Carbon balance

The concept of carbon balance has a logical appeal when predictions are required for forest productivity in relation to changing climatic conditions. Increasing temperatures and higher levels of atmospheric carbon dioxide are climatic variables that demand an assessment oftheir likely effects on growth, photosynthesis, respiration, litter production and quality, soil respiration and other aspects of carbon sequestration in forests. As conditions become more severe with increasing altitude or latitude it is also logical to enquire if there is a limit above which trees fail to make a net gain in carbon over the year. As already discussed (Section 1.3.2), numerous studies have attempted to ascertain whether or not trees are limited in their growth by a carbon deficiency, particularly at high altitudes. There are numerous difficulties in obtaining an integral estimate of total photosynthetic and respiratory activity throughout the year in the severe conditions that exist at either the tundra-taiga interface or the montane timberline. Nevertheless, some studies have succeeded in estimating the extent to which increased atmospheric carbon dioxide levels in the atmosphere will benefit trees at the upper limits of their distribution. The alpine treeline where these experiments were carried out is not the tundra-taiga interface. However, the eventual significance of increased levels of carbon dioxide for tree growth at the northern limits of the boreal forest, like that in the Alps, is likely to be influenced by the interplay between biotic and abiotic processes, and the species of tree.

Experimentally it has been possible to provide carbon dioxide enrichment and combine this with foliage removal to test the effect of altered source-sink relationships on tree growth and leaf level responses in mature larch (Larix decidua) and mountain pine (Pinus uncinata) in situ near the treeline in the Swiss central Alps at 2180 m above sea level (Handa et al., 2005). A three-year long study found that elevated carbon dioxide levels enhanced photosynthesis and increased non-structural carbohydrate concentrations in the needles of both species. While the deciduous larch trees showed longer needles and a stimulation of shoot growth over all three seasons when grown in situ under elevated carbon dioxide, pine trees showed no such responses. After three years, the results suggested that for the deciduous larch carbon is limiting at the treeline, while for the evergreen pine it is not a limiting factor. It might be expected therefore that depending on the functional types of the treeline species increasing carbon dioxide availability may improve the carbon balance in some but not all cases. In complex cases it is sometimes useful to use modelling to simulate the likely outcome of changes in environmental conditions on some feature which is difficult to quantify in the field. A comparison of dwarf trees (stunted trees that retain an arboreal growth form) and krummholz trees (assumed in this study to have a mat-like growth form) has shown that these differences in canopy types have significant effects on simulated carbon balance. Dwarf tree canopies appear in computer simulations to have higher carbon balances for all leaf area index values above 4, while carbon balances for both canopy types decrease with increases in LAI. The results demonstrate the importance of canopy type in relation to survival at the alpine treeline (Cairns, 2005). Unfortunately, models tend not to include other important factors such as the distribution of carbohydrates, which varies with age in trees, and estimations based on one particular age of tree or tree seedling may not be representative of the forest as a whole. A resolution of this problem is obtained when an actual assessment is made in the field where it can be seen that there is usually no problem for trees at the timberline to maintain a surplus of non-structural carbohydrate throughout the year. Such investigations reveal that although temperature is related to the position of the timberline it does not reflect a negative carbon balance but is due instead to a shortening of the thermal time available for developmental processes (Körner, 2003). An explanation of just what processes are involved and how they are limited by temperature nevertheless remains elusive even though it has long been observed that arborescent growth is restricted by direct climatically caused damage before reaching an alti-tudinal limit set by an insufficient carbon balance (Holtmeier, 2003).

5.6.3 Carbon balance versus tissue vulnerability at the treeline

While carbon balance has much practical significance for forestry and other productivity studies it is not necessarily a deciding factor in relation to the survival of natural plant populations. Plants being autotrophic have an inherent capacity for matching carbon utilization with availability. As the growing season shortens or temperatures fall, growth can be reduced and an overall carbon deficit may be avoided. Woody plants become progressively smaller at high latitudes. Trees are replaced by scrub, and scrub is then replaced by woody plants with prostrate stems. The most northerly willows in the world, the polar willows (Salix polaris, S. reticulata) of Spitsbergen, are sometimes described as the arctic forest. They give no indication of suffering a carbon deficit and produce copious quantities of seed even at their most northern locations.

What sometimes is unavoidable, however, is a carbon deficit in certain vulnerable tissues. Root meristems that are depleted of carbohydrate by anaerobic respiration when flooded will also be likely to be deficient in antioxidants, making them vulnerable to post-anoxic injury when water tables drop. Buds that are depleted of sugars by intermittent warm periods in winter become sensitive to late frosts (see Section 9.8.2). It follows therefore that under natural conditions it is likely that the sensitivity of certain vulnerable tissues will determine the long-term viability rather than the carbon balance of the entire tree. However, once the essential tissues are damaged a reduction in the carbon balance of the whole tree will follow.

5.6.4 Winter desiccation injury

The length of the growing season is undoubtedly critical for the survival of trees and early research first carried out in Germany (Michaelis, 1934) considered that the length of the growing season and the development of the cuticle and scale leaves on overwintering buds was essential for surviving the dangers of winter frost desiccation (Tranquillini, 1979). Observations at the treeline in oceanic Scotland have not found evidence of excessive dehydration damage due to inadequate cuticle development (Grace et al., 2002). However, in more continental climates, significant water losses from trees that do not survive well at the timberline are frequently reported. Winter desiccation-induced foliage loss incurred by krummholz growth forms of subalpine fir (Abies lasiocarpa) at treeline locations in National Glacier Park, Montana, USA, for the winter of 1998/1999 affected an average 8.68% of the krummholz canopy (Cairns, 2001. Winter desiccation, however, was not found to be related to any single environmental variable. Nevertheless, when outliers were removed, winter desiccation showed a strong correlation with elevation (r = 0.97) and was highly predictable in relation to various habitat characteristics, e.g. elevation, slope and aspect. In general, injury increased with elevation and on more south-westerly facing hill slopes, while sheltered locations showed decreased winter desiccation. Within patches, most winter desiccation was found at the windward edge of patches. This trend was attributed to the presence of leading shoots above the mean canopy surface of the krummholz patch. In these more extreme environments high winds and ice particles can cause severe abrasion of cuticular surfaces (Hadley & Smith, 1989).

A further risk to the maintenance of the water supply to tree stems at montane treelines comes from the incidence of embolisms in the water conducting tissues. A study of the two dominant species of the European central Alps timberline, namely Norway spruce (Picea abies), and stone pine (Pinus cembra), compared the seasonal courses of embolism and water potential at 1700 and 2100 m during two winter seasons. Stone pine is the hardier tree and usually reaches higher altitudes than Norway spruce. Embolism was observed only at the timberline and only in Norway spruce. Both species showed a significant drop in hydraulic conductivity, but in stone pine critical levels in water potential were avoided. It would appear that water losses in Norway spruce make winter embolisms a relevant factor in defining the treeline position for this species (Mayr et al., 2003).

5.6.5 Overwintering photosynthetic activity

Studies on Picea abies and Pinus cembra near the alpine timberline have been able to explain the depression in photosynthetic activity that occurs during winter as

Fig. 5.27 Comparison of possible contrasting changes in distribution of Pinus sylvestris with varying climate conditions as compared with the temperature regime from 1961—1990. (a) Winter 4°C colder, summer 4°C warmer. (b) Winter 4°C warmer, summer 4 ° C colder. Note the retreat from western Europe with the imposition of warmer winters. The colour key relates to the probability of occurrence of pine. (For details see Crawford & Jeffree, 2007.)

Fig. 5.27 Comparison of possible contrasting changes in distribution of Pinus sylvestris with varying climate conditions as compared with the temperature regime from 1961—1990. (a) Winter 4°C colder, summer 4°C warmer. (b) Winter 4°C warmer, summer 4 ° C colder. Note the retreat from western Europe with the imposition of warmer winters. The colour key relates to the probability of occurrence of pine. (For details see Crawford & Jeffree, 2007.)

the result of oxidative stress due to the coincidental effects of high light and cold temperatures. Potential efficiency of photosystem II (F-V/F-M), light and CO2-saturated rates of photosynthetic oxygen evolu tion (P-max) and contents of the antioxidants ascor-bate and glutathione in the reduced and oxidized state were measured in sun-exposed and shaded needles at the timberline. It was found that needles with the

Fig. 5.28 The Siberian alder (Alnus fruticosa) on Paramushir Island, northern Kurils. In this oceanic environment the alder and willow scrub is found where dwarfSiberian pine (Pinus pumila) would be found in more continental regions. (Photo by courtesy of Dr P. Krestov.)

lowest photosynthetic activity contained high levels of antioxidants in the reduced state. It was therefore concluded that the reduction of F-V/F-M during winter is not the result of oxidative stress, but is instead an injury-preventive down-regulation of PS II (Stecher et al, 1999).

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