The Tundrataiga Interface

Will climatic warming allow the boreal forest to advance onto the treeless tundra? This is one of the most tantalizing questions that can be asked in any discussion in relation to vegetation margins. The zone between the northern limit of the boreal forest (taiga) and the southern extent of the arctic tundra is the world's only circumpolar vegetation boundary and stretches for 13 400 km around the northern hemisphere and across three continents (Figs. 5.1-5.3). It is probably more exact to refer to this boundary as a zone, for in many places, and particularly in Russia, there is a well-developed interface region. This variable interface ecotone is best developed in Siberia where it is referred to by Russian ecologists as lesotundra (Russian les, forest). The term forest-tundra has also been adopted in describing similar ecotones or zones in eastern Canada created over the last 3000 years by deforestation as a result of the combined action of forest fires and climatic cooling (Asselin & Payette, 2005).

Such is the length of this boundary that it cannot be expected to have the same appearance or behave uniformly throughout its entire length. In some places it can be seen as a relatively abrupt change from forest to open tundra while in others, and particularly in Siberia, there is a lesotundra transition zone that can be several hundred kilometres deep. The forest element can be either evergreen or deciduous and the tundra can vary from ancient tundra-steppe communities to fell-field and bogs (see Table 5.1 for definitions).

The taiga-tundra interface has two unifying properties. The first is its recent origin. Throughout the Tertiary Period the lands bordering the Arctic Ocean were largely forested and little or no tundra existed. The second property is that the Holocene development ofthe circumpolar boreal forest has created a new major vegetation conflict zone. When temperatures rise it is possible for trees to advance over the tundra, changing not only the vegetation but also the microclimate and soil conditions. As conditions cool, trees can be suppressed by rising levels of permafrost, or possibly by being engulfed by bogs (paludification). This relatively new zone of vegetation conflict has never been static throughout its brief 10 000-year Holocene history, migrating often slowly as climates have changed. The question for this chapter is how it may move in response to ongoing and future climatic change. Will trees be able to advance everywhere in the Arctic or are there regions, or 'no go areas', such as bogs and deep permafrost zones where trees will not readily advance?

5.1.1 Migrational history of the tundra-taiga interface

To understand the nature of the tundra-taiga interface we have to examine its past history and its present ecological condition. By the mid-Pliocene, the Cenozoic temperature decline had begun and the warmth-demanding trees of the earlier Tertiary period had been replaced in the Arctic with genera that are typical of the present boreal forest. Nevertheless, at 83° N in Peary Land (north-east Greenland), despite this dramatic cooling, a heterogeneous forest-tundra with trees, heathlands, and well-vegetated lakes, along with some present-day arctic species, is believed to have been present up to 2.0-2.5 million years ago at the Pliocene-Pleistocene transition (Bennike & Bocher, 1990). Forests then largely disappeared from these high latitudes and throughout the Pleistocene remained far to the south. However, an analysis of temporal-spatial patterns in pollen records suggests that some of the refugia that are known to have existed and supported herbs and shrubs in ice-free regions at high latitudes during the Pleistocene may have also allowed pockets of trees to survive during the Last Glacial Maximum (Brubaker et al., 2005). The subsequent Holocene tree readvance, even during the warmest periods of the Holocene, never restored forest as far north as it formerly existed during the Pliocene, with the result that for the past 10 000 years a tundra-taiga interface or forest-tundra has existed at varying distances from the North Pole.

The Pleistocene ice sheets were never ubiquitous throughout the Arctic (see Chapter 2) and, particularly in regions bordering the North Atlantic and in Beringia, a number of ice-free refugia have been identified geologically (Section 6.4). Extinctions therefore will not have been uniform and reimmigration will have been influenced by the existence of refugia, the alignment of mountain chains and availability of ice-free coastal habitats. A fall in sea level will have exposed a greater area of the Arctic shelf, and provided an extensive polar desert which would have served both as a glacial refu-gium and as a pathway for subsequent plant migration. During the Last Glacial Maximum much of the Siberian

Canada Vegetation Map
Fig. 5.2 Position of the North American forest tundra in relation to the other major subdivisions of the boreal forest. (Reproduced with permission from Payette et al., 2001.)

I_] Arctic desert I I Tundra M Lesotundra I I Taiga i 1 Montane

Fig. 5.3 The position of the lesotundra in Russia in relation to the major divisions of the boreal forest and principal cities of northern Russia. (Reproduced with permission from Vlassova, 2002.)

I_] Arctic desert I I Tundra M Lesotundra I I Taiga i 1 Montane

Fig. 5.3 The position of the lesotundra in Russia in relation to the major divisions of the boreal forest and principal cities of northern Russia. (Reproduced with permission from Vlassova, 2002.)

arctic coast was free of ice due to a lack of precipitation. Thus, the Taymyr Peninsula, like most of Siberia, lay bare of ice in a precipitation shadow at a time when there was still large-scale ice coverage in Scandinavia and north-western Europe.

Just as the severity of the Pleistocene cold period varied, so the Holocene has also altered climatically over the past 10 000 years. Early in the Holocene, a period of rapid warming reached its maximum approximately 7000 years BP. The culmination of this warm period is variously referred to as the Hypsithermal, Xerothermic or Climatic Optimum (the warmest post-glacial period when temperatures rose to as much as 2 °C above present). The date and duration of the Hypsithermal also differed from one region to another. In south-western Saskatchewan there was a warm dry Hypsithermal

Trees at the tundra-taiga interface Table 5.1. Terms, and abbreviations used in relation to arctic and subarctic treelines

Arctic and subarctic treelines

Bowen ratio

Cenozoic temperature decline

Cryoturbation Fell field

Holocene Hypsithermal

Koppen's Rule Krummholz

LAI (Leaf Area Index)

Last Glacial Maximum

Lesotundra or forest tundra or tundra-taiga interface

Lichen woodland

The arctic treeline defines the northerly limit to where trees grow and beyond which there is only tundra (barren or treeless land). The subarctic treeline marks the upper altitudinal limits of boreal forest at high latitudes although isolated and small groups of trees can be found further north especially at lower altitudes.

The ratio of energy fluxes from one medium to another by sensible and latent heating respectively. For plants sensible heat loss and evaporative heat loss are the most important processes in the regulation of leaf temperature. The ratio of the two provides the Bowen ratio as used in plant ecology.

The cooling in global temperatures, particularly at high latitudes, over the past 60 million years which has probably been caused by tectonic uplift and the removal from the atmosphere of carbon dioxide by newly exposed rocks. The physical mixing of soil materials by the alternation of thawing and freezing. An area of tundra with frost-shattered stony debris interspersed with finer rock particles supporting only sparse vegetation with much open ground and subject to freeze-thawing activity in the soil.

The geological epoch within the Quaternary period, from c. 10000 BP to the present.

The post-glacial period when temperatures were warmer than at present. The time interval varies with location (see text) but lies approximately within the range 9000-2500 years BP.

A long-established rule of thumb often used to relate climatic tolerance of forest with temperature in North America and Europe, which states that trees do not survive when the mean temperature of the warmest month fails to rise above 10 °C.

(German krumm, crooked, bent, twisted; Holz, wood) is used to describe trees with distorted and prostrate or stunted forms that are frequently found at or just beyond the treeline either on mountain slopes or at the tundra-taiga interface. Sometimes this growth form is genetically determined while in other cases it is due entirely to phenotypic plasticity or death of terminal buds.

The ratio of total upper leaf surface of a crop, a natural stand of vegetation, or an individual plant, divided by the surface area of the land on which the plant or plants grow.

The time of maximum extent of the ice sheets during the last glaciation. The timing of this event varied with location (see text).

An interface region sometimes several hundreds of kilometres wide where pockets of tundra and taiga intermingle. This variable interface ecotone is best developed in Siberia where it is referred to by Russian ecologists as lesotundra (Russian les, forest). The term forest-tundra is also used in describing similar ecotones or zones in North America.

Open forest with the open ground dominated by lichens. Frequently these are areas with shallow permafrost which limits the root zone available to trees and shrubs.

Little Ice Age A period of cooling lasting approximately from the fourteenth to the mid nineteenth centuries. There is no generally agreed start or end date. Some authors confine the period to 1550-1850. This cooler period occurred after a warmer era known as the Medieval Climatic Optimum. There were three minima, beginning about 1650, 1770, and 1850, each separated by slightly warmer intervals.

Mono- and polycormic Refers to woody plants depending on whether they have a single or many basal stems stems.

Paludification The growth of bogs.

Fig. 5.4 Comparison of present and past northern limits for tree survival in northern Siberia. Present-day distribution of the boreal forest (brown) is based on the vegetation map produced by Grid Arendal and published by the World Wide Fund for Nature. Mid Holocene limits to forest trees are regional generalizations from locations of fossil remains (green is evergreen - pine and/or spruce spp.; red is tree birch; purple is larch) based on Kremenetski et al., 1998. Modern limits for the northern survival of individual tree species (colours as above) are also taken from Kremenetski et al., 1998, as drawn by Callaghan et al., 2002.

Fig. 5.4 Comparison of present and past northern limits for tree survival in northern Siberia. Present-day distribution of the boreal forest (brown) is based on the vegetation map produced by Grid Arendal and published by the World Wide Fund for Nature. Mid Holocene limits to forest trees are regional generalizations from locations of fossil remains (green is evergreen - pine and/or spruce spp.; red is tree birch; purple is larch) based on Kremenetski et al., 1998. Modern limits for the northern survival of individual tree species (colours as above) are also taken from Kremenetski et al., 1998, as drawn by Callaghan et al., 2002.

period between 6400 and 4500 BP (Porter et al., 1999), while in north-west Montana the equivalent warm dry period lasted from 10 850 to 4750 BP (Gerloff et al, 1995). In western Norway the Hypsithermal is dated to between 8000 and 6000 BP (Nesje & Kvamme, 1991) when hazel (Corylus avellana) reached its most northerly expansion at the Norwegian Nordkapp (71° N). In Russia the early Holocene also saw a spread of spruce, larch, pine and tree birch to northern latitudes where they are no longer found (Fig. 5.4). In Siberia between 8000 and 4500/4300 BP, Picea abies ssp. obovata was farther north than at present and various Larix spp. were further north than now between 10 000 and 5000/ 4500 BP. Tree birches (Betula pubescens) reached the present-day shoreline of the Barents Sea in the Bolshezemelskaya Tundra (68° N) and in the Taymyr Peninsula (72° N) between 8000 and 9000 BP, and in the Yamal Peninsula (Fig. 5.4) the tree birch limit was near 70° N by 8000 BP (Kremenetski et al, 1998).

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