Geoscience Reference
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50
58
Size of pollen grains (
μ
m)
Fig. 2.2
Thevariationinsizeofpollengrainsfromtwodiferentspeciesofthesamegenusmayoverlap,impeding
identiication.Shownaredataforlodgepolepine(
Pinuscontorta
),withanaveragepollen-grainsizeof
50
m.Statisticalanalysisofthediference
ofmeansofasampleagainstthosefromaknownspeciesmayberequiredforidentiicationpurposes.
m,andwesternwhitepine(
Pinusmontecola
),ofaveragesize58
μ
μ
decays quickly. So, a sample of fossil pollen dominant in spruce (
Picea
spp.) could
have come from a pure spruce forest or a mixed forest of spruce and poplar; there is no
way of knowing. This means that pollen analysis rarely, if ever, provides a complete
picture of a past ecosystem. Furthermore, some sites are better for conserving pollen
and spores than others. The cold, acidic, anaerobic (oxygen-deficient) water of a peat
bog is a better environment for preservation than warm, eutrophic (nutrient-rich)
waters where bacteria thrive.
Considerable expertise by those who study pollen, palynologists, is needed to
correctly identify which species particular pollen comes from. For example, the
pollen grains of junipers (
Juniperus
spp.) and arborvitaes (or cedars,
Thuja
spp.) are
indistinguishable. In other cases, although it may be possible to identify the genus of a
pollen grain, it may be difficult to tell the species. Sometimes the grains from different
species belonging to the same genus are identical except for a slight difference in size.
Furthermore, pollen grains from a single species do have natural variability in size
and if the range from a single species overlaps with the range of another species in
the same genus, say lodgepole pine (
Pinus contorta
) and western white pine (
Pinus
montecola
), the identity of the grain becomes uncertain. In fact these two species
have average pollen-grain lengths (excluding bladders) of 50 and 58
μ
m, respectively
±
(error,
m). In difficult instances where there is doubt a statistical analysis of the
difference of means is required (see Figure 2.2).
Consequently pollen and spores as palaeoclimatological indicators really only
come into their own as a means of confirming (but not necessarily disproving) the
picture portrayed by other palaeoclimatological proxies, be they local or hemispheric,
and in this sense can assist in helping ascertain not only whether there was genuine
climatic change of such-and-such a degree but whether it was a local variation or part
of a more global climatic change. It is arguably better (as with species; see below) to
think of pollen-and-spore evidence not in terms of climatic proxies but as evidence
contributing to a picture of the effects of climatic change. We shall return to this
later when discussing past ecosystems as being palaeoclimatic ecological analogues
8
μ
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