Last modified 8th March 1998

GLOBAL VEGETATION CLASSIFICATION SCHEME


Details of the vegetation scheme used for the map reconstructions.


THE GLOBAL ATLAS OF PALAEOVEGETATION

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The use of any vegetation categories to represent the world of the distant past is a risky business; one can never be absolutely certain what the vegetation was really like in structural terms, and very often it clearly had certain attributes of floristic composition that do not occur today. However, it does still seem worthwhile trying to narrow down the unnecessary confusion and loss of information that is a continual problem for Quaternary ecologists. Different workers use different labels when they each talk of essentially very similar vegetation types. For example, what one person in North America describes as 'desert' may be what another in Africa would include in the category of 'steppe'. Only if they each give additional details of the structure and physiognomy of the vegetation that they have in mind will they both be certain when they are talking about the same thing.

For the purposes of clarification, some general details are given here to show what is to be bourne 'in mind' for each of the map categories. In implementing this scheme for the mapping, considerable efforts have been made to push the diverse range of literature sources on Quaternary palaeovegetation into a coherent framework. Most importantly, the editors have endeavored to ensure its use by showing this scheme to local/regional experts when asking for their opinions on the palaeovegetation maps.

Note that this is simply an ad hoc scheme, and a careful reader will no doubt find gaps and inconsistancies within the scheme itself and in its application to the palaeodata summarized in the main section of this paper. Amongst the many unknowns of the world of the past, it is also possible that vegetation-climate relationships might have changed somewhat (e.g. due to changed CO2 levels), in which case the temperature limits might not be as appropriate as they are today. Thus, one could suggest that the limit of rainforest at the LGM might (purely hypothetically) in fact have been 20° C for the coldest month, rather than 15.5° C as for the present. The scheme presented here is based on a general knowledge of the literature on plant biogeography, applied to a specific and urgent task. This scheme is thus ad hoc and imperfect, but it is hoped that it will have at least some role in reducing the amount of confusion that currently exists within the literature concerning Quaternary palaeovegetation.

DETAILED KEY TO VEGETATION TYPES.

Detailed key to vegetation types, corresponding to numbers on regional maps. A brief description of the physiognomy of each vegetation type is given, together with the nearest corresponding vegetation type/s on the global map of Olson et al. (1983). Note that the descriptions of cover are partly based on a concept of 'cover strata', selectively taking the percentage cover above a particular horizontal plane, as if the leaves and branches above it were casting down a shadow onto this imaginary plane. This principle is illustrated in the figure shown below. The approximate height and cover values used here are not arbitrary; they have been checked with a range of vegetation scientists and are based on those presented by J.M. Adams at a UNEP workshop in Charlottesville in January 1991. They were generally accepted at the workshop as being 'representative' cover values for each major vegetation category. The temperature limit for desert and and semi-desert is taken to correspond approximately to the geographical limit for 'hot and warm desert' presented on the Olson et al. (1983) ecosystems map, and used as a basis for their carbon storage categories; for our palaeovegetation maps this limit is assigned using estimates of the land temperature depression in each region. 'Subcategories' are also given to enable the more specific subdivision of vegetation types if the data appear to allow this. Not all of these are utilized in the regional maps presented here, because the resolution in the fossil record is not always seen to justify this.


TROPICAL AND SUBTROPICAL (mean temperature of coldest month above 15.5C for forest vegetation, and mean temperature of coldest month above 10C for all other physiognomic types).


1. Tropical rainforest (evergreen or semi-evergreen forest of humid tropics, usually tall)

(corresponds to Olson seasonal tropical forest and broad-leaved humid forest. Also includes swamp forest).

Leaf cover above a level 8m off ground, >60% . No more that 50% loss of canopy leaf cover at any one time during average year.

(1a= Rainforest of well-drained soils <25% loss of canopy leaf cover, 1b=semi-evergreen forest 25-50% loss of canopy leaf cover,


2. Monsoon or dry forest (medium height, deciduous or mainly deciduous forest of warm climates)

(not corresponding to an Olson category; approximately a sub-division of seasonal tropical forest)

Leaf /branch cover above a level 8m off the ground, >60% during peak month of leafiness. >50% loss of canopy leaf cover at some stage in average year, except for Australian Eucalyptus dry forests, where limit of rainforest is defined by where Eucalyptus becomes >50% of canopy.


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

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

Leaf /branch cover above 8m off ground, 60%-20%.


4. Tropical thorn scrub and scrub woodland (low, woody, usually deciduous)

(Incorporates Olson succulent and thorn woods, and also overlaps with warm or hot shrub and grassland)

Leaf /crown cover above 8m off ground less than 20%, but total leaf cover between 0.8-8m off ground greater than 20%.


5. Tropical semi-desert (sparse scrub or sparse grassland)

(Corresponds to subdivision of Olson desert and semi-desert)

Less than 2% vegetation cover above 80cm off the ground. 25-4% vegetation cover between 0 and 80cm off the ground, during an average year.


6. Tropical grassland (fairly closed grassland without many trees or shrubs)

(Corresponds to subdivision of Olson tropical savanna and woodland, and overlaps with warm or hot shrub and grassland)

(6a= dense sward tropical grasslands, 6b= sparse sward tropical grasslands)

Leaf/branch cover above 80cm off ground less than 2%. But total cover above ground level, greater than 25%.


7. Tropical extreme desert (very sparse vegetation, or completely barren)

(Corresponds to subdivision of Olson desert and semi-desert)

Total cover above ground level, less than 4% at any time during average year.


9. Savanna (dense grassland with a scattering of trees and/or bushes)

(subdiv of Olson tropical savvanna & woodland).

(9a=Tree-dominated savanna, 9b=bush-dominated savanna)

Leaf/branch cover above 80cm of the ground, 2-20%.


HIGHER LATITUDE AND MONTANE (mean temperature of coldest month below 10C, or below 15.5C for forest vegetation).


10. Warm temperate evergreen forest (fairly tall, usually many broadleaved evergreen/semi-deciduous angiosperm trees but conifers also tend to be abundant, in moist climate).

(Corresponds to subdivision of Olson temperate broadleaved forest)

(1a= warm temperate forest on well-drained soils, 2b= same, but as swamp forest)

Cover above 8m off ground, greater than 60%.


11. Cool temperate giant coniferous rainforest (very tall, closed conifer forest; usually Pseudotsuga or Seqouia)

(Subdivision of Olson southern continental taiga)

Greater than 50% cover above 40m off ground.


12. Montane tropical forest (evergreen, adapted to cool temperatures)

(not corresponding to an Olson category)

(12a=lower montane forest, 12b=upper montane forest).


13. Mediterranean sclerophyll woodland or forest (mixture of sclerophyllous and deciduous trees & bushes)

(Corresponds to a subdivision of mediterranean types of Olson)

(13a= Med sclerophyll forest or woodland, 13b= Med sclerophyll scrub)


14. Cool temperate forest (closed forest. Includes mixed conifer-broadleaved forest)

(Corresponds to subdivision of Olson temperate broadleaved forest)

Cover above 8m greater than 60%. Less than 50% of this cover by needle-leaf trees. Greater than 50% of broadleaved leaves lost in winter.


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

(Corresponds to southern taiga of Olson)

Cover above 8m greater than 90%. Greater than 50% conifer cover.


16. Mid taiga (conifer or broadleaved forest with a relatively open canopy)

(Corresponds to Olson main taiga)

(16a=main taiga on well-drained soils, 16b = main taiga on bog or swampy soils; not distinguished here)

Cover above 8m 60-90 %.


17. Open boreal woodlands (various open woody vegetation types; coniferous or

broadleaved) (Corresponds to Olson northern & maritime taiga)

Cover above 8m, 20-60%.


18. Semi-arid temperate woodland or scrub (various open woody vegetation types; coniferous or broadleaved, in temperate climates). (Corresponds to Olson warm conifer and other woodlands )

(18a = temperate woodland, 18b = temperate scrub).

Cover above 8m, less than 20%. Cover 0.8-8m, above 20%.


19. Tundra (mainly herbaceous or with low shrubs)

(Corresponds to Olson tundra, but also includes herbaceous bogs).

(19a = sparse tundra, alpine and high polar 19b=dense tundra)

Cover above 80cm, less than 2%. Cover above ground level, greater than 4%.

(19a; sparse tundra - 4-25% ground cover.

19b; dense tundra - 25-100% ground cover.)


20 Steppe-tundra (a no-analog glacial age vegetation, probably sparse vegetation, herbaceous with a few low shrubs. Resembling both present-day steppe and tundra in certain aspects)

(No Olson analogue)

(20a = 'tundra-like' relatively rich in tundra plants 20b = 'steppe-like' richer in steppe plants).

Vegetation cover 10-50%, averaging about 30-40% across most of Eurasia? Highest in moister 'tundra-like' areas, lower in dry 'steppe-like' areas. (estimated by analogy with present-day plant communities in various regions, on basis of pollen, palaeosoil and geomorphological evidence).


21. Polar and alpine desert (very sparsely vegetated with only low herbaceous plants)

(Corresponds to Olson polar or rock desert)

Vegetation cover less than 4%.


22. Temperate desert (very sparsely vegetated, cold winters)

(Subdivision of Olson cool desert and semi-desert types)

Vegetation cover less than 4%.


23. Temperate semi-desert (sparse shrubland or grassland)

(Subdivision of Olson desert and semi-desert)

(23a=grassy temperate semi-desert, 23b= shrub-dominated temperate semi-desert; not distinguished here)

Less than 2% cover above 80cm off the ground. 4-25% total above ground cover.


24. Temperate and montane steppe (grasslands and other herb-lands, closed or fairly dense sward)

(Corresponds to subdivision of Olson cool grassland/scrub)

(24b=sparser, short-grass steppe, 21a=dense tall-grass steppe)

Leaf/branch cover above 80cm less than 2%. Vegetation cover above ground, greater than 25%.


25. Forest steppe (mainly herbaceous, but with clumps of trees or bushes in favourable pockets)

(Corresponds to subdivision of Olson wooded tundra and timberline. Also includes wooded bogs)

(25a = moister climate types, closed herbaceous vegetation, 25b = drier climate types, open herbaceous vegetation; not distinguished here).

2-20% leaf/branch cover above 80cm, on a broad scale.


26. Forest tundra (mainly herbaceous or low shrub, with a scattering of trees and bushes).

(Corresponds to a subdivision of Olson wooded tundra and timberline).

2-20% leaf/branch cover above a level 80cm off ground, on a broad scale.


27. Bog/swamp (of tropical or high latitude zones) (>50% surface water cover for 6 months or more of year). (corresponds to Olson Swamp and Bog).

27a) herbaceous bog 27b) wooded bog 27c) swamp forest forest growing on soils with >50% surface water cover for 6 months or more of year, (not distinguished here)


28. Ice sheet and other permanent ice.


29. Lakes and open water.

(29a=fresh water, 29b=saline water)