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This paper presents data on and discusses the degree of postglacial stability of the northern Canadian tree limit. Holocene climatic changes have been both directional and synchronous for this region, though deteriorations/ameliorations either (1) lagged from west to east, (2) were buffered by local geographic factors, or (3) were not registered in the fossil record owing to the inherent persistence of the vegetation at some sites. The present northern tree limit in eastern Canada is in equilibrium with the prevailing climate, with the tree populations successfully regenerating both sexually and vegetatively. These tree stands are still occupying their maximum Hypsithermal positions. In contrast, the tree limit, tree line, and intervening forest-tundra ecotone in central and western Canada are out of equilibrium with today's climate. These populations are not sexually regenerative and apparently became established during warmer climates where the tree line was north of its present position. As such, they are susceptible to destruction by further climatic deterioration or anthropogenic disruption. [ABSTRACT FROM AUTHOR]

In the southwest Yukon, valley walls mantled by talus and glacial deposits have been modified by a variety of catastrophic processes. Avalanches, landslides, mudflows, and retrogressive slope failures have been triggered by a variety of internal and external causes. These flow mechanisms produce a range of landforms in the paraglacial environment that dominate the landscape, particularly in the higher mountain zones. Models of rock glacier formation resulting from retrogressive slope failures, high hydrostatic pressures, and avalanche activity are proposed. [ABSTRACT FROM AUTHOR]

It is now well established that Precolumbian farmers of Mexico and Central America were responsible for widespread environmental degradation before the arrival of Europeans. Relatively little is known, however, of the chronology, severity, and exact geographic distribution of these anthropogenic landscape impacts. This article addresses questions of Holocene land use and anthropogenic environmental change in the upper Rio Paz Valley, El Salvador, before and after the arrival of the Spanish in the sixteenth century. A lacustrine sediment sequence from Chalchuapa records the impacts of prehistoric agriculturalists on a mid-elevation tropical watershed during the past 3700 years. The 9.72 m sediment core from Laguna Cuzcachapa contains unequivocal evidence for human manipulation of the landscape: high levels of maize and agricultural weed pollen, charcoal, magnetic susceptibility, and sediment flux. The abundance of maize pollen (>6 percent) even in the basal sediments of the core indicates that an intensive system of maize cultivation was well established in Chalchuapa by 3700 cal yr BP. Proxy indicators of human disturbance continue to exhibit high values from 3700 cal yr BP to the present, interrupted only by two hiatuses: one centered on the ca. AD 430 Tierra Blanca Joven eruption of the Ilopango volcano, and another during a period of population decimation following the arrival of Europeans in the early sixteenth century AD. The periods of greatest anthropogenic disturbance include ca. 3700–1600 cal yr BP (Preclassic/“Protoclassic”) and ca. 1350–1000 cal yr BP (Late Classic and Early Postclassic Periods). [ABSTRACT FROM AUTHOR]