OFR 151.pdf - CRC LEME

2. Principal components analysis (PCA)
This technique is widely used in ecological and palaeoecological research to identify groups
of associated or correlated taxa (Birks and Birks 1980, Birks and Gordon 1985). The
appropriate routine is included in statistical software used to generate Late Quaternary pollen
histograms, for example Tilia.
PCA can be useful in the reconstruction of both Cretaceous and Tertiary palaeoclimates in
two ways. Firstly, it can help link taxa whose ecology is uncertain with taxa whose climatic
preferences have been established by other means. Secondly, it can help identify areas with
similar bioclimates by identifying recurrent groups of taxa.
3. Biome analysis
The biomization method was developed to resolve problems arising from direct comparisons
of Holocene and Quaternary palaeoecological data from very widely separated sites. Rather
than focus on shared taxa (if any), the technique utilises the well-known phenomenon of
functional convergence in the ecophysiology of unrelated plants growing in similar physical
environments. This convergence may be expressed at the molecular or whole plant level.
Examples are the evolution of the C4 photosynthetic pathway in unrelated plant families
occupying drier, warmer habitats and the evolution of a drip-tip on the leaves of many
unrelated rainforest angiosperms.
The method involves: (1) assigning plants to plant functional types (PFTs); (2) defining
biomes in terms of the PFTs of the constituent taxa; and (3) using observed bioclimatic
relationships to reconstruct patterns of climate at a given time in the past. Mathematically,
the method involves calculating the affinity (affinity score) of each fossil flora to each biome
defined by PFTs and assigning the flora to the biome (and climate) with which it has the
highest affinity (references in Prentice and Webb 1998). The formula used for Holocene
microfloras combines presence/absence and relative abundance data weighted by threshold
values, which take account of the different representation of the parent plants by pollen or
spores. Prentice and Webb (1998) discuss how the technique can be applied to the mid
Holocene. Horrell (1991) has used an analogous approach to reconstruct global climatic
zones for the Maastrichtian.
Holocene floras are a special case in that almost all microfossil taxa can be assigned to plant
functional types in the certain knowledge that the fossil plants possessed the same
ecophysiological adaptations as living representatives. Only ~200 of the >300 described
Tertiary fossil species have presumed NLRs whilst many more undescribed taxa have not
been systematically recorded because of small size, simple morphologies and lack of (known)
biostratigraphic value. Nevertheless sites that preserve both plant macrofossils and
microfossils confirm the general applicability of modern relationships (phylogenetic and
ecophysiological) to Tertiary situations (Macphail et al. 1994), even if this may not be the
case for the earlier Cretaceous floras. Accordingly, with appropriate resources it would seem
possible to develop a mathematical model along the lines of Prentice and Webb's (1998)
formula, which could assign Tertiary and possibly Late Cretaceous floras to biomes defined
by PFTs. Logically, any formal quantitative analysis would be empirical and begin with
either the Pliocene microfloras, because of the significant number of taxa with moderately
well known biome relationships, or the Oligo-Miocene macrofloras, because their
ecophysiological adaptations to primary forcing factors such as humidity can be directly
observed.
The method reduces often very large numbers of fossil taxa into more manageable ecophysiological
types using the basic physiognomic characteristics of life-form, leaf-form and
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