Climatic Change And Forest Migration

5.3.1 Boreal migrational history

In the early Holocene, around 9000 BP, tree migration rates of 1 to 2 km yr~\ have been reported both in Europe and Canada, causing a substantial and rapid reduction in the area of the tundra biome (Huntley & Birks, 1983; Ritchie, 1987; Huntley & Webb, 1988). However, since the passing of the Hypsithermal period, cooler weather has prevailed over the past 6000 years and this has been reversed only recently due to the current global climatic warming trend. Since the Hypsithermal temperature optimum, the extensions and contractions of the boreal forest in response to climatic oscillations have been geographically diverse. In some areas, small reductions in temperature have been followed by marked vegetation changes due to a parallel onset of more oceanic conditions. Across northern Russia (including Siberia), boreal forest advanced to or near the current arctic coastline between 9000 and 7000 BP and retreated to its present position between 4000 and 3000 BP (MacDonald et al., 2000). The length of the cool period and the consequent substantial forest retreat that has taken place in some areas has allowed the long-term development at high latitudes of a variety of treeless plant communities including vast expanses of bog (Figs. 5.13-5.15). Consequently, there is now a high-latitude landscape that differs from that which existed in the early Holocene and which may prove resistant in certain areas to forest recolonization.

Fig. 5.13 Map of northern Russia showing the West Siberian Lowlands (between approximately 60° and 90° E) with the location of the area shown in Figs. 5.14-5.15.
Fig. 5.14 Aerial photograph taken flying north over the West Siberian Lowlands at 75° E between 64° and 66° N showing an advanced state of paludification with lichen-rich open forest surrounded by bogs. (Photo S. Kirpotin.)
Fig. 5.15 Aerial photograph taken flying north over the West Siberian Lowlands at 75° E between 64° and 66° N showing the ultimate state of paludification. (Photo S. Kirpotin). (Figs. 5.13-5.15 reproduced with permission from Skre et al., 2002.)

This is a fundamental habitat change between what existed in the early Holocene and the communities that are now in place. The modern wetland vegetation in particular is well established and through its homeostatic properties will present a considerable barrier to forest migration. The northward movement of the rainbelts as in Russia, the rise in winter temperatures and the greater input of nitrogen to high latitude regions will all serve to encourage bog growth and thus hinder the establishment of forest.

It is therefore not possible to base predictions of the potential responses to current climatic warming based on the early Holocene rapid advance of trees that took place when the ice sheets melted after the last glaciation. In particular, the large expanses of arctic bogs that have developed in Siberia may prove to be an unsurmountable barrier to a poleward readvance of the boreal forest. Even in northern temperate zones this same trend to increased paludification can be found. In Shetland (Scotland's Northern Isles) a similar contemporaneous change took place and has been considered as accelerating a natural tendency for an autogenic succession from forest to bog under oceanic conditions rather than as a direct response to a decrease in temperature (Bennett et al., 1992).

Plant migration in a modern world is therefore likely to differ from the recolonization that took place after the Pleistocene tabula rasa. Similarly, in northern Sweden and in the Kola Peninsula there was a regression from a boreal birch and pine forest to subarctic birch woodland tundra c. 3000 BP (Gervais et al., 2002) which has existed to the present day. The change appears to be due not so much to a change in temperature, but to the onset of a more oceanic climate with greater soil leaching and erosion and the establishment of heath vegetation in place of forest (Holmgren & Tjus, 1996). Such phenomena indicate that sustaining forest cover depends not just on adequate thermal conditions, but also on water relations, with excessive humidity being just as detrimental to tree survival as drought.

These various forest retreats, due either to increasing oceanicity, or decreasing temperatures or both, were all gradual processes. During cooling periods treeline-altitude decrease has generally lagged behind changes in solar activity levels by 400 to 500 years as in the Sierra Nevada (Scuderi, 1994, and for up to 650 years in the mixed forests of southern Ontario (Campbell & McAndrews, 1993). Similarly, there can be expected a lag in the advance of forests with climatic warming. In Scandinavia, responses to temperature increase in birch forest are less than would be predicted from the meteorological record (Holmgren & Tjus, 1996; Dalen & Hofgaard, 2005). Visual reports of the establishment of tree seedlings above existing treelines in Scandinavia (Kullman, 2002b), might suggest that current climatic warming is already favouring an upward and polewards migration of the treeline. Nevertheless, close examination usually shows that the extent of the change falls short of what might be expected given the degree of climatic warming that has taken place over the past 100 years. Often the density of the saplings above the treeline is low and the fact that they have been recorded is possibly due to more thorough searching than before climatic warming and plant migration became active research issues. Whether the seedlings will survive long enough to form a close forest canopy still remains an open question. Many cases can be seen where in the past seedlings have advanced up mountains (Fig. 5.16) in periods of favourable weather only to be killed at a later date by the return of adverse weather or the increased exposure they encounter as they rose above the shelter of the surrounding vegetation (Holtmeier, 1995).

In north-western Canada and Alaska the arctic treeline limits are now at higher latitudes than at any previous time during the Holocene. These favourable conditions for tree survival at high latitudes are evident in a general stability at the treeline coupled in many cases with a tendency to migrate northwards or upwards. In north-western Canada there has been increased establishment of white spruce (Picea glauca) resulting in increasing density, but no substantial increase in the altitude of the treeline during the past 150 years (Szeicz & MacDonald, 1995). These findings match a similar study along the coast of the Hudson Bay (Payette & Filion, 1985). Observations in the Seward Peninsula in western Alaska indicate a treeline advance with an increase of approximately 2.3% in the area of tundra that has become covered with trees over the past 50 years (Chapin et al., 2005). A continuation of these trends in shrub and tree expansion, it is suggested, could further amplify the effects of atmospheric heating.

Moving from general to particular trends, Alaska provides examples of how factors other than temperature are likely to affect the eventual outcome of climatic warming on forest expansion at high altitudes. Sampling of 1558 white spruce at 13 treeline sites in the Brooks and Alaska Ranges has found both positive and negative growth responses in the same broad geographical area to climate warming, an observation which challenges the widespread assumption that arctic treeline trees grow better with warming climates. High mean temperatures in July decreased the growth of 40% of white spruce at treeline areas in Alaska, whereas warm springs enhanced the growth of an additional 36% of the trees while 24% showed no significant correlation with climate (Wilmking et al., 2004). Further investigations on the effect of drought stress on radial growth of white spruce in the western part of the study area showed that a high number of trees

Fig. 5.16 Rowan (Sorbus aucuparia) maintaining the tree form above a krummholz scrub (Pinus mugo) at an alpine treeline in Bavaria photographed in 1976. Note the Norway spruce (Picea abies) in the distance which have colonized the treeline scrub zone during an earlier period and are now showing a high rate of mortality.

Fig. 5.16 Rowan (Sorbus aucuparia) maintaining the tree form above a krummholz scrub (Pinus mugo) at an alpine treeline in Bavaria photographed in 1976. Note the Norway spruce (Picea abies) in the distance which have colonized the treeline scrub zone during an earlier period and are now showing a high rate of mortality.

responded to recent warming with an increase in growth; while in the eastern part, trees responded predominantly with decreases in growth. These patterns coincided with precipitation decreases from west to east and local water availability gradients, therefore pointing to drought stress as the controlling factor for the distribution of trees (Wilmking & Juday, 2005).

Trembling aspen (Populus tremuloides) is the most important deciduous tree in the North American boreal forest and this species is also showing signs of suffering from increasing drought. A study in western Canadian aspen forests showed that during 1951-2000 the aspen forests underwent several cycles of reduced growth, notably between 1976 and 1981, when mean stand basal area increment decreased by about 50%. Most of the growth variation was explained by interannual variation in the climate moisture index in combination with insect defoliation. The results of the analysis indicate that a major collapse during the years 2001-3 in aspen productivity is likely to have occurred during the severe drought that affected much of the region at that time (Hogg et al, 2005).

Similar situations have been reported using sequential aerial photography in examining the distribution of the subalpine fir (Abies lasiocarpa) at the Glacier National Park (Montana, USA) where over a 46-year period no altitudinal changes in the location of alpine treeline ecotone were observed (Klasner & Fagre, 2002). There was nevertheless, over this same period an increase in the area covered by stunted subalpine fir (krummholz, sensu lato) and an increase in tree density. In a similar study, at and near the subarctic and arctic treeline in three regions in Alaska it was found, contrary to expectations, that after 1950 warmer temperatures were associated with decreased tree growth in all but the wettest region of the Alaska Range (for location see Fig. 5.17). Growth declines were most common in the warmer and drier sites, and thus give further support to the hypothesis that drought stress may accompany increased warming in the boreal forest (Lloyd & Fastie, 2002; Holtmeier & Broll, 2005).

In Alaska, a comparison of treeline movements based on palaeoecological studies has provided a regional analysis of patterns of recent treeline advance, and given an estimate of how much lag exists between recruitment onset and forest development beyond the Alaskan treeline (Lloyd, 2005). Treeline advance has been ubiquitous throughout the region, but asynchron-

Fig. 5.17 Map of Alaska showing the relative positions of geographical features referred to in the text.

ous in time, occurring significantly earlier in the White Mountains in interior Alaska than in western Alaska or the Alaska Range. The mean lag between initiation of recruitment and forest development was estimated at approximately 200 years, similar to that predicted from contemporary modelling studies. It appears that although a continued advance of white spruce forests is the most likely outcome of future change, variability in the rate of forest response to warming will be modified by problems with seed dispersal and tree establishment which may be further hindered by the slow warming in highly permafrost-affected sites. There appears therefore to be considerable resistance to both expansion and retraction of boreal forest in response to climatic warming, both at present and in the past.

Treelines, it would appear both from this and other studies, represent ecological boundaries of great complexity that develop over long periods of plant-environment interaction. Consequently, it seems doubtful that forest migration will respond either directly or in direct proportion to any degree of short-term climatic change.

It has also been observed that there is substantial variability in response to climate variation according to the distance from the treeline. Inverse growth responses to temperature were more common at sites below the forest margin than at sites at the forest margin. Together, these results were taken to suggest that inverse responses to temperature are widespread, affecting even the coldest parts of the boreal forest. As growth declines, as is common in the warmer and drier sites, it would appear that increased warming if accompanied by increased drought stress will nullify any positive responses to temperature amelioration from climatic warming (Lloyd, 2005).

In northern Quebec, a study of logs buried in peat underlying stunted clonal spruce enabled an assessment to be made of the stability of the forest limit during both warm and cold periods of the late Holocene. This study strongly suggests that the forest limit has remained stable during the last 2000-3000 years BP (Lavoie & Payette, 1996). The stability of the forest limit during warm periods (c. 2000 BP, early medieval times, and this century) and cold periods (c. 3000 and 1300 BP, and the Little Ice Age) of the late Holocene demonstrate that mechanisms allowing forest limit advance or retreat are not easily triggered by climatic change. A summary of expected changes to the boreal treeline in response to present warming trends has been given in the Arctic Climatic Assessment Report (ACIA, 2005). In North America where black and white spruce are the principal tree species at the tundra-taiga interface it was concluded that it is possible that the northern white spruce tree limit in Alaska and adjacent Canada would not advance readily (Juday, 2005). For black spruce, in Alaska at least, it would appear that warm years would result in strongly reduced growth and that it is unlikely that black spruce would survive in some types of sites.

5.4 FIRE, AND PALUDIFICATION AT THE TUNDRA-TAIGA INTERFACE

Many studies have questioned whether or not climate alone is the main controlling factor for the northern limit of forests and have suggested alternative scenarios. One notable argument is that the effect of natural fires may be particularly severe at the tundra-taiga interface (Fig. 5.18). At high latitudes, the interactions between vegetation and disturbances such as forest fire are particularly important, as the changes induced in the vegetation may produce significant alterations in local climatic conditions. Natural forest fires may be so severe as to prevent the vegetation from returning to its original forested state. Furthermore, the frequency of natural forest fires makes it essential that trees retain a capacity for subsequent regeneration (Laberge et al., 2000). In this connection it is relevant to note that in

Siberia along the tundra-taiga boundary there is an especially high frequency of thunderstorms, suggesting an increased fire probability due to the warmer conditions of the forest increasing evapotranspiration from the adjacent bog (Valentini et al., 2000). The removal of trees, whether by fire or any other agency (e.g. insect attack) will lead to rising water tables and hence will promote paludification and thus hinder regeneration. Fire can also aid bog formation as particles of ash and carbon deposited into the soil profile can reduce drainage and therefore help to initiate peat growth (Mallik et al., 1984).

5.4.1 Post-fire habitat degradation

Tree growth on peatland is more successful in regions with cold climates than in oceanic areas with warm winters. When the winter is cold the roots become fully dormant and do not suffer the adverse effects of prolonged exposure to anaerobic conditions as their metabolic processes are much reduced. Tree colonization of peatlands in cold climates is therefore widespread but frequently regeneration presents problems and deteriorating environmental conditions can cause a disappearance of trees from formerly forested bogs. In a study covering the Little Ice Age period in northern Quebec (Arseneault & Payette, 1997) it was found that black spruce continued to colonize the peatlands as long as the adjacent well-drained sites were occupied by seed-producing forest.

The need for neighbouring forest is two fold. In addition to the need for a seed source, there is also a requirement for shelter to reduce the damaging effects of winter snow-drifting conditions. When neighbouring areas succumb to periodic fires then both the seed source and shelter are removed. Under these conditions the surviving trees suffer dieback of supranival stems. The trees then finally succumb to drowning in permafrost-induced ponds. The post-fire degradation and disappearance of the conifer stands from the peatlands represents the ultimate stage of a positive feedback process triggered by a modification of the snow regime at the landscape scale. The region is dominated by cold ocean currents, with the result that in summer frosts are frequent and cause a stunting of the growth of trees regenerating in open spaces, hence the frequent occurrence of krummholz (S. Payette, pers. com.). The same sequence of events will probably not

Fig. 5.18 Remains after forest fire in black spruce (Picea mariana) forest, Quebec.

occur in non-oceanic areas with lower levels of precipitation. This scenario appears to explain the southern position of the treeline in Quebec as well as the anomalous occurrence there of lichen-spruce forest at a latitude of only 47° N (Fig. 5.19).

In more continental subarctic Finland the aftereffects of fire on local climate and its implications for forest regeneration were studied after a widespread forest fire in the Tuntsa area of Finnish Lapland in 1960 (Vajda & Venalainen, 2005). Here it was found that fire-induced deforestation increased the wind velocity by 60%, which changed the soil thermal regime through a 20-30 cm reduction in snow cover, lowered the evapotranspiration and diminished the Bowen ratio to 0.4. In this case the absence of forest recovery after fire appears to have been due to the imposition of an increase in climatic adversity for the trees in an area which was already at the boundary for tree survival.

5.4.2 Treelines and paludification

Studies of treeline position and movement in the northern hemisphere are most commonly carried out

Fig. 5.19 View of an open frost-prone site at the southernmost occurrence of lichen spruce forest in Quebec at 47° N. The arrow indicates a krummholz tree several decades old that has been regularly stunted in growth by frost damage, as shown in the progressively enlarged insets. Note also the high degree of lichen diversity.

Fig. 5.19 View of an open frost-prone site at the southernmost occurrence of lichen spruce forest in Quebec at 47° N. The arrow indicates a krummholz tree several decades old that has been regularly stunted in growth by frost damage, as shown in the progressively enlarged insets. Note also the high degree of lichen diversity.

in regions where an uninterrupted boundary between tundra and taiga can be found extending over many hundreds of kilometres. This natural bias towards areas with extensive treelines tends to emphasize continental areas and neglects the influence of oceanic conditions on the position of the treeline. Proximity to the ocean can have both positive and negative effects on the growth of trees. Freedom from drought and reduction in exposure to frost enables active forestry and agriculture to be carried out at many locations north of the Arctic Circle. However, there are also many aspects of the oceanic environment which have a negative influence on long-term tree survival and can depress the position of the treeline and impoverish soil conditions. Oceanic cooling of the growing season and its negative effect on long-term tree survival has been noted in a number of sites including the western shores of the Hudson Bay, Fennoscandinavia, and the Siberian lowlands (Crawford, 2000; Crawford et al., 2003).

5.4.3 History of paludification

The end of the twentieth century witnessed a pronounced increase in the intensity of maritime conditions which varies with the periodic behaviour of the North Atlantic Oscillation (Fig. 5.20). This change in climate has also had a noticeable ecological impact at high latitudes, altering the seasonal spatial dynamics of plant growth and the seasonal spatial dynamics and social organization of musk oxen (Forchhammer et al., 2005). Over the last three decades, the phase of the NAO (Fig. 5.21) has been shifting from mostly negative to mostly positive index values, and an increase in oceanicity is also evident (Fig. 5.20), as measured by a reduction in Conrad's Index of Continentality (the converse of oceanicity).

The recent trend to increased oceanicity in the Siberian Arctic can be viewed as a continuation of a long-term trend from continentality to oceanicity. Examination of peat stratigraphy and carbon dating of buried tree stumps (as mentioned above) has shown that there has been a marked southward movement of the Siberian treeline over the past 6000 years, probably as a consequence of the mid-Holocene sea level rise and the flooding of the Arctic Ocean onto the Siberian lowlands and the consequent imposition of cool, moist summers (Kremenetski et al., 1998).

The early to mid Holocene expansion of arctic waters altered the delicate balance between temperature and air humidity so that in both arctic and subarctic regions bogs began to replace forest over a wide area in northern Quebec (Payette, 1984), Scandinavia, Scotland (Tipping, 1994), and in the Siberian lowlands (Kremenetski et al., 1998). A marked climatic deterioration, commonly termed the 8200 BP event (probably due to freshwater fluxes in the final deglaciation of the Laurentide ice sheet), appears to have been accompanied by a reduction in tree cover throughout Europe (Klitgaard-Kristensen et al., 1998).

A trend to more oceanic conditions with cool moist summers appears to have continued in many areas probably as a result of these early Holocene rises in sea level. The Hudson Bay and Arctic Ocean would therefore have begun to exert a greater maritime influence on climate in Quebec and northern Siberia respectively. Examination of the changing abundance of wetland forests located at the arctic treeline (northern Quebec, Canada) during the last 1500 years (Payette & Delwaide, 2004), has shown a remarkable coincidence of events such as the mass mortality of wetland spruce and the post-fire failure of forest regeneration during the late 1500s, suggesting that this was connected with local flooding, probably attributable to greater snow transportation and accumulation after fire disturbance. This same study demonstrated a marked increase in paludification with the highest water levels recorded so far (nineteenth and twentieth centuries), causing lake and peatland expansion. The conclusion that any future moisture increases in these subarctic latitudes in oceanic eastern Canada will result in important spatial rearrangements of wetland ecosystems.

The late Holocene retreat of the Eurasian treeline coincides also with declining summer insolation (McFadden et al., 2005). Although this decline in insolation would have been global, the effect appears to have been greater in arctic Siberia due to the proximity of the Arctic Ocean. The increase in the extent of arctic water as a result of sea level rise would have created at northern latitudes a more oceanic climate with cool moist summers, which would allow permafrost levels to rise and facilitate the development of bogs.

By c. 5000/4500 BP the northern limit of tree birch in Russia had retreated from its Hypsithermal maximum to more or less its present limit and this was mirrored in the southward retreat of other tree species

Fig. 5.20 North Atlantic Oscillation (NAO) anomalies for 1900 to 2000 (grey columns with superimposed lowest line in black: data from Hurrell & VanLoon, 1997) compared with anomalies of annual mean temperature °C (green) and annual temperature range °C (red) for (a) a restricted part of the West Siberian Lowlands, lat. 62.5° to 67.5° N, long. 62.5° to 67.5° E, showing a significant relationship between NAO and the annual mean temperature. The temperature range anomaly is typically of opposite sign to the mean anomaly (b) Spain, lat. 36° to 44° N showing weak relationships between NAO index and mean temperature but a stronger relationship between NAO and annual temperature range. (Reproduced with permission from Crawford et al., 2003.)

Fig. 5.20 North Atlantic Oscillation (NAO) anomalies for 1900 to 2000 (grey columns with superimposed lowest line in black: data from Hurrell & VanLoon, 1997) compared with anomalies of annual mean temperature °C (green) and annual temperature range °C (red) for (a) a restricted part of the West Siberian Lowlands, lat. 62.5° to 67.5° N, long. 62.5° to 67.5° E, showing a significant relationship between NAO and the annual mean temperature. The temperature range anomaly is typically of opposite sign to the mean anomaly (b) Spain, lat. 36° to 44° N showing weak relationships between NAO index and mean temperature but a stronger relationship between NAO and annual temperature range. (Reproduced with permission from Crawford et al., 2003.)

(Fig. 5.22). This treeline retreat was accompanied by development of dwarf shrub tundra and extensive bog growth. The considerable antiquity of the peat in the upper layers of bogs in the region of the Pur-Taz rivers of western Siberia (for location see Fig. 5.6) has been taken to suggest that much of the peat growth took place at an early date (Peteet et al., 1998). However, comparison of methane emissions from the Holocene Optimum (5500-6000 BP) with modern levels from forests of northern Eurasia, suggests that the area of

Fig. 5.21 Changes in continentality across arctic Siberia from the 1960s to the 1990s as calculated by Conrad's Index (see below). There is a notable increase in oceanicity in western Siberia which gradually changes to an increase in continentality in the far east. (Map prepared by Dr C. E. Jeffree using temperature data from the Climatic Research Unit 0.5° gridded 1901-1995 Global Climate Dataset). Conrad's Index of Continentality K — (1.7A/sin (0 +10)) —14 where K is the index of continentality; A the average annual temperature range; and 0 is latitude. The scale provides values approximating to nearly zero for Thorshavn (Faroe: 62° 2' N, 6° 4' W) and almost 100 for Verkoyansk (Russia: 67° 33' E, 133° 24' E).

Fig. 5.21 Changes in continentality across arctic Siberia from the 1960s to the 1990s as calculated by Conrad's Index (see below). There is a notable increase in oceanicity in western Siberia which gradually changes to an increase in continentality in the far east. (Map prepared by Dr C. E. Jeffree using temperature data from the Climatic Research Unit 0.5° gridded 1901-1995 Global Climate Dataset). Conrad's Index of Continentality K — (1.7A/sin (0 +10)) —14 where K is the index of continentality; A the average annual temperature range; and 0 is latitude. The scale provides values approximating to nearly zero for Thorshavn (Faroe: 62° 2' N, 6° 4' W) and almost 100 for Verkoyansk (Russia: 67° 33' E, 133° 24' E).

tundra and the proportion of wetlands within the boreal forest zone is probably greater now than at any time in the past (Velichko et al., 1998). Examination of the recent growth of bogs suggests that bogs appear to engulf the forest both in the maritime regions of Canada and northern Siberia as well as in other locations, particularly near the present tundra-taiga interface. The situation is complex, as at this interface between forest and bog there is (as mentioned above) a cyclical alternation between tree cover and wetland vegetation due to the rise and fall of permafrost levels (Fig. 5.9). The predictions therefore that are made in relation to the movement of northern vegetation types in relation to climatic warming probably do not take fully into account the homeostatic properties of bogs in retarding the northward migration of the boreal forest.

5.4.4 Bog versus forest at the tundra-taiga interface

A lengthy debate has taken place, particularly among Russian ecologists, as to whether in certain locations climatic warming, instead of causing an advance of the treeline, will result in a continuation of the retreat that began after the Hypsithermal (Callaghan et al., 2002). Those who believe that climatic warming causes an advance of the treeline northwards subscribe to the eventual overriding influence of climatic factors. There is, however, in the north Siberian lowlands a powerful argument that climatic warming can cause a retreat of the treeline, based on the belief that the presence or absence of trees is due to edaphic factors influenced by the proximity of the Arctic Ocean and the long-term

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