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3. Tropical woodland (relatively low, open tree canopy, usually deciduous)

(Corresponds to subdivision of Olson et al. tropical savanna and woodland).

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


Introduction

This open woody type of vegetation is widespread in the present-day world in the tropics (and locally in the drier areas of the subtropics), but is often extensively altered by humans burning it and taking the wood for fuel, and grazing their animals on the undergrowth. Such woodland areas have probably been inhabited and deliberately burnt by humans for many thousands of years (especially in Africa). In earlier times large 'bulldozer' herbivores (Kortlandt 1982) such as elephants may have caused extensive degradation of this vegetation relative to its present state in which these herbivores are reduced by hunting, poaching or competition with smaller domesticated animals. However, pollen records indicate that woodland vegetation did in fact occur in roughly the same localities in Africa and elsewhere in the tropics in pre-neolithic times. The subtle changing role of various degradational factors, ancient and modern, must be guessed at when trying to make any reconstruction of past 'natural' carbon storage for this ecosystem.

Suggested summary values for the preanthropogenic state

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

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Litter

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

(1.) ad hoc estimate

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Vegetation

Storage (tC/ha) Location Author(s)
30 tC/ha Tropical savanna and woodland Olson et al. (1983)
70 tC/ha Tropical dry forest & woodland Olson et al. (1983)
50 tC/ha Semi-arid woodland/low forest Olson et al. (1983)

(1.) Based on global literature survey. 20 (low) - 30 (medium) - 50 (high) tC/ha. This estimate includes savannas, which are also dealt with as a separate category in the present data summary, and their inclusion would tend to 'dilute down' the carbon storage for woodland. Hence the true carbon storage of woodland vegetation taken alone might be higher than the figures given in this range.

(2.) For tropical dry forest and woodland, described as 'mostly drought deciduous' and in the overall category of 'woodland/savanna/tall scrub', the values are 20-40-90 tC/ha in vegetation. The medium value of 70 tC/ha may be the most accurate, considering that even in the preagricultural state these woodlands were subject to frequent disturbance by fire or by large herbivores.

(3.) 20 (low) - 50 (medium) -100 (high-side estimate) tC/ha in vegetation estimated by Olson et al. This category may not be expressly tropical however, coming under their category of 'other dry woodlands'.

Conclusion; a figure of 70 tC/ha is accepted here for the vegetation component, given that 'woodland' usually refers to relatively high densities of trees.

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Soil

Storage (tC/ha) Location Author(s)
60 tC/ha (1.) Tropical savanna and woodlands Zinke et al. (1984)
112 tC/ha (2.) Seasonally dry tropical woodland Zinke et al. (1984)
70 tC/ha (3.) Tropical woodlands and savannas Schlesinger (1985)
60 tC/ha (4.) Tropical woodland, Kenya Turay & Hall (in press)
32 tC/ha (4.) Tropical woodland, Kenya. Turay & Hall (in press)
69 tC/ha (5.) Tropical very dry forest Zinke et al. (1984)

(1.) 69 samples, SD = 4.5. Confusingly, this includes savannas, which probably differ quite significantly in soil carbon storage from woodlands (e.g. some savannas have very rich chernozem-like soils).

(2.) 22 samples, SD = 5.3. Probably in fact refers partly to monsoon/dry forests (all the samples are from South America or Asia, none from Africa).

(3.) Based on literature survey, Again, includes savannas in with the woodlands.

(4.) This estimate is based on 7 samples, all of which are at 20, 30 or 40 tC/ha except one sample from a humic nitrosol which falls at 220 tC/ha. The resulting average of 60 tC/ha is thus weighted heavily by this one anomalous value. I suggest that this very high value represents a pocket of wet soil conditions (e.g. an ox-bow) that is unrepresentative of the woodland environment in general. Thus, this value is excluded and an average taken from the remaining values. The resulting preferred value from Turay & Hall's data is 32 tC/ha.

(5) Based on a global survey of 124 samples, SD = 4.9 . This is from the Holdridge category between 1000 and 500mm annual precipitation and a biotemperature above 23C. Roughly speaking, this would seem to apply to typical tropical woodland environments (Walter 1971). The number of samples here, and the fact that the limits on the samples category seem to fall fairly close to what most would regard as being 'tropical woodland', make this value a good candidate for the woodland category.

Conclusion; A figure of 70 tC/ha is suggested for the soil organic reservoir, mainly on the basis of the Zinke et al. value for tropical 'very dry forest', but higher than the Kenyan soil value.