Boreal Forest Productivity At High Latitudes

The general question of whether or not trees are responding positively to climatic warming in boreal regions can elicit different answers depending exactly on whether the response is measured as a northward expansion of forest or an improvement in tree growth. In terms of forest expansion there is considerable inertia for movement in much of the tundra-taiga interface in response to climatic warming. The most noticeable advances appear to be a spread north in sheltered river valleys as seen in Alaska where the continued advance of white spruce (Picea glauca) forests is the most likely scenario of future change. There are, however, limiting factors due to restrictions on spruce establishment in highly permafrost-affected sites, together with problems in seed dispersal and early establishment which may cause spruce populations to exhibit non-linear responses to future warming. It follows therefore that an uncritical extrapolation from recent trends may be unwarranted (Lloyd, 2005).

In terms of growth rates there has been a noticeable improvement in boreal tree growth in recent years throughout the northern world, from Mongolia (Jacoby et al., 1996) to Siberia (Briffa et al., 1995) and from central Europe (Spiecker et al., 1996) to North America (Lavoie & Payette, 1994). Although this improved growth is usually coincident with increased temperatures, there are also cases which are associated with a demonstrable influence of increased precipitation (Graumlich, 1987; Whitlock, 1993) as well as higher levels of atmospheric carbon dioxide. Growth trends of Scots pine (Pinus sylvestris) at its northernmost extent in the northwest Kola Peninsula may be an indicator of changes in the carbon cycle of terrestrial forest ecosystems. Using a method which removed age trends from the data, a time-series analysis of annual radial increments in wood over the last few decades compared with an earlier period of registered warming (maximum around 1920-40), revealed elevated growth, particularly for younger trees. In this northerly site the higher

Temperature CC( Ring-width Index utdovl

Temperature CC( Ring-width Index utdovl

19« 1950 1940 1970 1980 1990 2000

- Ring-width index — Temperature

Fig. 5.25 Relationship of radial growth of black spruce at Caribou-Poker Creeks Research Watershed to the mean of April (growth year) and February (two years prior) temperature, smoothed with a 5-year running mean. N.B. The temperature axis is inverted. (Reproduced with permission from Juday, 2005.)

19« 1950 1940 1970 1980 1990 2000

- Ring-width index — Temperature

Fig. 5.25 Relationship of radial growth of black spruce at Caribou-Poker Creeks Research Watershed to the mean of April (growth year) and February (two years prior) temperature, smoothed with a 5-year running mean. N.B. The temperature axis is inverted. (Reproduced with permission from Juday, 2005.)

global level of atmospheric carbon dioxide is the most probable reason for the marked recent increase in radial increment growth of the younger populations of Scots pine (Alekseev & Soroka, 2002).

It is important to note that although seasonal changes in temperature can have a strong influence on tree growth they can, depending on species and site conditions, show very different responses. In a study of radial stem growth of Alaskan black spruce (Picea mariana) at four different sites in Alaska three showed a negative response to warming and one a positive response. At one of the sites (Caribou-Poker Creeks Research Watershed) where the growth response was negative, warmer temperatures could promote the onset of photosynthetic activity in early spring when the ground was still frozen, causing desiccation and damage to the needles early in the growing season (Fig. 5.25). By contrast at another site at Fort Wain-wright the growth of the trees was positively correlated with winter temperatures (Fig. 5.26). It was also noted that this was one of the few species and site types in Alaska where empirically calibrated growth rates can be inferred to improve under projected higher temperatures (Juday, 2005).

A further complication in assessing whether or not the tundra-taiga interface is on the move or not arises from the changes in land use that have taken place in marginal areas over recent decades. The long history of extensive grazing by domestic animals is now changing with the abandonment of many northern and upland pastures. In many places in northern Europe, including Scandinavia, trees can be seen spreading over former summer pastures.

A comparison of birch forests in different regions of Scandinavia (Dalen & Hofgaard, 2005) has shown that growth rates varied through time and between regions, with an apparent decrease in the north since the 1940s. Although Scandinavian treelines are expected to advance in response to climate warming, this was not evident as a general pattern for all regions. Seasonally different climate patterns, browsing, and abrasion are all involved. These regionally different patterns have to be taken into account in predictions of future responses to avoid overestimation of ecosystem responses to climatic change. Nevertheless, there still appears to be a noticeable limit to the growth and establishment of birch forests (Betula pubescens) at higher altitudes in Norway.

Examination of the birch treeline position showed that both the number of trees and their basal areas decreased continuously with increasing altitude from 300 m below the treeline (Hofgaard, 1997). The number of birch saplings also decreased from about 150 m below the treeline towards higher altitudes. Viable but browsed populations of birch were present along the whole length of all transects, irrespective of aspect and geological substrate, with saplings present up to summit positions at 420 m above the treeline. Due to browsing by sheep, mean height of saplings established above the treeline was only 0.2 m. In situations such as this it has to be concluded that future vegetation responses to diminished grazing pressure are likely to override responses forced by changing climate (Holtmeier, 2003).

5.6.1 Physiological limits for tree survival at the tundra-taiga interface

Research into the limitations to tree growth in recent years have centred on the acquisition and utilization of resources for growth. Consequently, attempts to make generalizations as to the limit of tree growth have been mainly related to aspects of the growing season as they affect the performance of the whole tree. The

Fig. 5.26 Relationship of radial growth of black spruce (Picea mariana) at Fort Wainwright, Alaska (n — 20 trees), to a 4-month climate index (mean of monthly temperature at Fairbanks, Alaska, in January of growth year and January, February, and December of previous year). Scenario lines show projections of the 4-month climate index using two models (ECHAM4/OPYC3 projecting strong winter warming for the Fairbanks grid cell and CSM_1.4 a lesser degree of winter warming. (Data from Juday & Barbour, 2005, and reproduced with permission from Juday et al., 2005).

Fig. 5.26 Relationship of radial growth of black spruce (Picea mariana) at Fort Wainwright, Alaska (n — 20 trees), to a 4-month climate index (mean of monthly temperature at Fairbanks, Alaska, in January of growth year and January, February, and December of previous year). Scenario lines show projections of the 4-month climate index using two models (ECHAM4/OPYC3 projecting strong winter warming for the Fairbanks grid cell and CSM_1.4 a lesser degree of winter warming. (Data from Juday & Barbour, 2005, and reproduced with permission from Juday et al., 2005).

hypotheses that have been put forward as to the major causes for the absence of trees at high latitudes and altitudes have therefore revolved around the question of whether limitations in available thermal time affect the ability of the trees to make a net gain in carbon, or whether it is the time that is necessary for growth and development that is crucial to survival in the marginal conditions of boreal and alpine treelines. Both these hypotheses seek to find generalizations that can be used to make global predictions as to whether or not any particular climatic regime will be either favourable or disfavourable for tree survival.

Despite the emphasis that carbon balance studies give to the holistic reaction of trees to their environment, there are other more specific aspects of tree physiology that are affected by climate and thus have consequences for survival and distribution limits. Certain specific tissues or parts of the tree can suffer physiological and metabolic damage by adverse climatic conditions. If the susceptible tissue is part of a vital component (organ)

even just intermittent stress exposure can reduce the viability of the whole tree. Examples of this type of localized injury, with widespread consequences for long-term survival, are found in apical buds, stem cambium, and root meristems. All these tissues can be fatally damaged by unfavourable conditions such as drought, flooding, frost, and attacks from pathogenic organisms which can lead to the eventual demise of the entire tree.

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