Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 47

corresponding to the occurrence of the earliest fossil seeds ( Archeosperma ). Pollen
and spore walls of each plant species have a distinctive shape with characteristic
apertures and these can be used to identify the types of plants represented by pollen
in a sample from a given site. A microscope must be used for such identification
because pollen and spores are small, typically between 10 and 200 m. First, though,

pollen and spores must be isolated from sediments and rocks using both chemical and
physical means, before they are mounted on microscope slides for examination and
identification. For most geological and environmental applications of pollen and spore
analysis scientists count and identify grains from each sample using a microscope
and generate pollen diagrams of the relative (percentage) and absolute abundance of
pollen in samples from a site's stratigraphic sequence (the geological age column): it
is not just the presence or absence of a species used in this form of palaeoclimatology
that is important, but its abundance. Typically, the results of spores and pollen analysis
from several species are pulled together to establish a picture of the changing plant
ecology, and hence palaeoclimate, of a site.
Whereas dendrochronology provides an annual indicator of climate, pollen and
spore analysis does not have such a high resolution. In some circumstances, spe-
cifically in well-dated sediment cores of high resolution, it is possible to obtain
high-resolution records of botanical change at a decadal scale or less (in some cases
to within a year) and also to document community changes over the last few centuries
and even millennia. However, this is rare. Typically the resolution achieved is of
several years or even a few decades. Even so, it is important to bear in mind that the
presence or absence of pollen or spores is not a hard-and-fast identifier of climate, as
other ecological factors need to be considered.
For example, if the climate warms it will take a little while for a new species
to establish itself, especially if the climate change is marked and sharp, so often
necessitating species migration over a long distance. Indeed, there may also be other
factors impeding migration. Suppose climatic cooling affects a northern boreal forest;
even if the cooling is not marked enough to kill any trees, some species, such as black
spruce, may stop growing cones and producing pollen. Black spruce pollen will then
be absent from the record even though the species is as abundant as it was before,
and it may even multiply by the growth of new roots where drooping branches touch
the ground; so the trees will not die out even if they cannot seed. Yet, simply by being
there they are impeding the arrival of new species whose pollen might be a better
indication of the climate.
Aside from resolution and ecological problems, other difficulties with pollen and
spore analysis as climate indicators include those of human land use. Humans tend
to clear land for various purposes, from intensive urban landscapes to semi-natural
systems such as agriculture. Either way, the natural community is no longer there.
Consequently spore and pollen analysis to elucidate past climates is best from sites
in areas where there has been minimal human interference. (Although, of course, this
particular problem can be turned on its head with pollen and spore analysis being
used for archaeological purposes as an indicator of human activity.)
One also needs to recognise the problem of pollen decay. Pollen of some species
are less resilient than others. For example, white pine pollen is resilient to breakdown
whereas larch ( Larix spp.), aspen ( Populus spp.) and poplar (also Populus spp.) pollen

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