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Last modified Tuesday 25th February 1997
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INTRODUCTION TO THE REVIEW AND ATLAS OF PALAEOVEGETATION
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[1]VIEW NEW SET OF PALAEOVEGETATION MAPS (PRELIMINARY) 
[2]Details of the vegetation categories used in the maps

[3]THE GLOBAL ATLAS OF PALAEOVEGETATION

[20]The time-slices
* [21]The last glacial maximum
* [22]The early Holocene, 8,000 years ago
* [23]The mid-Holocene, 5,000 years ago
* [24]The 'present-potential' vegetation

The time-slices

Although time is of course a continuum, it has been necessary here to
concentrate the reconstructed scenarios around particular slices of
time that seem particularly significant in relation to the processes
taking place in the global system. The perception of what is or is not
a particularly 'significant' time slice depends on what aspects of the
global system one is interested by, and in this respect the choice has
been biased by an interest in the relationship between vegetation
ecology and carbon storage on land. Another criterion has been the
relative 'stability' of vegetation at each time slice, avoiding trying
to make anything of the confusing and blurred picture from times such
as the late glacial/earliest Holocene in which the world seems to have
been a mosaic of landscapes and vegetation, all at various stages of
rapid change. At such times, a slight inaccuracy in the dating or a
gap in the data could make a vast amount of difference.
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The last glacial maximum. This earlier time-slice, the last glacial
maximum (LGM), is placed at around 18,000 years ago in _radiocarbon_
years. Note that due to changes in radiocarbon abundance at different
times in recent Earth history (due to changes in the carbon cycle or
in cosmic ray flux) the ages given by radiocarbon dates may be wrong.
It is now thought that 18,000 years ago in radiocarbon terms
corresponds to about 20,000-21,000 years ago in 'real' years (Bard _et
al_. 1990).

The LGM is generally defined as the stage during the last glacial
cycle at which the greatest mass of ice was present on Earth, showing
up in ocean carbonates as a peak of 18O. It is also thought of as
being the time at which at which other components of the
ocean-atmosphere system were at their most 'glacial' (e.g. lowest
global temperatures, lowest atmospheric CO2 concentration, and
apparently greatest aridity in many continental regions) (Crowley &
North 1991).

In fact, there are numerous signs that not all attributes and
processes reached their peak of 'glaciality' (in the sense of maximum
cold, maximum ice extent, and maximum difference in water balance
relative to the present) at the same time during the last general
glacial phase; example Colinvaux _et al_. (1989) suggest that the
lowest temperatures and maximum glacier extensions in tropical uplands
may have occurred several thousand years before those at higher
latitudes. The most recent evidence is suggesting that in fact the LGM
in terms of the 'ice maximum' was not the maximum in terms of global
cold and aridity. This extreme in terms of climate may have coincided
with Heinrich Events (surges of icebergs) that affected the whole of
the planet's climate, and that what has been thought of as single
glacial maximum may actually be two fairly similar intense cold
episodes, separated by a short-lived but slightly milder episode that
corresponds to the maximum land ice extent itself. If this
interpretation is correct, the global cold and aridity maximum
described here may need to be shifted slightly in time to lie both
just before and just after the LGM _sensu stricto_. The 'LGM' as
described here should be seen as the 'globally most arid and cold
conditions to have occurred simultaneously during the last 20,000
radiocarbon years' and not in terms of 'greatest global ice extent'.

In cases where there is no proper dating evidence available, it is too
easy to assign all ambiguously dated 'cold' and 'arid' events in the
palaeoenvironmental record to the time around LGM, simply because
well-dated evidence from elsewhere shows cold or aridity at that time.
Having said this, in a situation where one has to scramble for tenuous
clues as to the true nature of palaeoenvironments at around the LGM,
one can also intuitively understand that this 'circular' search for
relatively poor quality evidence to reinforce a picture based on good
quality evidence might lead to the best overall predictions in the
longer run. For example, if one has a whole series of undated lake
cores from across a broad region, each showing a single marked arid
event, it seems reasonable to suggest in a tentative way that this
arid event occurred synchronously throughout the region. If a
neighbouring region has dated information that also shows a single
well-marked arid event, a reasonable preliminary hypothesis (in the
absence of any other information) is that the arid events in both
adjacent regions occurred simultaneously. Carbon dating of evidence
from around the world suggests that in most areas (though by no means
all!) this approach would in fact be correct in identifying the
conditions that occurred simultaneously at approximately the LGM*.

It is important to bear in mind that in some areas for which there is
continuous well-dated evidence, climates only a few thousand
radiocarbon years before or after 18,000 14C years ago were very
different from the LGM itself, often being much moister (and as
mentioned above, the 'true' LGM in terms of land ice extent may have
been a brief milder phase between two closely placed and more extreme
cold events). The ambiguous use of the word 'glacial' in the
literature to include periods thousands of years before and after the
LGM often confuses readers, who tend to assume that this refers to the
LGM itself. This is not the place to resolve fundamental issues of
stratigraphy, but authors should perhaps take note of this problem.
They could perhaps take care to specify when they are talking about
the last glacial period 'before', 'after' or 'at around' the LGM.

*In some cases this method could confuse the Younger Dryas with the
LGM, but then again the vegetation conditions of the two periods seem
to have been generally similar in terms of direction of vegetation
change, but with the LGM being markedly more extreme in most areas.

In this review, wherever undated or ambiguously dated evidence is
cited, it is pointed out as such and treated cautiously, and presented
as only _possibly_ relevant to the overall picture. Certainly, in some
areas where other evidence is lacking it has been allowed it to
influence the vegetation map reconstructions presented (this fact is
explicitly stated wherever it is used). Of course, when relying on
review sources or expert overviews one cannot always be sure what
quality and types of evidence individual experts are willing to accept
as convincing. For the most part it is only possible to report on
their final recommendations or conclusions about palaeoenvironmental
conditions, and not all the detailed contortions of the
decision-making process which allowed these experts to arrive at their
conclusions.
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The early Holocene, 8,000 radiocarbon years ago. This second time
slice is intended to represent the world whilst it was reaching the
final stages of deglaciation (some remnants of the North American ice
sheets still remained but they were melting fast; Denton & Hughes
1982), as vegetation and soils were approaching a relatively steady
state after the rapid changes that had taken place during the previous
millennia. In terms of 'real' years, the 8,000 radiocarbon year
timeslice was probably nearer 9,000 years ago, though there are still
considerable uncertainities.

There are some indications that a sudden drought episode set in
northern and eastern Africa (and possibly elsewhere) either just
before or just after 8,000 14C years ago (Gasse & van Campo 1994), but
this time slice is intended to represent the moist conditions
immediately prior to this particular climate switch. Although the
broad climatic background was much more similar to that of today,
relative to the LGM, climates in most areas do seem to have been
noticeably different from those of the present. There is less
ambiguity in the dating for this time, and data are generally much
more abundant, so there is greater confidence that the biome
reconstructions presented here are reliable.

In many areas of the world, the indications are that the 8,000 14C
years ago vegetation was already quite similar to today's, at least in
the sense of broad biome structure (e.g. temperate deciduous forest as
opposed to closed boreal forest) although not necessarily in terms of
detailed community composition (e.g. the relative abundance of
particular tree species). Wherever published review sources agree in
emphasising that the vegetation was broadly similar to that existing
today, little emphasis in our review is placed on the nature of the
evidence used to justify this conclusion, although it is given some
cursory mention. Instead, the emphasis is on the nature and validity
of those sources of data apparently indicating a significant
_difference_ from the present-natural vegetation. For the many regions
where there are no indications of conditions being significantly
different (on the available evidence, which is often not of ideal
quality), we have merely filled in 'present-natural' vegetation
boundaries as the most reasonable surrogate. Knowing that many parts
of the world were not so very different from today, it seems better
for the sake of global-scale studies to make a reasonable but
conservative guess rather than leaving swathes of the land surface
unclassified (e.g. it would be unrealistic to make a global
biogeochemical study that did not include the whole land surface; one
has to put some values into the sums). It may eventually turn out that
significant differences were present, in which case the mapped
vegetation boundaries will need to be altered accordingly. For the
time being, it is important that the user of the maps consults the
accompanying literature review for each region to know just how much
weight to give to the reconstructions within each region.

Although agriculture was certainly present in several parts of the
world by 8,000 years ago, it generally seems that it was not a
significant modifier of vegetation on anything more than a localised
scale (e.g. see Tallis 1990, and discussions in the text below). Thus,
on these maps there is no attempt made to mark out any areas of
croplands or other land cleared for agriculture or settlement.
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The mid-Holocene, 5,000 years ago. By around 5,000 14C years ago, it
seems that climate and potential vegetation patterns were
significantly closer to those of the late 20th century than those that
existed 8,000 14C years ago. Hence, this time interval provides a
state _relatively _similar to the 'present potential', the difference
being that it actually existed and is not a hypothetical state in the
way that the present-potential is.

There certainly would have been important differences from the
vegetation patterns that would potentially exist at present (e.g. the
Sahara was much more densely vegetated than now at 5,000 14C years
ago), and there has been an attempt made here to map those differences
that appear in the literature and have been pointed out by
contributors to this database. However, in many areas, it seems that
the world's vegetation at 5,000 years ago was very similar to the
'present natural' vegetation that is mapped in atlases and ecology
textbooks (although some slight differences are usually apparent).
This is hardly surprising in some ways, for the idea of what was the
natural state of the world's present vegetation is sometimes inferred
from pollen records of whatever vegetation seems to have existed
before agriculture caused major landscape modifications. However, even
apart from this circularity it does seem that over much (but not all)
of the Earth's surface, climates around 5,000 years ago were very
similar to those of today (e.g. Crowley & North 1991, Williams _et
al_. 1993), and the vegetation that existed was very similar in
character to that which now survives in regions not subject to intense
agricultural activity, and in remnant pockets within otherwise mainly
agricultural areas.

By around this time, farming populations or cultures were spreading
rapidly outwards across various regions, probably from independent
centres of innovation (Tallis 1990). Although the vegetation of the
world had undoubtedly been influenced in some important ways by
preagricultural human activities (such as burning of semi-arid
vegetation by hunter-gatherers) since well before the LGM, clearance
for agriculture seems likely to have been of relatively localised
importance in terms of overall vegetation ecology and structure at
5,000 years ago. There certainly seem to have been some 'false dawns'
for extensive agricultural impact seen in the environmental record;
for example, the elm decline observed around 5,000 years ago in
north-western Europe (and earlier in southern and central Europe) was
initially ascribed to a sudden and widespread increase in animal
herding, but it now seems far more likely that disease and/or climate
was largely to blame (Rackham 1980, Huntley & Birks 1983).

There were some exceptions to the generalisation if one focuses in on
specific areas; the first irrigation schemes in Mesopotamia began at
around 5,000 years ago. In southern Greece, some large areas of
cultivation were probably present by 5,000 years ago, but even here
the main phase of forest loss and soil erosion began about 1,000 years
ago later (Tallis 1990). However, concentrating on general patterns
across broad areas of the Earth's surface rather than just taking the
few isolated exceptions, it seems that the overall impact of
agriculturists on vegetation at 5,000 years ago was still fairly
light. Tallis (1990) concludes that pollen diagrams from around the
world do not tend to show a significant deflection attributable to
agriculture until around 4,000 years ago, even in such agricultural
'cradles' as the middle east. This view seems borne out by my own
survey (the text below) of the literature and of the opinions of
palynologists. In many regions of the world, the first signs of
_extensive_ (rather than simply sporadic) modification of landscapes
by herders and farmers appear between around 4,500 and 3,000 years
ago, and these signs have increased towards the present.

The climatic shift that appears to have caused the 'elm decline' in
Europe was quite possibly part of a much more extensive episode of
cold or aridity that occurred around this time. Recent evidence from
the Greenland ice cores shows a sudden fall in atmospheric methane at
around 5,000 years ago (Chappellaz _et al_. 1993), probably reflecting
some temporary decrease in biological activity in tropical or boreal
latitudes. In the context of this event, the maps here should be
regarded as representing the system immediately before the onset of
the cold and/or drought event.
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The 'present-potential' vegetation. At the present, large areas of the
world have an extremely modified and fragmented semi-natural
vegetation cover (e.g. Olson _et al_. 1983, Milanova _et al_. 1994)
that makes it difficult to know what things could have been like if
agriculture had not arisen, and if human populations had not exploded
as they have done. Over large areas of Europe, there is nothing but
cropland with barely a tree in sight. Over much of India and
Bangladesh, the vegetation is so influenced by crop-growing, grazing
and fire that it is completely a matter of speculation as to what it
would otherwise be like (Milanova _et al_. 1994). Likewise at the
fringes of the rainforest zone of Africa and Amazonia, one sees
domestic cattle grazing and regular fires set to provide them with
fodder. Yet everywhere in atlases and textbooks one sees maps of the
'present potential'; the vegetation that was 'meant' to be here but
which does not in fact exist.

What is one to make of this? Can we really know what the present
vegetation would be like if human history had been different? The
answer is, both 'yes' and 'no'. In many areas, the vegetation seems
almost untouched by human influence, or at least we see no obvious
intervention occurring. In many other areas, there are enough
remaining fragments of what appears to be a sort of semi-natural
vegetation to allow plant geographers to suggest what the vegetation
would become like if humans were to suddenly be removed from the face
of the Earth. However, there are also many places that are obviously
too heavily grazed, burnt, trampled and ploughed for anyone to know
how far things would change without these influences. Sometimes the
only way that plant geographers can support their opinions is to refer
to pollen evidence from before the time of agriculture, making a
tangled web of the real past and the imaginary present.

What is the point of presenting such 'present-potential' maps here if
one knows that they represent a non-existent world? Their benefit
comes in the need for inter comparisons. At a basic level, the
widespread familiarity of 'present potential' maps - for all their
faults - allows any reader familiar with seeing such maps to use them
as an anchor point for comparison with the world of the past. There
are also other pragmatic reasons for using them. Very often within the
Quaternary literature, individual authors refer to the boundary
between one vegetation type and another having shifted by a certain
amount under Holocene or LGM conditions. To know where the starting
point is, one has to connect up the pieces of remnant vegetation, then
allow for where the authors think the natural vegetation boundary
would have lain, and from this draw a shifted boundary for the past.
In other areas, the necessary early or mid Holocene vegetation
evidence is completely lacking, so that the best one can do in the
reconstruction (other than either leaving the area completely blank or
assigning a completely arbitrary vegetation cover) is to throw
backwards in time a present-potential vegetation map and hope that it
is not too inaccurate, whilst making some alterations to allow for
broad climatic shifts that seem to have been present. This is what the
editor has been forced to do in some places here by the inherent gaps
in the data.

Hence, for the sake of comparison, and honesty, the editor has
presented here a vegetation map of the 'present-potential' world,
representing a compilation of the Olson _et al_. present-actual maps
and various regional present-potential vegetation maps (e.g. the now
famous vegetation maps of P.E. Preston-James in the Times Atlas;
1992). There has been no attempt to indicate which areas are subject
to most intense human activity through anthropogenic burning, grazing,
forestry, agriculture _etc_. Milanova _et al_. (1994) have produced a
comprehensive set of maps indicating the approximate intensity of
modification by humans, and by consulting these the reader can make up
his or her mind as to how much to trust the true accuracy of the
'present-potential' maps presented here. Generally, the greater the
human modification at present then the less trustworthy the
'present-potential' reconstruction actually is. Thus, whilst one can
be almost certain that the position of the forest - tundra boundary in
Canada is not primarily a result of human activity (at least in the
position it was in the 1950s, and before greenhouse warming begins to
act too strongly), one should strongly suspect that the forest-savanna
boundary in central Africa is very much a product of the agricultural
human populations that live throughout this zone.
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