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Fig. 5.6 (Above) Location of the northern limits to the boreal tundra forest and West Siberian Lowlands, as recorded by a Normalized Difference Vegetation Index (NDVI) satellite image recorded in May 1998 while the tundra was covered in snow. The snow-covered region does not register on the NDVI scale as there is no photosynthetically active visible vegetation. The evergreen forest emerging above the snow gives positive readings, shown by various shades ofgreen with the more intense colours indicating more positive values. Note the southern depression of the evergreen treeline in the region of the Siberian Lowlands where forest has been replaced by the development over the past 6000 years by a very extensive bog, creating probably the largest example in the world of paludification of former forest. (Opposite) Detail of the transition zone between forest and tundra as seen in the normalized vegetation index recorded in May 1998 (1 km resolution). Colourscale: blue = 0; blue-green = 0.11—0.25; dark-green 0.26-0.40; bright green = 0.41—0.64. It is considered (see text) that this mosaic represents a self-renewing cyclic process taking place over hundreds of years as patches offorest develop on land that dries out after being raised by frost-heave and then reverts again to bog as tree cover cools the underlying ground. (Images prepared from 8 km resolution Pathfinder data set — US Geological Survey, EROS Data Center, Sioux Falls, South Dakota.)

Fig. 5.6 (cont.)

shown in Fig. 5.6 (detail) reveals a mosaic of tree and bog cover that extends north-south at this location for approximately 600 km from the south-eastern shore of Obskaya Guba (Russian guba, bay or inlet) to the east-west flowing section of the River Ob'. This mosaic is considered by Russian ecologists to be the result of a self-renewing cyclic process taking place over hundreds of years. Cryoturbation causes the soil surface in localized areas to rise above the general level of the bog (Chernov & Matveyeva, 1997). In some regions this permits the active layer (the layer of peat in a bog that dries out during the growing season) to dry out suffi ciently to allow the establishment of trees. The trees persist only for a period until they shade the ground and cause the permafrost to rise and favour once again the growth of mosses as opposed to trees. Under these conditions determining the possible movements of tree limits in relation to climatic oscillations is almost a cartographic impossibility in a transition zone that can be as much as 600 km wide (Fig. 5.6).

The many and varied changes that take place in vegetation at the tundra-taiga interface therefore make it difficult to formulate pan-arctic generalizations as to the probable extent of treeline movement at present or in the foreseeable future, as the climatic history of the different regions in relation to trees is highly varied (Skre et al., 2002). A similar phenomenon is seen on a smaller scale with the east Siberian dwarf pine (Pinus pumila) which frequently shows a very patched distribution (Figs. 5.7-5.9).

The extent of the permafrost zone is much more extensive in Eurasia than in North America (Fig. 5.10). The large expanse of Siberia that is underlain by a permanently frozen subsoil can be related to a number of ecological features, including the widespread growth of bogs during the Holocene and the presence of patchy woody scrub vegetation over large areas of drier ground. When making comparisons between North America and Eurasia in relation to treeline movements it is important to note that it is customary in North America to distinguish between the arctic and the subarctic treeline (Fig. 5.11). This is due in part to the extent of the Precambrian Shelf which covers a large part of northern Canada with a hilly terrain of sufficient elevation to produce subarctic treelines. It is these subarctic treelines that show the greatest tendency to respond to climatic change, while the more northerly arctic treeline has remained relatively stable over the past 3000 years (Payette et al, 2001).

5.2.2 Krummholz and treeline advance

The term krummholz (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. The dwarfed or prostrate tree forms often have the appearance of being severely pruned by wind and

Fig. 5.7 Aerial view of patches of dwarf Siberian pine (Pinus pumila) growing in northern Kamchatka. The lighter green trees are Betula ermanii growing on the south-facing slopes while tall forbs meadows cover the narrow valleys, where snow cover still exists at the beginning of July and returns again in October. (Photo Dr P. Krestov.)

Fig. 5.7 Aerial view of patches of dwarf Siberian pine (Pinus pumila) growing in northern Kamchatka. The lighter green trees are Betula ermanii growing on the south-facing slopes while tall forbs meadows cover the narrow valleys, where snow cover still exists at the beginning of July and returns again in October. (Photo Dr P. Krestov.)

Fig. 5.8 Close-up detail of bush of Pinus pumila. (Photo Dr P. Krestov.)

ice blast. Sometimes this growth form is genetically determined while in other cases it is due entirely to phenotypic plasticity.

The dwarfing of woody species in marginal habitats is a worldwide phenomenon and can be observed from the fringes of the boreal forest to the montane forests of the tropics. Typical American species that can be found as dwarf forms at the treeline are subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii), limber pine (Pinus flexilis), lodgepole pine (P. contorta) as well as the famous long-lived bristlecone pines of the Rocky Mountains (P. aristata, P. longaeva and P. balfouriana).

In North America black spruce (Picea mariana) is the main krummholz-forming species and shows a greater plasticity in form than white spruce (P. glauca). Black spruce is also better adapted to poorly drained acidic soils and high permafrost tables. Due to its particular krummholz habit of prostrate growth it is able to survive the dangers of wind throw in shallow soils and is more able to propagate vegetatively by layering than white spruce (Gamache & Payette, 2005). In Europe the mountain pine (Pinus mugo) is common throughout the Alps and the Apennines while the dwarf Siberian pine (P. pumila) is extensive in the more eastern regions of Eurasia. The mountain birch (B. pubescens ssp. czerepanovii) is the dominant treeline species for much of northern Europe from Scandinavia to Iceland. It is not strictly a krummholz species as its many limbs normally

Pinus pumilu - young growth from seed caches

Thaw depth

Permafrost__

Spreading by layering

Permafrost__

Spreading by layering

Death of Pinus pumita from lo» temperatures and waterlogging

Death of Pinus pumita from lo» temperatures and waterlogging

Fig. 5.9 Interaction between dwarf Siberian pine (Pinus pumila) and site conditions in the lesotundra (tundra-taiga interface) in northern Siberia. (Reproduced with permission from Holtmeier, 2003.)

maintain the persistence of the leading shoots. In exposed conditions these leading shoots can adopt a prostrate growth form (see also Figs. 9.13-9.14). Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) can also exist in the krummholz form which they commonly adopt as a phenotypic response to exposure. In terms of latitudinal extent across the Eurasian Arctic, the dwarf Siberian pine is outstanding for the amount of terrain that it occupies and for being the most northerly pine species in the world, reaching 72° N in Yakutia (Fig. 5.12).

A Canadian study of regeneration patterns of black spruce (Picea mariana) along a 300-km latitudinal transect across forest-tundra in northern Quebec compared forest-tundra interfaces at different latitudes; one from the southern forest-tundra and two regions from the northern forest-tundra, including the arctic treeline (Gamache & Payette, 2005). The results showed that during the twentieth century the southern forest-tundra treelines rose slightly through establishment of seed-origin spruce, while some treelines in the northern forest-tundra increased the height of stunted spruce (krummholz) already established on the tundra. Despite these improvements in spruce reproductive success in the twentieth century in the southern forest-tundra, there was little evidence that recruitment of seed-origin spruce was controlled by 5- to 20-year regional climatic fluctuations, apart from the effects of winter precipitation. It was therefore concluded that local topographic factors rather than climate have influenced the recent rise in treelines. In particular, the effect of black spruce's semi-serotinous cones (retaining seeds in protective structures) and the difficulty of establishment on exposed, drought-prone tundra vegetation appeared to explain the scarcity of significant short-term correlations between tree establishment and climatic variables. Furthermore, the age data of the trees indicated that the development of spruce seedlings into forest was being retarded by the harsh wind-exposure conditions.

In Scandinavia, the upper treeline is formed mainly by mountain birch, which is not a well-defined taxon due to the high degree of polymorphism and introgressive hybridization which has created many widely different morphological types of krummholz. The Nordic mountain birches (Fig. 9.13) are therefore composed of many varieties, hybrids and subspecies of Betula pubescens and sometimes also B. pendula and the dwarf birch B. nana (Wielgolaski & Sonesson, 2001). All these combinations are usually referred to as the subspecies B. pubescens spp. czerepanovii (formerly spp. tortuosa). Within this taxon a variety of growth forms can be found (Vare, 2001; Holtmeier, 2003). These can be either one-stemmed (monocormic) or many stemmed (polycormic). In the former case the trunk sometimes extends itself close to the ground in a twisted form, often developing semi-upright knees, and is sometimes considered as the variety appressa (see Section 9.4). This type is probably genetically fixed. Polycormic forms of mountain birch are found typically in areas with nutrient deficiencies, or where there has been disturbance or attack from the autumn moths Epirrata autumnalis and Operophtera brumata, which

Fig. 5.10 Contemporary distribution of permafrost in the northern hemisphere classified into three categories: (1) sporadic is green; widespread discontinuous is blue; continuous is purple. (Reproduced with permission from Brown, 1997.) Light grey is no permafrost. (Reproduced with permission from Brown, 1997.)

Fig. 5.10 Contemporary distribution of permafrost in the northern hemisphere classified into three categories: (1) sporadic is green; widespread discontinuous is blue; continuous is purple. (Reproduced with permission from Brown, 1997.) Light grey is no permafrost. (Reproduced with permission from Brown, 1997.)

feed on the leaves of the mountain birch and have population peaks approximately every 10 years. In the polycormic forms improved growth can be achieved with nutrient application and it has therefore to be assumed that here the krummholz form is phenotypic.

Irrespective of how the krummholz arises it has two principal advantages for tree survival at the tim-berline. First, there is the advantage in having foliage near the ground as this decouples low stature plants from air temperatures which are cooler than temperatures near the ground (Grace et al., 2002). Secondly, the ratio of photosynthetically productive tissues to non-productive tissues is increased, especially in the polycormic (many stemmed) form of krummholz, as is also the dependence for survival on one upright tree trunk.

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