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15. Southern taiga (needle-leaf conifers, tall very dense forest)

(Corresponds to southern continental taiga and cool conifer of Olson et al. and the 'dark taiga' of Russian authors)

References directly cited in these pages (does not at present include secondary citations)


Introduction

This forest type is typical of the more southerly and moist parts of the boreal zone, presently between about 45 and 60 N. in North America and somewhat further north in western Russia. The canopy is dominated by evergreen needle-leaf conifers (species include members of the genus Thuja in North America, and Picea in both North America and Russia), packed so densely that almost no light can penetrate to the forest floor (hence the term 'dark taiga', applied by Russian ecologists) (Walter 1971).

There is considerable interest in the role of boreal forests in general as a potential source or sink of carbon in the modern world. However, large scale logging is now widespread in the boreal zone, tending to reduce biomass and soil carbon.

Conversely, fire prevention and fire control policies are also operated in North America (Apps et al. 1993). Thus to some extent, the resulting carbon storage estimates that are being published as representative of present-day boreal forests may have incorporated areas of forest altered by human disturbance factors (so tending to underestimate carbon storage in the primaeval state). On the other hand, fire prevention might have allowed forest biomass and litter levels in certain areas to exceed what would be normal in a primaeval state, with lightening-induced fires supressing carbon accumulation (tending to cause the carbon storage of the primaeval state to be overestimated).

The true significance of natural fires in suppressing boreal forest biomass remains a controversial area. The available record of estimates of forest fire frequency since 1918 (Auclair et al. 1996) suggests that there was about a ten-fold decline in the volume of wood lost to forest fires in the USA between 1920 and the 1960's onwards. However, for Canada there is less of a decline, and for the former USSR there is no clear trend in loss of wood to fires over the same period. Since the latter two regions contain most of the boreal forest mass in the world, this may suggest that the decline in fire disturbance of boreal forests is more a USA-based than a global phenomenom. The same summary graph does show however that there was a dip in fire losses during the period between 1950 and 1970 when much of the important early work on forest biomass was being carried out in the USSR and Canada, perhaps tending to lead to inflated estimates.

Storage (tC/ha) Ecosystem component
25 tC/ha Dead standing trees, coarse woody litter, leaf litter and other debris
140 tC/ha Above and below-ground vegetation
135 tC/ha Soil organic carbon
300 tC/h Total carbon storage

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Litter

Storage (tC/ha) Location Author(s)
(1.) 42 tC/ha Coarse woody debris Harmon pers. comm.

(1.) Based on knowledge of site studies of forests in a natural state, but still an ad hoc estimate.

Conclusion; a more conservative figure of 25 tC/ha for combined woody debris and leaf litter is used here, based on discussion with boreal forest ecologists (e.g. S.P. Payette, Univ. Laval. E. Zelikson, at Moscow State University). Fire frequency will tend to be a controlling factor on debris accumulation, with a fire return time of 100->200 years suggested for boreal spruce forests by Wein & MacLean (1983). However, the suggested more frequent fire return times of Apps et al. (1993) would greatly depress debris accumulation.

Vegetation

Storage (tC/ha) Location Author(s)
110 tC/ha (1.) Southern continental taiga Olson et al. (1983)
120 tC/ha (2.) Taiga Duvigneaud (s.)
140 tC/ha (2.) Taiga Rodin et al. (1975)
506 tC/ha (3.) Old-growth. NE USA. Andrews (s.)
30 tC/ha (4.) Boreal east, Canada. Apps et al. (1992)

(1.) Global review, based on a range of studies. 60 (low) - 110 (medium) -140 (high) tC/ha. It is likely that these values were intended to refer to the Pacific temperate rainforests as well as the more continental southern taiga. Presumably, many areas that have been subject to logging are also included in the sources of these estimates for present-day carbon storage.

(2.) These values may refer to samples taken in this zone of the taiga, being too high for taiga in general, or a mixture of 'southern taiga' and 'mid taiga', thereby diluting down the values for southern taiga.

(3.) Study by Andrews cited in Harmon & Hua 1992 of southern taiga old-growth forest 450 years old since the last burn, obtained figures for above-ground living material varying from 245 - 535 tC/ha. There is the need to add an extra % for belowground roots; if these form an extra 30% of total (proportion of roots = 0.25) then living plant material is 318- 695 tC/ha. (the simple mean of these two extremes is 506.5 tC/ha). It is necessary to question whether the natural burn frequency would allow this average biomass to be generally representative of the natural state, but Harmon & Hua appear to view it as representative of the preanthropogenic state.

(4.) A study for Forestry Canada on Canadian carbon in forest ecosystems (Apps et al. 1993) presented figures for the 'boreal east' forest zone of Canada (approximately corresponding the the area of the 'southern taiga' defined here. They suggest only around 30 tC/ ha in biomass; obviously much lower than all previous studies. The basis of this large discrepancy needs to be examined further.

Conclusions; The contradictions between the carbon storage values given by different authors are striking. The main cause of this disagreement seems to be the difficulty of allowing for the natural fire frequency. There appears to be little prospect at present of obtaining a thorough analysis of these problems until more work has been published. However, given that most closed southern taiga is fairly tall and very dense, it seems reasonable to suppose that it might have a carbon storage value approaching that of cool temperate forest. Hence, I have preliminarily suggested a relatively high value of 140 tC/ha in vegetation for the preanthropogenic state.

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Soil

Storage (tC/ha) Ecosystem component Author(s)
206 tC/ha (1.) Boreal forest Schlesinger (1985)
230 tC/ha (2.) Boreal moist forest (250 - 500 mm) Zinke et al. (1984)
198 tC/ha (3.) Boreal wet taiga (500 - 1000 mm) Zinke et al. (1984)
147 tC/ha (4.) Cool conifer forest Zinke et al. (1984)
123 tC/ha (5.) Mid continental southern taiga Zinke et al. (1984)

(1.) Based on global data review.

(2.) Based on global review of standardised soil samples. 127 samples, S.D. 23.

(3.) 5 samples, S.D. 8.2.

(4.) 717 samples, S.D. 11.4.

(5.) 179 samples. S.D. 12.0.

Conclusion: For southern taiga on non-peatbog soils, a mean value of values 4) and 5) is suggested; 135 tC/ha.