta, Alnus forests expanded upwards to the modern tree line at 2800-3000 m. The tree line remained stable near Chirripo Paramo throughout the Holocene (Horn, 1993). By the middle Holocene, Podo-carpus-Quercus forests had developed at La Chonta and La Trinidad, indicating humid conditions with tree line at its present altitude (Islebe et al., 1996a). Over the last 4000 years, Podocarpus has declined at La Chonta and La Trinidad, but the only charcoal evidence for natural or anthropogenic fires is at Chirripo Paramo. Thus, the early to middle Holocene (10,800-5000 B.P.) was wetter than today, with a drier period from 5000-1500 B.P. A lack of shifts in the Holocene tree line suggests stable temperatures (Islebe et al., 1996a).

In the Gatun basin in the Panama Canal Zone, mangrove vegetation prevailed during the early Holocene (Bartlett and Barghoorn, 1973). The decline of Iriartea and the absence of Symplocos and Ericaceae at the end of the early Holocene suggest that temperatures attained their present values at this time. At La Yeguada, in central Panama (Bust et al., 1992; Piperno et al., 1990), the pollen record indicates the presence of moist tropical lowland forest since 12,700 B.P. with Pilea, Trema, and Cecropia. Drier conditions developed during 80004700 B.P. at the Gatun basin; pollen of Ilex and Myrica also suggest more seasonal climates. At La Yeguada, diatoms, phytoliths, and lower lake levels indicate a relatively dry phase from 8200-5500 B.P. (Piperno et al., 1990). A decline in Poaceae and an increase in forest taxa (Pilea, Cecropia, and Myrtaceae) may indicate human activity (Bush et al., 1992). During the late Holo-cene, important changes in vegetation occurred in the Gatún basin, but their climatic significance is unclear because of human impacts documented by Zea pollen in the cores.

19.3.4. Caribbean

High-resolution analyses of 818O from ostracode shells and geochemistry, pollen, and charcoal data provide evidence indicating the climatic and environmental history for the region of Lake Miragoane, Haiti, for the last 12,400 years (Hodell et al., 1991; Brenner, 1994b). The period 12,400-9300 B.P. was drier than today with low lake levels; montane shrubs and xeric palms dominated the vegetation. A sudden change occurred at 9200 B.P., when Chenopodiaceae-Amaran-thaceae increased and 818O decreased, suggesting that the climate became increasingly wetter from 9200-2500 B.P. Open forest with shrub elements dominated from 9200-6250 B.P.; it was replaced by more mesic forest with Ambrosia and Poaceae in the understory from 6250-4000 B.P., and then by Moraceae forest with Pseudolmedia, Trophis, Chlorophora (Maclura), and Cecropia from 4000-2500 B.P. It was during the latter interval that the lake reached its maximum depth, and the climate was warm-wet and seasonal. In the late Holocene, at Lake Miragoane, the climate became drier as lake levels declined and open, dry forests expanded, although some relicts of the previous mesic forest elements persisted. The pollen record from Church's Blue Hole on Andros Island in the Bahamas also indicates this dry period with increases of Pilea, Dodonaea, and dry shrubs (Kjellmark, 1996).

19.3.5. Summary for México, Central America, and the Caribbean

In spite of the limited number of records, dating problems, and marked human influence especially during the late Holocene, climatic trends can be interpreted from pollen data. Evidence for increased moisture is documented for the early Holocene in central México, the Petén lowlands, Central America, and the Caribbean. At Lakes Chalco and Quila in México's east-central sector, montane Pinus-Quercus-Alnus forests developed from 12,000-8000 B.P. A dense Abies forest existed from 11,000-8000 B.P. (Sosa-Najera et al., 1998; Lozano-Garcia and Ortega-Guerrero, 1994; Almeida-Leñero, 1997). Glacier advances occurred on Iztac-cihuatl Volcano at 12,000-10,500 and 8800-7400 B.P. (Vázquez-Selem and Phillips, 1998). Pollen records from the west-central sector indicate warmer/wetter conditions with a reduction of Cupressaceae pollen at Lake Pátzcuaro (Watts and Bradbury, 1982; Bradbury, 1997) and an expansion of mixed Pinus-Quercus-Alnus forest at the shallow marsh of Lake Zacapu (Xel-huantzi-López, 1994). Quercus increased at several sites during the early Holocene: at Puebla at 9500 B.P., Lake Chalco at 11,400-8300 B.P., Lake Quila at 9400-7000 B.P., and Lake Zacapu at 9000-6100 B.P. High Quercus pollen levels may indicate increased summer temperatures. Although summer rain probably was intensified, it was apparently insufficient to increase lake levels, as is suggested by the diatom records from Lakes Chalco, Pátzcuaro, and Zacapu (Caballero-Miranda, 1995; Bradbury, 1997; Metcalfe, 1995; Fritz et al., 2000). Higher temperatures apparently canceled the effect of higher precipitation, and lower lake levels resulted. Seasonal-ity of precipitation with increased monsoonal (summer) precipitation resulted in more mesic vegetation, but low lake levels may relate to low winter moisture instead.

In the Yucatán Peninsula, the records start at ca. 9000 B.P., when lakes filled as a consequence of rising of sea levels (Hodell et al., 1995), and vegetation records are available from the early Holocene. In the Petén lowlands, the existence of tropical forests composed of Moraceae-Urticaceae after 12,300 B.P. suggests a moist early Holocene (Islebe et al., 1996b). At Salpeten, pollen data indicate that a cool climate continued during the earliest part of the Holocene, and at Lake Quexil temperate mesic forest prevailed. The tropical mesic forest was completely established by 9500 B.P. (Leyden et al., 1994).

The early Holocene was also wetter in Central America. At La Chonta in Costa Rica, upper montane rain forest was established by 10,500 B.P. At La Yeguada in Panama, lowland moist tropical vegetation dominates the entire Holocene pollen record. In the Caribbean, the high-resolution record from Lake Miragoane indicates increasing precipitation during the early Holocene.

In the mid-Holocene, a trend toward a drier climate is evident in central México, although the beginning, duration, and culmination differ among sites. In records from the basin of México, a reduction in Abies and Alnus and abundant charcoal occurred from 7380-5740 B.P. (Sosa-Najera et al., 1998; Lozano-García and Ortega-Guerrero, 1994). Relatively low lake levels continued, although a trend to higher levels did occur (Caballero-Miranda et al., 1999), suggesting higher evaporation rates. At Lake Quila, less humid conditions led to a reduction of Alnus forest and the development of a Pinus hartwegii forest from 7800-6300 B.P. (Almeida-Leñero, 1997). In west-central México, a re duction of Alnus occurred at Lake Cuitzeo at 6820 B.P., while Chenopodiaceae-Amaranthaceae peaked (Veláz-quez-Durán, 1998), and a reduction of planktonic diatoms occurred at Lake Pátzcuaro at 5740 B.P. (Bradbury, 1997). At Lake Zacapu, Alnus decreased after 7160 B.P., while Chenopodiaceae-Amaranthaceae increased from 7380-4500 B.P. (Xelhuantzi-López, 1994). Diatom data indicate a dramatic reduction in water levels.

At San José Chulchacá in the northern Yucatán Peninsula, the record begins at 8120 B.P., showing dry vegetation and low lake levels. After a dry early Holocene, conditions became somewhat wetter, with the presence of Brosimum, which indicates mesic forest, by 7600 B.P. Episodes of dry conditions between 7000 and 5280 B.P. and between 3000 and 1350 B.P. bracketed a wetter episode. At Lake Cobá, pollen diversity was low from 8370-6480 B.P. After that time, an increase and diversification of dry semideciduous forest occurred that lasted until 4400 B.P. At Salpeten in the Petén lowlands, a trend to a moister climate with Brosi-mum occurred in the early Holocene (Leyden, 1987; Leyden et al., 1994). At Petén Itzá, high forest dominated, but began to decline at ca. 6360 B.P., probably because of human activities; however, the decrease in 818O values provides a clear signal of a moist mid-Holocene climate (Curtis et al., 1998).

In Panama, the records indicate dry conditions at the Gatún basin from 7790-4500 B.P. (Bartlett and Barg-hoorn, 1973), while at La Yeguada a relatively dry phase occurred from 9000-5740 B.P. with a reduction of lake levels. Human activities confuse the climate signal in the pollen records (Bush et al., 1992). The early Holo-cene wet conditions continued into the middle Holo-cene at La Chonta (Islebe et al., 1996a), and at Lago Chirripó no evidence exists for a mid-Holocene vegetation change (Horn, 1993). In the Caribbean, the wettest conditions occurred from 7790-4500 B.P. with the development of mesic forest (Hodell et al., 1991).

Thus, for the middle Holocene, the climate was more arid in central México from 6820-5740 B.P., while in the Yucatán Peninsula and Petén lowlands, somewhat moister but fluctuating conditions occurred. In Central America and the Caribbean, the data indicate humid conditions during this time.

Most of the pollen spectra that cover the late Holo-cene show a significant human influence for the last thousand years, including the presence of Zea pollen, reductions in arboreal cover, changes in vegetation composition, and increases in charcoal particles. The human effect on vegetation varies in timing and intensity and masks the climatic signal in some of the Mesoamerican sequences. In central México, a pattern of human disturbance is evident in several basins for the last 3500 years (Metcalfe et al., 1989). Nevertheless, a trend to moister conditions is suggested from synchronous increases in lake levels at Lakes Texcoco, Chalco, and Tecocomulco in the basin of México at 3500 B.P. (Caballero-Miranda et al., 1999). However, episodes of less humid climates are recorded at Lakes Pátzcuaro and Zacapu for 1200-900 B.P. (Curtis et al., 1996; Metcalfe, 1995).

In the northern part of the Yucatán Peninsula, the wettest period occurred between 6000 and 3000 B.P., interrupted by several dry episodes. A period of humid conditions occurred from 5280-3500 B.P., but drier conditions returned from 3500-1340 B.P. (Leyden et al., 1996). In the central part of the peninsula, the Lake Cobá record shows herbaceous and successional associations at 3500 B.P. related to human activities (Leyden et al., 1998). Drier conditions during the late Holocene are evident at Lakes Chichancanab, Punta Laguna, and Miragoane in Haiti (Hodell et al., 1995; Curtis et al., 1996). Oxygen isotope records document a period of aridity from 1790-930 B.P. in two of the Yucatán lakes (Punta Laguna and Chichancanab), which coincides with the collapse of the Maya civilization and other Mesoamerican cultures (Curtis et al., 1996). Nevertheless, this signal of dryness is not evident in the palynological records, probably because of predominant human influences (Bradbury, 1982). The openness in vegetation in the Petén lowlands during the late Holocene also indicates human disturbance (Islebe et al., 1996b; Curtis et al., 1998).

Panamanian palynological records indicate strong human influences on the vegetation during the late Holocene. Bush et al. (1992) suggested that significant human disturbance in La Yeguada began as early as 11,000 years ago. In the Lago Chirripó record in Costa Rica, high concentrations of charcoal during the late Holocene in subalpine and lower montane forests are indicative of human influences (Horn, 1993). The La Chonta pollen record also points to a drier climate for this period, with an episode of more arid conditions at ca. 1200 B.P. (Islebe et al., 1996a).

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