3.0 Methodology

3.1 A top-downwards approach

Throughout the process of putting together the LGM map, the emphasis has been on a pragmatic top-downwards approach. Regional experts were consulted on the basis of their own published reviews and maps; these experts discussed the problems and uncertainties in the evidence with the authors, and recommended key papers for them to read. The authors then returned to each contributor in order to check that what has been written in the database is factual and reasonable. More extensive description and discussion of the methodology used here can be found in Adams and Faure (1997).

In summary, the LGM map is based on the following sources of data:

Sedimentological processes are often dependent on vegetation cover, either in the area where the sediment is being deposited or the area from which it is being eroded. A particular sediment grade or type of depositional structure can often give clues as to the type of vegetation that once existed there, although it is possible to be misled by processes in the past giving the same result by different means.

More tenuously and controversially, there are biogeographical clues based on the present-day distributions of animals and plants, which may be partly a legacy of the changed vegetation and climate conditions which existed during recent glacial phases, although not necessarily the most recent one. Generally, present-day biogeographical evidence must be regarded as taking lowest priority because it is the most ambiguous and the least direct. Here, in the debate that has given rise to the maps, biogeography has been used only to back up or dispute patterns suggested on the basis of palaeoenvironmental evidence, not as a primary source of ideas and opinions.

The approach used to produce the initial QEN maps and the maps presented here is thus highly interdisciplinary, in contrast to most previous attempts at broad-scale vegetation reconstruction for the Holocene and Last Glacial Maximum.

The problems of dating the evidence

An additional problem is that accurate dating is often lacking from deposits of palaeoenvironmental significance. This is especially so for well-oxidized or terrestrial sedimentary deposits such as sand dunes where there is little surviving organic matter that can be used for 14C dating. Fortunately the range of direct and correlative dating techniques is expanding (for instance, optical dating of the quartz in sand dunes is now becoming widely used), and the accuracy of existing methods is also improving. Nevertheless, many sites which have been taken as revealing conditions under the Last Glacial Maximum remain poorly dated or even totally lacking in any real dating control. Obviously, such sources of evidence must be treated with more caution than those which have been reliably dated, and the decision whether to accept them or not is ultimately subjective and, of course, potentially clouded by one's own preconceptions of what past conditions were like. The editors have, for the most part, left these difficult decisions to local experts who have themselves studied the Quaternary geology of each area, and have drawn the map boundaries on the basis of their detailed advice.

The drawing of geographical boundaries in relation to climate

In all published palaeovegetation maps based on point sources of data, extrapolation or interpolation based on knowledge of present-day vegetation-to-climate relationships have been important. Each piece of data relating to climate or palaeovegetation provides a basis from which to deduce the climate/palaeovegetation for other adjacent areas. To some extent, the maps presented here are based by proxy on the vegetation-climate relationships that individual contributing experts have assumed, and on the pattern by which they feel climate would have varied across the region.

Such factors are difficult to control and to describe, for the experts who contributed to each individual palaeovegetation map rarely state the assumptions which they are using. However, where such information is provided, it is included in the QEN database.

The boundary between tropical and temperate vegetation types is reconstructed on the basis of palaeotemperature estimates from various sources. The distribution of the tropical-temperate boundary is assumed here to follow the minimum monthly temperature limits detailed in the vegetation scheme at the end of this article. In fact, the precise position of the demarcation of 'tropical' from 'temperate' vegetation is fairly arbitrary if one looks at the problem objectively (see detailed discussion in Adams 1993), but there is no doubt that there is a general gradient in vegetation ecology and composition as one travels away from the equator. It is always necessary to draw the tropical versus temperate line somewhere, and in the maps here the boundary for most tropical vegetation types is set at a coldest mean monthly value of 10°C. This follows the general correspondence between global present-day poleward vegetation boundaries and the present-day temperature isotherms (e.g. see the Times Concise World Atlas (1992) which presents good general maps of both). For tropical rainforest vegetation, a somewhat higher temperature limit of 15.5°C (as the mean temperature of the coolest month of the year) seems to correspond well to the map boundaries drawn by various experts (I.C. Prentice, pers. comm.).

For the boreal-to-temperate transition, a more generalized definition based on the prevalence of cold climate conifers or birch woodland (Betula) is used as the basis for demarcating the boundary, and a similar floristic type of definition is used for tundra as distinguished from temperate steppe.

In drawing the palaeovegetation maps, the general assumption is made that the overall pattern of isotherms remained approximately the same, but that temperature was lowered (for the LGM) or raised (for the early Holocene) by a particular amount in each area. This seems generally reasonable with the exception of certain areas close to ice sheets or where large shelf areas had appeared: all general circulation model (GCM) reconstructions give generally similar qualitative spatial patterns to those existing at present, albeit shifted quantitatively, and that at around the outer boundaries of the tropics the temperature lowering at the LGM was at least 5.5° C (10°F) (e.g. Crowley and North 1991; Broecker 1995). Thus for the majority of vegetation types, the tropical-temperature boundary is moved equatorwards from the present 10°C (50°F) isotherm for the coldest month, to what is presently the 15.5°C (60°F) isotherm. Another important factor to include in extrapolating from scattered data points to produce a palaeovegetation map is the likely pattern of variation in precipitation. From finding evidence of a particular vegetation type having existed at a particular site in the past, it is possible to arrive at a rough quantitative estimate of precipitation for that place. From this it is reasonable to extrapolate the possible pattern of rainfall across a wider area, and to turn this back into an estimate of vegetation cover. If more than one data point is available, it is possible to interpolate between these. Extrapolation/interpolation of past rainfall patterns from proxy data points can be carried out using a numerical model (which may be coupled to a GCM). Although this brings with it an element of consistency, this is no guarantee that the model itself is correct, judging by the problems that are evident in many GCM models (see discussion by Crowley and North 1991, Kagan 1995 and Broecker 1995), and a certain degree of scepticism would be healthy.

Here, the approximate relative precipitation patterns are generally assumed to have been similar to those of today (the present rainfall distribution being taken from such sources as the Times Atlas (1992) and other regional climatological maps), but with a shift in the amounts. The amount by which rainfall varied is dictated by the indications from regional sources of terrestrial palaeoevidence, except where there is evidence to the contrary. Assuming this, the boundaries of palaeovegetation types are extrapolated across from areas where relevant data have been found, assuming a broadly similar distribution of rainfall maxima and minima to those which occur today. In fact, there is good evidence that even during the LGM, most areas that are relatively moist today tended to be relatively moist during the LGM (even if altered substantially overall to drier and cooler conditions, they were generally still moister relative to their surroundings). Hence, extrapolation along these principles seems perfectly reasonable in most areas (though not all; see the text of the QEN website for exceptions).

Web-based expert consultation as a source of information for the maps.

Once the preliminary version of the map was ready, it was made available online for review. Requests for comments on the map were sent out to approximately 20 Quaternary vegetation experts for different parts of the world. The global map was split into six regions, allowing reviewers to access their zone of specialization easily. An announcement of the availability of this review page was also made through the QUATERNARY list server (Canadian Research in Quaternary Science), to ensure a wide targeted audience. This process was very efficient, with much valuable feedback from palaeovegetation specialists. The comments helped us correct vegetation categories or extensions of certain areas, and revealed various additional literature sources


© Internet Archaeology URL:
Last updated: Mon Dec 3 2001