CGRG Bibliography of Canadian Geomorphology
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Author : Agar, T.; and White, J.
Date : 1997.
Title : The history of Late Tertiary floras and vegetation change in Beringia based on the fossil records of northwestern Canada, Alaska, and northeastern Asia.
Publication : Beringia Paleoenvironments Workshop Sept., 1997: Abstracts and Program. Edited by: Scott Elias and Julie Brigham-Grette . Workshop sponsored by the U.S. National Science Foundation.
Issue :
Page(s) : 5-6.
Abstract
The present-day vegetation of Beringia consists of lowland tundra, alpine tundra, shrub communities, and boreal forest/taiga. The taxonomic composition of the modern forests of east Beringia (Alaska and NW Canada) and west Beringia (in NE Asia) differ markedly at the species level (e.g., Picea glauca, P. mariana, Larix laricina, Betula papyrifera, Populus balsamifera, P. tremuloides in east, Larix dahurica, Pinus pumila, Betula mid-dendorfii, Chosenia, Populus tremula, in west). How long have these forests been separated? When did the modern Beringian floras and vegetation types develop, and from what were they derived? What was Beringian vegetation like prior to the onset of Pleistocene glacial climates? What other events caused earlier changes in Beringian vegetation? Answers to these and other challenging questions about this fascinating region are emerging from ongoing studies of the fossil record of plants from numerous localities across Beringia. The fossil evidence shows that the modern forest flora of Beringia is a small remnant of far more diverse ancestral forests that existed in the region during the Tertiary. Most species of Beringia’s modern trees and shrubs are descendants of taxa that first appeared in the region during the Paleocene or Eocene. Spore-producing plants now found in Beringia have even longer ancestral histories in the region. Herbaceous families and genera that compose modern tundra and forest understory communities of Beringia have much shorter fossil records, because they evolved more recently than most trees and shrubs. Most herb taxa first appeared in Beringia during the Miocene, Pliocene, or Pleistocene. Studies of the floral history of Beringia during the late Cenozoic are not only useful for understanding the origins of modern high northern latitude vegetation, but also provide insights into ecologic responses of various genera to a variety of climatic and environmental changes. The records of fossil floras through time are also of great biostratigraphic value and have practical applications in regional correlation and mapping. The late Cenozoic fossil record of Beringian plants is not as well known as in many more accessible regions of the world, in spite of several decades of research by paleobotanists and palynologists from Russia, USA, and Canada. Although many gaps remain in the fossil records of Beringia, the emerging floral history is becoming increasingly detailed. Improvements in age estimates of fossil-bearing deposits in the region by means of isotopic methods, fission-track dating, and magnetostratigraphy have greatly facilitated interpretation of the regional histories, and allow correlation and comparision of widely separated fossil assemblages. The great changes seen in the history of floras of Beringia during the Miocene and Pliocene are the result of multiple influences (e.g., varying orbital parameters of Earth; changes in large-scale oceanic circulation; the rise of the Tibetan Plateau, and the rise of high mountain ranges in southern Alaska and NW Canada). During nearly all of the Cenozoic, land connections of varying extent existed between NE Asia and Alaska. These long-persisting land connections permitted essentially continuous exchange of plants and animals between the American and Eurasian continents for millions of years. The flooding of the Bering Strait by marine waters about 4 to 3 Ma created a significant obstacle to biotic migration. This geographic change, along with subsequent cooling climates, substantially limited exchange of terrestrial forest plants between NE Asia and NW North America during the late Pliocene and Quaternary. During the early Miocene (24-16 Ma), Beringian forests may have been a mixture of broadleaf deciduous and coniferous taxa, but the record of leaf fossils suggest there were largely separate lowland broadleaf communities and upland conifer forests. Beringian fossil assemblages of probable early early Miocene age (24-18 Ma) include such taxa as Metasequoia, Pinus, Picea, Larix, Cupressaceae, Ulmus, Quercus, Cocculus, Cladrastis, Carya, Nyssa, Populus, and Alnus. Estimated mean annual temperature (MAT) in central eastern Beringia at this time was probably ca. 7 o C, compared with modern MAT in that region in the range of -3 o to -8 o C. During the late early Miocene, beginning about 17.5 Ma, the largest magnitude global warming event of the past 26 Ma or more began. The causes of this warming event are uncertain, but one possible cause is greenhouse warming of the Earth from massive eruptions of basaltic lava that formed the Columbia plateau during the late early to middle Miocene. The influence of this warming event was felt worldwide in both oceanic and terrestrial realms. Taxonomically diverse broadleaf deciduous forest communities spread over the lowland land-scapes of Beringia, spreading northward well north of the Arctic Circle. Conifers appear to have dominated upland, coastal and northernmost habitats. The taxa present across Beringia were part of a temperate flora that occupied much of the land areas adjacent to the North Pacific from northern China and Japan to the Pacif-ic Northwest of the U.S. Many of the tree and shrub genera that occurred in Beringia during this warming event now grow in China, Korea, northern Japan, the Pacific Northwest and eastern North America. Tree and shrub taxa common in Beringia during this warm interval include Carya, Quercus, Acer, Juglans, Pterocarya, Ulmus, Zelkova, Fagus, Liquidambar, Cercidophyllum, Alangium, Tilia, Castanea, Castanopsis, Betula, Myrica, Alnus, Salix, Ericaeae, Ilex, Sequoia, Metasequoia, Taxodium, Picea, Pinus, Larix, Thuja, and Ginkgo. Climatic estimates, based upon temperature requirements of modern representatives of the Beringian flora from this time interval, suggest MAT’s in the range of 12 o to 7 o C for southern to northern Beringia, respectively. Fossil floristic data suggest that precipitation during the growing season was substantially higher (ca. 3x) than modern precipitation in interior Beringia. Global cooling began during the middle Miocene, soon after 15 Ma, marking the beginning of a long interval of middle Miocene to late Miocene temperature decline, interrupted by several warm temperature oscilla-tions. The cooling trend was manifested by both declining winter and summer temperatures, but the decline in summer temperatures was more dramatic. This resulted in a rapid elimination of many broadleaf taxa from Beringian forests. Fossil evidence shows the persistence of some “temperate” broadleaf taxa such as Pterocarya, Ulmus, Carpinus and Ilex. Some of the most cold-adapted broadleaf tree and shrub genera were evolving rapidly at this time, however, and were able to adapt to the progressively cooling regional climates (e.g., Salix, Populus, Betula, Alnus, Ericaceae). The decline in temperatures favored expansion of conifer forests at the expense of the remnants of northern hardwood forests during late middle Miocene to early late Miocene. Late Miocene forests of Beringia included Pinus, Picea, Larix, Abies, Pseudotsuga, Tsuga, Glyptostrobus, Metasequoia, Betula, Corylus, Alnus, Myrica, Populus, Salix, and Ericales. Continued cooling during the late Miocene eliminated Metasequoia from the forest flora by ca. 10 Ma, but Glyptostrobus persisted until well into the Pliocene in some areas. Small amounts of Pterocarya pollen occur in many late Miocene samples in Beringia and suggest a hardy form of that broadleaf tree persisted in Beringia after most “temperate” trees had disap-peared from the region. During the latest Miocene, ca. 6.5-5.2 Ma, Picea increased in importance, at least in the forests of eastern Beringia, and this may have been the vegetation response to global cooling, manifested elsewhere by glaciation (e.g., southern coastal Alaska), and significant sea level drop. Early Pliocene forests of Beringia appear to have been compositionally little different from late late Miocene forests, including most of the same tree and shrub genera (Pinus, Picea, Tsuga, Larix, Abies, Betula, Alnus, Salix, Ericaceae). Pollen data suggest that Pinus became more abundant than Picea in early Pliocene time, and Tsuga and Abies were probably not abundant. The apparent expansion of Pinus populations at that time may have been a response to somewhat warmer climates. The rise of mountain ranges of southeastern Beringia (e.g., the Alaska Range) began in latest Miocene and early Pliocene time, and this created new, higher-elevation habitats. Rising mountains began to block the penetration of warm, moist air from the North Pacific into the interior. During the early Pliocene, herbaceous plants became more common in the landscape of Beringia, perhaps in response to cooling, drying, and the development of open forests and expanses of unforested uplands above altitudinal tree limit. Late Pliocene vegetation of eastern Beringia was composed primarily of conifer-dominated forests of Pinus, Picea, Larix, Abies (uncommon), Betula, Populus, Alnus, Myrica, Salix. The disappearance of Tsuga spp. from interior Beringia was probably caused by greater aridity and colder winters as mountain ranges in southeastern Beringia continued to rise. Cold climates between ca. 2.5-2.35 Ma resulted in the start of a major glaciation in Beringia (and in other high latitude regions). The cold climates of this interval led to the development of lowland tundra, forest tundra, and lowland permafrost (with ice wedges) in northern Beringia. This represents the earliest well-documented evidence for lowland tundra vegetation in Beringia, although it is likely that tundra-like vegetation appeared in alpine habitats, and possibly some high latitude lowlands earlier, perhaps during the latest Miocene. Glacial climates of the Pleistocene altered the vegetation of Beringia more profoundly than any climatic events of the Tertiary. After millions of years of adaptation to cooling climates, Beringian forests were essentially replaced by herb tundra, shrub tundra, and steppe-tundra during long glacial intervals. During inter-glacials, boreal forests recolonized from the south.
Bibliography of Canadian Geomorphology