Gerald A. Islebe

A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten, Guatemala

We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS 14C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to ∼9000 14C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by ∼9000 14C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (∼9000 to ∼6800 14C yr BP), pollen data suggest moist conditions, but high δ 18O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lakes water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in δ 18O values and associated change in ostracod species at ∼6800 14C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning ∼5780 14C yr BP may indicate early human disturbance. By ∼2800 14C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning ∼1100 to 1000 14C yr BP, following the collapse of Classic Maya civilization.

A Holocene vegetation history from lowland Guatemala

A 5.45-m core from Lake Peten-Itza, lowland Guatemala, contains a near-complete record of Holo cene sedimentation. The age-depth relationship for the core is based on AMS 14C dating of terrestrial wood fragments and provides a reliable chronology in this karst region where hard-water lake error has typically confounded sediment geochronology. In the basal part of the sequence, pollen of the Moraceae-Urticaceae group dominate, indicating the presence of widespread tropical forest during the early Holocene (c. 8600-5600 yr BP). Relative abundance of pollen of high forest taxa declined beginning as early as 5600 yr BP, indicating climatic drying or perhaps initial land clearance. Deforestation by prehistoric Mayan inhabitants is documented clearly in the pollen record beginning about 2000 yr BP (106 cal BC-122 cal AD, 95.4%, 2 sigmas) by the appearance of disturbance taxa (e.g. Ambrosia and Poaceae) and presence of Zea. Forest regrowth occurred following the Classic Maya collapse, c. AD 900, as reflected by a relative increase in Moraceae-Urticaceae pollen.

Vegetation and climate history of montane Costa Rica since the last glacial

Abstract New palynological evidence from the Cordillera de Talamanca (Costa Rica) is presented. The La Chonta-1 core (2310 m a.s.l) shows the development of montane vegetation during the late Quaternary. A shorter core (La Trinidad-III) shows the Lateglacial-Holocene transition, including the La Chonta stadial based on earlier published evidence. A soil section from the paramo belt at 3100 m shows vegetation recovery after fire. Modern pollen rain was studied along an altitudinal transect from 2100 m to 3800 m at Mt Chirripo. A comparison with other palaeoecological data of the region is given to elucidate climatic and vegetational changes throughout the Central American region. Data show a cooling of 7–8°C during the Last Glacial Maximum (LGM) for montane Costa Rica, which is in accordance with data from lowland Guatemala. A 1.5° to 2.5°C temperature drop is recorded during the Younger Dryas Chron in both Costa Rica and Guatemala, but apparently not in Panama. The Lateglacial-Holocene transition in montane Costa Rica is established at 10,400 BP. Between 9000 and 8500 BP moist forest developed in mountainous Costa Rica as well as in lowland Guatemala and Panama. Environmental change during the mid-Holocene seems more affected by changes in humidity than temperature change throughout Central America. Distribution maps of paramo and montane vegetation in Costa Rica are reconstructed for 10 ka, 14 ka and 18 ka based on currently available palynological data. These data indicate that during the LGM a paramo vegetation corridor existed between northern Costa Rica and probably northern Panama.

Climatic change during the Younger Dryas chron in northern South America: a test of the evidence

Abstract New AMS and palynological data are presented from the Colombian Andes to assess vegetational and climatic change during the Lateglacial–Holocene transition, with special emphasis on the Younger Dryas (YD) chronozone. The new evidence is compared and discussed with other Colombian cores and with data from other countries in the region. The Lateglacial climatic reversal in Colombia, known as the El Abra stadial, has an estimated lower boundary of 11,200 (conventional) and 10,900 (interpolated) 14 C yr BP, respectively. Although the El Abra stadial is assumed to be the equivalent of the European YD, the present data suggest that the El Abra stadial is a biostratigraphical signal representing both the YD chronozone and the earliest Holocene (11,000–9000 14 C yr BP). On the basis of new AMS dates and a re-evaluation of the pollen zones, we divided the Colombian El Abra biozone into two phases. From ca. 11,000 to ca. 10,500 14 C yr BP there is a sharp increase of subparamo and paramo pollen, reflecting a relatively cool phase during the YD chronozone (zone Y1). After ca. 10,500 14 C yr BP, a slight increase of arboreal pollen and the presence of Cactaceae (zone Z1) point toward a relatively milder but drier phase extending to ca. 9000 14 C yr BP in the earliest Holocene. Our conclusions add detail to the concept of astronomical forcing of contrasting rainfall changes in northern South America between 12,400 and 8800 14 C yr BP. We propose an environmental drought during the El Abra biozone, with subsequent erosion of the sediments deposited during that period, as a major factor explaining the poor presence of YD evidence in northern South America. We conclude that at hydrologically sensitive sites without a clear lithological change around 11,000–9000 14 C yr BP, a hiatus during the YD chronozone can only be detected if other cores are available with bracketed time control. We state that the start of the temperature decline in Central America and northern South America is related to the global YD cooling event. However, much work is still required to understand the duration and amplitude of the climatic reversal during the YD chronozone in this part of the globe.

Recovery of the forest ecosystem in the tropical lowlands of northern Guatemala after disintegration of Classic Maya polities

We employed paleolimnological methods to investigate tropical forest recovery and soil stabilization that followed abandonment of agricultural systems associated with disintegration of Classic Maya polities ca. A.D. 800–1000. We used lithological, geochemical, magnetic, and palynological data from sediment cores of Lake Peten Itza in the Maya Lowlands of northern Guatemala. Sediment core chronology was developed using radiocarbon dates on terrestrial wood and charcoal fragments. Our results indicate that in the absence of large human populations and extensive farming activities, Peten forests recovered under humid climate conditions within a span of 80–260 yr. Soil stabilization postdates pollen evidence of forest regrowth stratigraphically, and required between 120 and 280 yr. We conclude that the tropical forest ecosystem in the watershed of Lake Peten Itza had been reestablished by the early Postclassic Period (A.D. 1000–1200).

A cooling event during the Younger Dryas chron in Costa Rica

Abstract A fossil pollen record from the Costa Rican Cordillera de Talamanca presents the first AMS radiocarbon-dated evidence of a temperature decrease during the Younger Dryas Chron. This cooling event is named La Chonta stadial after the bog at 2310 m altitude, located at the actual lower to upper montane forest boundary. High-resolution pollen analysis revealed that between 11,070±130 (AMS 14C age) and 10,400 yr B.P. (interpolated age) vegetation comparable to present-day subalpine forest occurred about 300–400 m lower than in the previous warmer interval, and 600 to 700 m lower than at present time. The radiocarbon dates are close to the earlier estimated ages of 11,080-10,500 yr B.P. for this interval, based on interpolation and pollen concentration rates (Hooghiemstra et al., 1992). The downslope shift of the upper forest line indicates an estimated temperature drop of 2–3°C during the La Chonta stadial. The local vegetation development is indicative of a drier climate. From 10,400 to 9800±120 yr B.P. (AMS 14C age) subalpine rain forest was replaced by upper montane forest, a transitional period to Holocene environmental conditions.

Hurricane Gilbert and structural changes in a tropical forest in south‐eastern Mexico

Summary This paper describes structural change and damage to the natural tree vegetation in the Dr Alfredo Barrera Marin Botanical Garden, following Hurricane Gilbert in September, 1988. Post-hurricane data were compared with pre-hurricane data from the same plots, focusing mainly on composition and structure. Near complete defoliation of trees and plants of the understorey occurred and average foliage loss was estimated to be 4.5 t/ha. The density of trees decreased by 33%, and the basal area decreased by 12%. Small diameter trees accounted for the great majority of losses and trees in the largest diameter classes survived best. The average height of trees in the lower canopy decreased by 6% and for those in the upper canopy by 9%. The frequency of understorey plants decreased by 51% and their cover decreased by 70%. Floristic richness decreased only slightly after the hurricane. Resumen Este trabajo se refiere al cambio estructural y danos generales que sufrio la vegetacion natural del Jardin Botanico Dr Alfredo Barrera Marin, despues del paso del huracan Gilberto en septiembre de 1988. El estudio toma como punto de comparacion un trabajo previo realizado por Sanchez (1987), quien efectuo sus mediciones en tres parcelas que fueron establecidas y estudiadas tres anos antes del huracan. La mayor parte del dano incluyo la defoliacion cercana al 100% de arboles y plantas del sotobosque cayendo en promedio 4.5 t/ha de hojas. La densidad de arboles disminuyo 33%, significando un decremento de 12% en el area basal. Los arboles de diametro pequeno fueron derribados en su gran mayoria, mientras que los arboles de las clases diametricas mas grandes sobrevivieron. El promedio de altura para los arboles entre 3–8 m disminuyo 6% y para los arboles entre 8.1–16 m disminuyo 9%. La densidad de plantas del sotobosque disminuyo 51% y su cobertura tuvo un decremento de 70%. La riqueza especifica disminuyo ligeramente para despues del huracan.

Tropical forest communities in southeastern Mexico

A phytosociological study was carried out along a 450 kmtransect in tropical southeastern Mexico in order to identify plantcommunities.Five major plant communities were distinguished and described:Those communities reflect preference of different environmentalfactorssuch as soils, precipitation and local relief. Further, the major threats(hurricanes, fires, agriculture) to the vegetation of the study areaare discussed.

Forests, fields, and the edge of sustainability at the ancient Maya city of Tikal

Significance The rise of complex societies and sustainable land use associated with urban centers has been a major focus for anthropologists, geographers, and ecologists. Here we present a quantitative assessment of the agricultural, agroforestry, and water management strategies of the inhabitants of the prominent ancient Maya city of Tikal, and how their land use practices effectively sustained a low-density urban population for many centuries. Our findings also reveal, however, that the productive landscape surrounding Tikal, managed to the brink of its carrying capacity during the Late Classic period, did not have the resilience to withstand the droughts of the 9th century. These results offer essential insights that address the question of why some cities thrive while others decline. Tikal has long been viewed as one of the leading polities of the ancient Maya realm, yet how the city was able to maintain its substantial population in the midst of a tropical forest environment has been a topic of unresolved debate among researchers for decades. We present ecological, paleoethnobotanical, hydraulic, remote sensing, edaphic, and isotopic evidence that reveals how the Late Classic Maya at Tikal practiced intensive forms of agriculture (including irrigation, terrace construction, arboriculture, household gardens, and short fallow swidden) coupled with carefully controlled agroforestry and a complex system of water retention and redistribution. Empirical evidence is presented to demonstrate that this assiduously managed anthropogenic ecosystem of the Classic period Maya was a landscape optimized in a way that provided sustenance to a relatively large population in a preindustrial, low-density urban community. This landscape productivity optimization, however, came with a heavy cost of reduced environmental resiliency and a complete reliance on consistent annual rainfall. Recent speleothem data collected from regional caves showed that persistent episodes of unusually low rainfall were prevalent in the mid-9th century A.D., a time period that coincides strikingly with the abandonment of Tikal and the erection of its last dated monument in A.D. 869. The intensified resource management strategy used at Tikal—already operating at the landscape’s carrying capacity—ceased to provide adequate food, fuel, and drinking water for the Late Classic populace in the face of extended periods of drought. As a result, social disorder and abandonment ensued.

Holocene vegetation and water level history in two bogs of the Cordillera de Talamanca, Costa Rica

Pollen records of Holocene sediment cores from the Costa Rican Cordillera de Talamanca (La Chonta bog, 2310 m and La Trinidad bog, 2700 m) show the postglacial development of the montane oak forest zone from ca. 9500 to 1500 yr BP. During the early Holocene (ca. 9500–700 yr BP), alder vegetation covered the La Chonta and La Trinidad bogs and their adjacent hills. The upper forest line is inferred to be at 2800–3000 m elevation. A Podocarpus-Quercus forest characterised the middle Holocene (ca. 7000–4500 yr BP). The upper forest line is located at >3000 m reaching the present-day altitudinal distribution. A Quercus forest characterised the late Holocene (ca. 4500–1500 yr BP). Compared to modern conditions, the early Holocene has similar average temperatures, but the moisture level was probably higher. Pollen evidence for the late Holocene indicates drier environmental conditions than today. In order to improve the paleoecological interpretation, we described the local vegetation and used moss samples as pollen traps at both montane bogs along strong soil moisture gradients.