Forensic Science International 163 (2006) 173–182
www.elsevier.com/locate/forsciint
Consideration of some taphonomic variables of relevance to forensic
palynological investigation in the United Kingdom
Patricia E.J. Wiltshire *
Department of Geography & Environment, University of Aberdeen, Elphinstone Road, Aberdeen AB24 3UF, United Kingdom
Received 21 June 2006; accepted 13 July 2006
Available online 21 August 2006
Abstract
Palynology is a long established and respected branch of environmental science that has been applied to criminal investigation in a meaningful
way only in recent years. It has proved to be remarkably versatile in many kinds of criminal enquiry. It is not, however, an absolute science;
palynological data are on a par with the suites of symptoms which allow medical practitioners to make diagnoses. Taphonomic variability is the
main factor complicating interpretation of forensic palynological data. Palynological taphonomy may be defined as ‘‘all the factors that influence
whether a palynomorph (pollen, spore, or other microscopic entity) will be found at a specific place at a specific time’’.
If taphonomic variability is anticipated, and regularly tested, palynology will continue to keep its place in the armoury of useful forensic
methods. Some assumptions made by palynologists engaged in palaeoecology and archaeology have been shown to be untenable in the forensic
context. Palynological and botanical profiling of crime scenes has demonstrated anomalies which challenge received wisdoms. It has proved
impossible to obtain palynological population data because every site is unique—expectations of any palynological profile can only be crude. The
palynological status of any place must be tested every time. Without a body of analytical data from the actual crime scene, it is difficult to see how
any palynologist can hope to present credible arguments under cross-examination. The statements made in this paper relate mainly to work carried
out in the United Kingdom.
# 2006 Elsevier Ireland Ltd. All rights reserved.
Keywords: Palynology; Pollen; Taphonomy; Crime scene; Airspora; Shoes; Clothing; Vehicles
1. Introduction
The application of palynology to criminal investigation has
achieved momentum only in recent years. Those of us (and we
are few), who are routinely engaged in forensic investigations
have only our training, experience, and enthusiasm for
continued and painstaking research to enable us to be of real
practical help to the investigating authorities. None of us was
trained as a forensic palynologist, and there are no established
courses in forensic palynology. This is a subject not easily
taught as it requires a great deal of botanical, ecological, and
other environmental experience. The palynologist also needs to
have the personal resources necessary to cope with the
challenges presented by the police and the witness box. The
value of the forensic palynologist thus depends on the
individual skill and experience of the practitioner. As yet,
* Tel.: +44 1372 272087; fax: +44 1372 278934.
E-mail address: [email protected].
0379-0738/$ – see front matter # 2006 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.forsciint.2006.07.011
there is no absolute benchmark for validation. Accreditation
comes from repeated achievement of outcomes that prove to be
of real value in criminal investigation—ones that are
scrutinised by peers employed by the Defence, yet pass the test.
All competent palynologists, whatever their basic environmental training, will have knowledge of the literature that will
aid interpretation of their data. However, when viewed
critically, there is surprisingly little published information
that is of practical value in the forensic context. Much of it is
highly theoretical [1], or based on remarkably small data sets
[2,3] in which case they would certainly be insufficient to stand
cross-examination in the British courts. To date, the most
useful papers are those which demonstrate principles through
case studies [3–5]. Even here, the actual numerical and
statistical data presented are sparse because the authors are
attempting to present general principles. However, because of
the dearth of results, they are often difficult to evaluate
critically.
Palynological data are probabilistic, and their quality is
constantly under scrutiny in British criminal cases. Generally,
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little credibility is given to analyses where data are few but, in
some cases, the results are clear and provide compelling evidence
[6]. In others, even where extensive results are obtained, they can
be so complex and varied that they defy interpretation. Whatever
the nature of the case, standard protocol should ensure that
sufficient comparator (control) samples are collected to facilitate
an adequate understanding of palynological patterning at scenes
of crime, and/or the palynological profiles associated with
critical items. A comparator sample can be taken from any
surface, matrix, or object which might have been contacted by
any exhibit being investigated. Palynomorph assemblages from
comparator samples are for comparison with those from the
pertinent exhibits. Some investigations are highly complex and it
is often necessary to obtain comparator samples from surfaces,
matrices, or objects other than those associated with the crime
scene itself. This is because of the phenomenon of multiple
deposition of palynomorphs during normal use or wear of the
exhibit. Such ‘‘innocent’’ comparators are necessary for
elimination purposes.
Over the last few years in Britain, forensic palynology has
helped to:
1. Link offenders or their belongings with places and other
objects (trace evidence).
2. Predict the location of places from the pollen assemblages
associated with suspects or their belongings (search and
location).
3. Estimate the length of time human remains, or objects, have
lain in situ.
4. Differentiate murder sites from the places victims are
dumped.
5. Give information on the peri-mortem fate of victims.
6. Provide limited temporal information on post-mortem
interval through gut analysis.
7. Establish food intake prior to death through gut analysis.
After being engaged in forensic palynological investigations
in all parts of Britain for many years, the author has collected a
large database of palynological profiles from very many
comparator samples taken from crime scenes, and other places
pertinent to specific criminal cases. This record also includes
profiles obtained from a wide range of objects, and from human
remains. While there are too many data to present here, some
idea will be given of the extreme spatial heterogeneity of pollen
and spore assemblages.
The body of results accrued over the years has made it clear
that the only truly predictable aspect of the spatial patterning of
pollen spectra is its unpredictability. This is due to the mass of
taphonomic variables inherent in the pollen and spore
accumulation in any natural or man-made environment. No
crime scene can be taken for granted. It must be evaluated both
by eye, and through analysis. Furthermore, very little accurate
prediction can be made of the profiles that may be retrieved
from various objects and from cadavers. The success of
palynology in forensic investigation depends on rigorous
analysis, the botanical and ecological experience of the analyst,
and the cooperation of investigating officers.
In terrestrial environments, spores are released from the
sporangia of mosses, liverworts, and ferns (and their allies).
Pollen grains are released from the male cone sporophylls in
conifers, and from the anthers of flowering plants. The spore
needs to land on a suitable substrate for germination and
development of the next generation. The pollen grain’s function
is to reach the female and facilitate fertilisation of the ovule;
this leads to seed production. The spores of mosses and ferns,
and the pollen grains of conifers and flowering plants, lack
motility and are carried away from the sporangium or anther by
vectors [7–10].
The method of dispersal evolved by any plant species must
ensure that sufficient spores or pollen will reach their functional
destination; dispersal mechanisms will, therefore, influence the
level of production. Successful plant spores will germinate to
form new generations of mosses and ferns. Successful pollen
grains will reach the female members of the species and fertilise
the ovule within the ovary. But the vast majority will simply
land on the ground (or some other surface) and die very quickly.
The dead spore or pollen grain might decompose and disappear,
or remain intact for variable lengths of time.
Palynological taphonomy may be defined as all the factors
that influence whether a palynomorph (pollen, spore, or other
microscopic entity) will be found at a specific place at a specific
time. It is obvious that this is highly complex and involves a
multiplicity of variables, including those discussed below, such
as production, dispersal, and survival. For any one situation,
many of these variables will remain unknown.
2. Production and dispersal
Obviously, the numbers of spores and pollen grains
produced by plants, and their mode of dispersal, will affect
the frequency with which they are encountered away from the
parent plant. Most pollen is transported various distances away
from donor plants and, in temperate ecosystems, the main
vectors are insects (entomophily), and wind (anemophily).
A great deal of effort has been made to gain some insight
into the amount of pollen produced by various species of plant.
Some workers have estimated the absolute numbers of grains
produced in each anther, by each flower, or shoot [11,12] whilst
others have estimated relative pollen production of plant
species and even attempted to apply correction factors for the
various plant taxa [13–15].
Generally, pollen production in entomophilous taxa is
relatively low, and the plants’ resources are channelled into
features which encourage insect vectors. These involve
production of nectar, perfume, colour, and a tendency towards
zygomorphic flowers to support, or even trap, the insect
during its visit. Some, such as Salix (willow), have exposed
anthers so that wind can also play an additional role in
dispersal, but the reliance on insects of others, such
Hyacinthoides non-scripta (bluebell), and Digitalis purpurea
(foxglove) means that their pollen rarely, if ever, moves far
from the immediate vicinity of the plant [16]. In plants such as
Aristolochia (birthwort), Arum (arum lily), the Orchidaceae
(orchids), and others, their extreme adaptation to entomophily
P.E.J. Wiltshire / Forensic Science International 163 (2006) 173–182
means that their pollen is seldom out of contact with the donor
plant, recipient plant, or the vector. It might only be released
to the ground when the flower dies and decomposes. If the
pollen of such plants were found in a forensic context, it could
be of significant relevance. Certainly, the pollen of insectpollinated plants often provides excellent forensic indicators
of contact.
Because of the high risk of failure in locating the female
stigmatic surface, anemophilous plants such as Pinus (pine) and
Poaceae (grasses) produce very large amounts of welldispersed pollen. It is not surprising, therefore, that they are
often well represented (even over-represented compared to the
absolute abundance of parent plants) in the air, especially
during peak periods of their respective pollen release. Unlike
insect-pollinated taxa, the morphology of such plants has
evolved to ensure free dispersal of pollen away from the parent
without impediment.
Even though wind-pollinated plants can be over-represented in the air and on the ground, they may still be important
markers in forensic investigation. Certainly, if there is a large
amount of pine pollen in a sample, it is highly likely that there
is at least one mature pine tree close to the pollen site. When
prolific pollen producers are represented at low level, their
presence is sometimes considered to be a ‘‘long distance’’
component, and relatively unimportant. Dispersal characteristics of individual taxa within specific stands of vegetation
have received attention by many palaeoecologists [17,18]
and, over the last 40 years, a very large body of data has been
assembled from various parts of the world. However, in some
of these studies, data sets from disparate investigations have
been amalgamated, while in others the pollen was collected
over very large areas with relatively few samples in each
specific stand of vegetation. Even where an attempt was made
to address pollen dispersal over time in a relatively small area,
the sample interval was crude (only 3 sample sites over
1.5 km) [19]. To be of use in the forensic context, sampling
frequency needs to be high. In a series of studies with
students, Wiltshire (unpublished) showed that, at two sites in
different parts of the United Kingdom where surface samples
were taken at 5.0 m intervals along transects, pine pollen
dropped from values in excess of 80.0% of total pollen and
spores (TPS) to about 4.0% within 50 m of the woodland
edge. Pine could hardly be regarded as a ‘‘long distance
component’’ here since there were dense stands of mature
trees within a very short distance of the pollen site. However,
the pollen of some plants is certainly known to travel very
considerable distances from source. For example, Ambrosia
pollen was found in deposits on the Isle of Skye in the
Hebrides [20]. The only possible source was the USA. It has
also been found in organic deposits at the top of the foreshore
of the River Mersey in Liverpool, United Kingdom [21]. In
the forensic context, these occurrences are so rare that they
have little relevance for most criminal investigations.
Whether the vector is wind or insects, in every case most
of the pollen produced falls near the parent; and local
obstacles and obstructions have enormous impact on the
distance travelled by the pollen grain.
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2.1. Relevance of pollen production and dispersal in the
forensic context
It is useful for the forensic palynologist to have some
knowledge of pollen production, especially where an investigation involves searching for important places such as clandestine
burial sites. Obviously, if an exhibit which had had contact with
a specific place had very high levels of a certain pollen taxon, it
might be assumed that the plant was relatively abundant in the
place in question. This may or may not be the case but, in the
absence of any other information, it would be the only
intelligence available to the scientist trying to find the location.
But, the practising forensic palynologist must be aware of
anomalies, and much of the information regarding pollen
production (considered to be of paramount importance by
palaeoecologists attempting to reconstruct ancient environments) is largely irrelevant to forensic investigation; accurate
information on the pollen production of any plant species is
unimportant. Furthermore, other than a broad understanding of
trends and tendencies, published data on pollen dispersal are
also largely irrelevant. It is the representation of taxa in the
whole pollen assemblage at the crime scene (or other pertinent
place) that is critical. Knowledge of this can only be gained by
vegetation survey, and adequate collection of comparator
samples from the relevant locations. A comprehensive analysis
of targeted comparator samples at the crime scene will give the
forensic palynologist an accurate picture of the within-site
heterogeneity in pollen patterning. Therefore, a planned and
adequate sampling strategy (as early as possible in an
investigation) is vital. The forensic palynologist cannot rely
on theoretical models for interpretation. Data must be obtained
which demonstrate, as comprehensively as possible, the
palynological profile at the crime scene.
The following recent criminal cases exemplify some of the
anomalous situations that could confound interpretation.
In a murder case in Wales [22], the victim was found buried
in a shallow grave in a dense plantation of Picea sitchensis
(Sitka spruce) at an upland location. It was chastening to find
that the comparator samples contained less than 1.0% spruce
pollen, even though hundreds of hectares of the landscape were
planted with this tree. Indeed, pine pollen was far more
abundant in the comparator samples, in soils both from the
surface and within the victim’s grave. Forestry records showed
that the woodland had been planted less than 40 years
previously so that most of the trees had not reached sexual
maturity. The large amounts of pine pollen in the surface soil
were derived from a single, small (though mature) pine tree
growing about 100 m from the deposition site of the victim. The
pine pollen in the grave profile could have been derived, at least
in part, from the single pine but also from trees growing in the
catchment before the spruce was planted. If the grave had been
sought from assemblages yielded by the footwear and spades of
the suspects, the pollen percentages would have provided a very
false picture of the extant vegetation at the site.
In a case in Birmingham, United Kingdom [23], a young girl
had been found dying in an alleyway opposite a schoolyard and
garden. Macroscopic leaf fragments from her hair and shoes,
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and the same evidence from the shoes of the accused, showed
that they had both stood, and been on the ground, close to a
young (but mature) Fagus sylvatica (beech) tree. There were
abundant beech fruits on the ground so it was obvious that the
tree had produced pollen (there were no other beech trees in the
vicinity). It was remarkable to find that none of the comparator
samples taken from that end of the garden contained even a
single beech pollen grain although other plants growing there
were well represented in the profile.
A similar situation prevailed inside a stand of mature Acer
pseudoplatanus (known in UK as sycamore) in Telford,
Shropshire [24] where there was an attempted rape of a young
girl. None of the comparator samples in, or immediately
adjacent to, the crime scene contained any Acer pollen grains at
all. In spite of the very large number of sycamore trees, only a
very low percentage of sycamore pollen was found on the
grassy area outside the stand of trees. It was even more
surprising that the comparator samples within the wooded area
were dominated by pine pollen (>60% total pollen). This had
been derived from a young pine tree growing slightly up a slope
on the edge of the sycamore woodland. In view of the dense
canopy, the pine pollen had probably blown in through the
sycamore trunk space. The observed assemblage could never
have been predicted, especially if the palynologist had not been
given the opportunity to visit the crime scene. An offender was
never identified and the case was not pursued further by police.
Another example of a false impression gained of the local
landscape by pollen assemblages came from comparator
samples collected from a badger sett near Stafford [25]. The
sett had been constructed very close to a boundary hedge in a
species-rich pasture. The landscape was largely open with
many small pastures and crop fields bounded by ancient hedges
of mixed, deciduous trees and shrubs. Immediately next to the
sett, the hedge had had remained untrimmed for some time and
consisted of Crataegus (hawthorn), Lonicera (honeysuckle),
Sambucus (elder), Acer campestre (field maple), A. pseudoplatanus (sycamore), Hedera helix (ivy), Rubus fruticosus
(bramble), and Rosa canina (dog rose). There were extensive
stands of Pteridium aquilinum (bracken) growing with and
through the bramble. Most significantly, there were two large
Quercus robur (oak) trees in the hedge, each about 60 m away
from the sett, and 120 m from each other. The comparator
samples yielded a profile that might have been mistaken for a
mixed, deciduous woodland. Although herbs were recorded,
the profiles were dominated by oak and the woody plants in the
hedge. Flowering of pasture plants was much diminished
through heavy grazing by stock animals, and the untrimmed
hedge provided a woodland signal. Again, without a field visit,
the palynologist could have been forgiven for providing
completely spurious information.
In cases where investigators are desperate to gain information on the location of a clandestine grave, and they have items
belonging to a suspect, the pollen assemblages retrieved from
those exhibits provide the only intelligence for finding the
human remains. In this kind of case, any information might be
valuable but, at all times, the palynologist must keep in
mind the unpredictability in production and dispersal when
interpreting data. There have certainly been cases where the
palynological profile obtained from offender’s belongings
have allowed an eerily accurate picture of the place where a
victim has been dumped [26], but there have been others where
it has proved impossible to envisage a location precisely even
though the nature of the landscape could be described in broad
terms [27].
2.2. The value of predictive modelling in forensic context
The mixture of spores and pollen grains in the air is known as
the ‘‘airspora’’ and it is envisaged as falling as ‘‘pollen rain’’.
The airspora tends to be dominated by plant taxa which exploit
turbulent air for dispersal; many trees and shrubs, but also
considerable numbers of herbaceous plants, are wind-pollinated and are frequent components of the airspora. The study of
airborne palynomorphs is fraught with difficulty, and there can
be very great variation in results. This phenomenon has been a
matter of study, modelling, and debate between palaeoecologists for many years [28–34], and it is becoming increasingly
clear in more recent research projects (such as the Pollen
Monitoring Programme and POLLANDCAL [35–38]) where
modern pollen data are being used to generate predictive
models. Just as with the early work on pollen productivity and
dispersal, current research is concerned with obtaining
information to enhance interpretation of pollen diagrams
covering long periods of vegetation history. In an attempt to
interpret past vegetation more accurately, it aims to relate
patterns of pollen deposition on the ground to modern
vegetation.
Researchers in the Pollen Monitoring Programme have set
themselves a huge task, even for the ‘‘broad brush approach’’ in
modelling at the landscape level. They use various kinds of
pollen trap to estimate pollen influx (number of grains
cm 2 year 2) for comparing estimates of the airspora at the
various sampling sites. Sample size is strictly standardised so
that meaningful comparisons can be made between estimates.
This quantitative method is considered to provide a more
objective basis for interpreting vegetation in the environs of the
trap.
Essentially, there are two main kinds of trap (and modified
versions of them)—the Burkard spore trap (modified from the
Hirst spore trap), and the Tauber trap (and its derivatives). The
Burkard trap is a volumetric instrument; measured volumes of air
are drawn in for known periods of time and the airborne
palynomorphs adhere to a sticky surface. The relative abundance
of palynomorph taxa in the airspora can then be compared from
place to place for specified periods. The Tauber trap is a nonvolumetric sedimentary sampler and relies on gravity to evaluate
the palynomorph composition of the air. Again, fallout from the
airspora can be compared from place to place. Details of these
mechanisms, and their relative limitations and merits, can be
obtained from various sources [39,40]. In essence, trapping
efficiency depends on wind velocity, relative size and mass of
particles, and size of trap aperture; there can be considerable bias
in the results depending on the kind of trap employed.
Furthermore, to allow comparability with other similar data
P.E.J. Wiltshire / Forensic Science International 163 (2006) 173–182
sets, traps must be placed in situations where there can be little or
no impediment to free flow of air, or free fall of palynomorphs.
It is vitally important to remember that, for reasons outlined
below, these kinds of study have little relevance to forensic
palynology.
2.2.1. Absolute counting techniques
Forensic data need to be expressed as percentages of the total
pollen and spores counted in a sample. In most cases, it is
impossible to obtain samples of known area, volume, or
concentration from police exhibits, and the forensic palynologist simply has to retrieve as much of the pollen load as
possible from the item. It would certainly be difficult to
standardise a pollen sample from footwear, a fur coat, a vehicle
foot well, or a pair of trousers. Therefore, since they demand
samples of quantifiable size, so-called absolute counting
techniques such as volumetric [41–43], weighing [44], and
exotic marker [45–47] methods, and estimation of pollen influx,
have had little or no value in forensic investigation.
2.2.2. Partial assemblages
Another reason for the limited value of the Monitoring
Programme to forensic palynology is that the database is
restricted to influx values for the major trees and ecologically
significant shrubs and herbs; the aim of the Programme is to
evaluate pollen deposition across transition zones between
forests and open landscapes. In forensic palynology, every
taxon at a crime scene has potential significance; wherever
possible, the presence of each one (either as a growing plant or
as a palynomorph in a comparator sample) must be noted.
Furthermore, obtaining influx data from comparator samples at
crime scenes, and other locations pertinent to the criminal
investigation, would be both impossible and irrelevant. Rather
than trying to estimate what might have fallen from the airspora
in the period during and after the crime has been committed, it
is important to know as comprehensively as possible the nature
of the assemblage transferred to an offender’s belongings. This
assemblage might have accumulated on a surface over many
years.
3. Other factors affecting palynological heterogeneity in
forensic samples
Many years of experience have shown that pollen and spore
assemblages at ground level are exceedingly heterogeneous and
unpredictable [48], especially in urban and domestic environments. This is a function of the contribution made from the
pollen rain at any one time of year, but mostly of the many years
of accumulation on the ground surface. The airspora will
change from day to day and year to year. In most cases, that on
soil, vegetation, and litter will yield a cumulative sample. This
means that pollen loads in soil and on vegetation will exhibit
less temporal variation than that observed in the pollen trap.
Experience has also shown that any area of ground will have
a unique palynological signature because of the multiplicity of
taphonomic factors affecting the persistence of the assemblage:
(a) accumulation history, (b) potential for promoting or
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inhibiting decomposition of palynomorphs, (c) effects of
precipitation, rain splash, and wind (d) action of animals, (e)
effects of people and barriers, and (f) many other unknown
taphonomic factors.
Obstacles will certainly affect palynological assemblages in
samples taken from traps or the ground [49]. Stands of
vegetation, walls, fences, or banks will distort airflow and bias
the results obtained from comparator samples at crime scenes.
Vegetation can also act as effective filters of pollen laden air,
while palynomorphs can become impacted onto walls, tree
trunks [50], and other obstacles. It has been shown that samples
from a wall can reflect the local vegetation better than those
taken from adjacent ground [51]. The effect of barriers on the
palynological profile at any one site is unpredictable.
4. Persistence of palynomorphs in soil, sediments, and
on vegetation
The persistence of pollen grains and spores on surfaces, or
within soils and sediments, depends on ambient conditions and
the robustness of the spore/pollen outer wall. This appears to
depend on the relative amounts of sporopollenin in the
outermost part of the pollen wall (the sexine). A number of
studies have shown that some pollen taxa are more vulnerable
to decomposition than others [52,53]. Some decompose so
quickly that there is little likelihood of them being found even
in comparator samples. These include the Juncaceae (rushes)
and Orchidaceae (orchids). Others such as the spores of
Lycopodium (clubmoss), Polypodium (polypody fern), Alnus
(alder), and Tilia (lime) appear to be much more robust and
resistant to decay; where conditions promote decomposition,
they are often over-represented and biased profiles can be
obtained. In recent years, frequent analysis of modern soils has
demonstrated that observed effects do not always agree with
published results. In forensic palynological study, an open mind
must be kept at all times; it is imprudent to adhere strictly to the
findings in published experimental work when interpreting
forensic material. They must only be regarded as a general
guide.
Where microbial activity is inhibited (for example by low
water potential (pF), low redox potential (Eh), and extremes of
pH), palynomorphs can persist for many thousands of years.
Pollen and spores are preserved in sedimentary rocks, ancient
sediments, lake and riverine sediments, peat and fen deposits,
and palaeosols. Indeed, the resistance of pollen and spores to
decay has allowed the reconstruction of prehistoric landscapes,
and the palaeoecological literature is very extensive. In the last
20 years or so, a very large body of publication has also accrued
from analysis of archaeological buried soils and features such
as pits, ditches, and water holes. It has been possible to
reconstruct not only a picture of ancient landscapes, but also
past farming practices and diet [54–59], and even ancient
shipwrecks [60]. Palynology has also provided information
about prehistoric cadavers [61], and even indicated the use of
medicines by Iron Age medical practitioners [62]. But,
preservation of palynomorphs in the forensic context can be
problematical.
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In every case, good preservation is only possible when the
physico-chemical environment within the matrix is inhibitory
to micro-organisms, particularly to bacteria such as the
Actinomycetes; these have the enzymes necessary for
hydrolysis of highly recalcitrant molecules [63,64]. Although
no sporopolleninase has yet been identified, its presence in
some of the Protista is certainly implied [65]. It would seem that
conditions which are conducive to pollen decay are the ones in
which Actinomycetes and other bacteria thrive.
In well-aerated, microbially-active soils, a whole season’s
load of palynomorphs can disappear within 4 months (or
possibly less time). For example, in Sussex, southern England,
the soil of a fertilised, ploughed field under wheat yielded a rich
assemblage of pollen in July; the pollen spectra reflected the
immediately local vegetation very closely. In February the
following year, further sampling, at the same locations within
the field, showed that all palynomorphs had decomposed in the
intervening period; not even corroded grains were observed in
the preparations [66]. On the other hand, a soil profile, under
Carpinus betulus (hornbeam) coppice in Hertfordshire, yielded
such a rich, well-preserved palynological assemblage (from the
surface to a depth of 50 cm) that detailed vegetation history of
the site could be constructed [67]. Comparison of the soil
profile’s pollen sequence with historical records indicated that
the soil had preserved at least a 100 years of vegetation history
at this crime scene. In contrast to the soil in the Sussex wheat
field, that in the Hertfordshire hornbeam coppice was an acidic,
nutrient-poor, tannin-rich compacted clay where microbial
decomposition was inhibited. It was of considerable interest,
therefore, that a subsequent experiment at the site, which
involved digging a grave and enriching the soil with a pig
carcass, created ‘‘islands’’ of intense bioactivity and pollen
decomposition [68–70].
Whether palynomorphs will be present in soils can
sometimes be deduced by the vegetation growing on them.
If the plants are mostly nutrient-demanding and favour light,
well-drained soils, the chance of good preservation is much less
than if the plants indicate an acidic, water-logged, badly aerated
conditions.
4.1. Residuality
It has been stressed that Burkard and Tauber traps are of little
value in most criminal investigations where knowledge of the
residuality of current and previous seasons’ palynomorphs on
surfaces (including those of soil and vegetation) is more
important. Soils, moss polsters, leaf litter, and surface foliage of
growing plants, including grassy turf, are all useful for studying
pollen assemblages at any one place [71,72]; they have the
advantage of representing longer periods of pollen rain than
traps. However, each kind of sample material will have inherent
biases for both the range and abundance of palynomorph taxa
[72].
Depending on the vulnerability or recalcitrance of the
individual pollen taxa at the pollen site, more than one, and
sometimes very many, years’ worth of pollen fallout can be
retrieved from moss polsters. Leaf litter can give many months’
record, while that yielded by surface vegetation (such as hedge
foliage or grassy turf) will depend on frequency of removal by
mowing, cutting, or even grazing [73,74]. In this case, the
palynological profile will represent the fallout between the
periods of removal. This can be of considerable advantage
when temporal aspects are relevant to an investigation. The
abundance of pollen is usually much lower in such samples, but
when taken between mowing, they can provide specific
information for the period in question. If the cutting frequency
is known, the pollen profile for that period can be measured
[75]. However, the results obtained from young vegetative
growth can be seasonally variable and, in much of Britain,
particular care must be taken when formulating sampling
strategies at crime scenes, especially in spring/early summer,
and where wind-pollinated trees are present in the catchment. It
has been found that pollen assemblages obtained from grass and
other plant clippings can exhibit dramatic differences within 1–
2 weeks, and that these effects can vary over relatively small
distances [75]. This means that sampling delays may cause
problems for comparing the palynological profiles from
exhibits with those from crime scenes.
Even if a flowering plant has been contacted and one taxon is
dominant in the palynological profile from an exhibit [76],
other taxa will be picked up; and although there are exceptions,
the offender rarely picks up only the current season’s pollen
deposition. The persistence of palynomorphs deposited from
the pollen rain will depend on their susceptibility to
decomposition. However, if they land in environment where
ambient conditions inhibit decay, they can remain in good state
of preservation for many years, and be available for transfer to
any object with which they come into contact.
When several or many seasons’ pollen deposition are
represented in a sample, differential decomposition might be
very obvious, and this can be useful for identifying pollen
sources on exhibits [77,78]. However, where the ambient
environment inhibits microbial growth, it is often difficult to
differentiate between old and new pollen [79]. It must also be
kept in mind that residual palynomorphs can be recirculated by
eddying of turbulent air as well as by rain splash and other
forms of physical disturbance. By their very nature, these
effects are localised and unpredictable, and contribute to the
heterogeneity of palynomorph assemblages encountered in
surface samples.
It is important to stress that failure to find a palynomorph
taxon in any sample does not necessarily mean that it is not
present; but if a taxon is found (and there was no likelihood of
contamination during laboratory preparation), it is obvious
that it was a component of the sample. The failure to find a
taxon might simply be a function of the numbers of pollen and
spores counted. A count of 300 grains is generally considered
acceptable and, after this number, the law of diminishing
returns operates [80,81]. Nevertheless, higher counts will
enhance the possibility of locating the rarer components of
any assemblage. It is often necessary to achieve counts of
several thousand palynomorphs per sample, especially if the
in situ vegetation is swamping more pertinent, underlying
signals.
P.E.J. Wiltshire / Forensic Science International 163 (2006) 173–182
5. Exhibits—some taphonomic considerations
It is possible that any object standing in the free air will pick
up airborne or soil/vegetation-borne palynomorphs. The object
might be a vehicle, article of clothing, footwear, rubbish bins, or
bicycle—indeed any article that might be important to a
criminal investigation. Any item can pick up pollen from a
polleniferous surface; even things made of plastic, such as
petrol cans and toys, retain palynomorphs for considerable
(though unknown) periods [82]. Digging implements such as
spades, shovels, and garden forks often yield information of
clandestine graves, or demonstrate links between offenders and
their victim’s dump site [22,26] even years after the event.
Well-preserved pollen has been retrieved from spades more
than 5 years after being used for criminal activity [78].
Unless an enquiry is intelligence-led by reliable witness
testimony, or compelling independent evidence from, for
example, foot or tyre marks, the actual surfaces contacted by an
offender’s belongings at a crime scene can only be surmised. It
is often the case that there is no knowledge of the offender’s
approach path so that obtaining comparator samples has to be
achieved by using ‘‘common sense’’ tactics. In such cases,
those surfaces most likely to have been contacted by the
offender are sampled. If a victim has been buried, then it is
known without doubt that the offender must have contacted the
grave and its immediate environs [16,22]. This, therefore,
would be an important target site for sampling. In view of the
high degree of heterogeneity in palynological assemblages at
any place, identification of an offender’s approach path to a
crime scene can be of pivotal importance for obtaining the most
relevant comparator samples [82]. At best, such samples can
only represent a very small part of any crime scene, so every
feasible approach path should be systematically sampled for
comparison with exhibits. It is often necessary to obtain large
numbers of such samples, especially when there is little
intelligence about the movements of the offender. But targeting
of the most likely areas can reduce the cost and time involved in
the investigation. These are always very important considerations by holders of police budgets.
The different kinds of object and exhibit that have been
analysed over the years are so many and varied that further
consideration must be given elsewhere. Only a small number of
objects that are frequently important in criminal investigation
will be discussed here.
5.1. Vehicles
Car owners in Britain are certainly aware of the heavy pollen
coatings that appear on their vehicles when parked near or
under flowering plants, particularly in late spring. But, in
criminal investigations, the importance of such accumulation
will depend on the length of time the pollen is retained. In
reality, cars are often cleaned assiduously (particularly if an
offender is ‘‘forensically aware’’), and there is little doubt that it
is easy to remove palynomorphs from car body work. Other
parts are more retentive [83] and vehicles have proved to be
important sources of palynological trace evidence; delicate
179
sampling has yielded profiles that have contributed towards
many convictions.
A famous case where palynological evidence from a car was
critical was that of murder of Holly Wells and Jessica Chapman
in the village of Soham [82], Cambridgeshire. Here, material
taken from the chassis, spare wheel, and foot wells demonstrated a good match with the crime scene profile. In a subjudice Metropolitan case, it has been possible to obtain a
convincing match between mud from the murder scene and
mud smears on the side walls of a car’s rear tyres [84]. The
results were very surprising considering the age of the vehicle
and the number of miles it had been driven. Even more
remarkable results were obtained in a recent Hertfordshire
murder case. Here the palynological profile from the offender’s
car foot wells matched that of the victim’s woodland grave 6
months after the offence, after having been driven from
England to Albania, and after having been cleaned several
times [83].
Wheel arches have rarely been a useful source of forensic
palynological evidence although a notable exception was in the
case of search for Charlotte Pinkney [85] in Devonshire, in the
west of England. Because of the long depositional history
represented in many wheel arches, they are not usually the
favoured parts of a car to be sampled. Recent developments in
retrieval techniques are yielding more promising results from
this part of motorised vehicles (personal observation). In the
experience of the author, air filters have been of little use in the
United Kingdom because of the small scale and high
heterogeneity of landscapes.
5.2. Fabrics, clothing, and footwear
Work in numerous criminal cases has shown that pollen and
spores can remain within the weave of dry fabric for many
years. Fears of spontaneous oxidation appear to be unfounded
when items are kept dry. If pollen is allowed to become damp, it
is likely that microbial decomposition is responsible for its
disappearance rather than progressive oxidation by the atmosphere. As yet there are no data on fabrics that have been kept
damp, other than general observations of items becoming
mouldy. This is because police are always requested to dry
items as quickly as possible to prevent microfungal growth.
Fungal spores cause problems by obscuring pollen grains on the
prepared pollen slide, and also making it difficult to assess the
fungal component in the original matrix.
The abundance of palynomorphs retrieved from clothing
varies according to the retentive qualities of the fabric, and the
assiduousness of cleaning after the event. A single attempt at
cleaning generally fails to remove palynomorphs entirely. Rich
assemblages, suitable for comparison with those obtained from
crime scenes, have been retrieved from clothing even when they
have been dry cleaned [86] or washed [87] immediately after the
crime. Experience has shown that dressed cotton is the least
retentive material while wool and a wide range of synthetic
fabrics are much better. Nylon and synthetic fleece fabrics are
particularly good at both at picking up pollen, and retaining it for
long periods [75]. Some fabrics are such good traps of pollen and
180
P.E.J. Wiltshire / Forensic Science International 163 (2006) 173–182
spores that it is actually quite difficult to remove them to facilitate
analysis. Certainly, out of very many cases, only one has
demonstrated secondary transfer from one fabric to another and,
in this case, the original garment was heavily laden with
palynomorphs, and there was firm and prolonged contact
between the garment and the fabric car seat cover which received
the pollen [75].
The popularity of training shoes with highly structured soles
has facilitated the use of forensic palynology in criminal
investigations. It is very difficult indeed to remove palynomorphs
completely from such footwear, even when it has been put
through the washing machine (as is common practice in Britain).
Length of retention is very variable, even between training shoe
types and styles. However, in one case, a pair of men’s Gucci fine
leather shoes yielded the profile of a burial site after being worn
by the offender continuously while on remand for 4 months, and
having been cleaned several times [79]. In another case against a
multiple rapist [75], it was possible to differentiate two distinct
crime scenes from a single pair of training shoes. In yet another
criminal investigation, evidence of a specific British mixed,
deciduous woodland was retrieved from the offender’s shoes
many months after he had given them to his sister in Albania. She
had worn them for working in the fields, but the mixed
palynological profile that was obtained from them clearly
reflected both the Albanian and British soils [83].
It must be remembered that the assemblage obtained from
the sole of a shoe (or any other object) will never match any
crime scene perfectly. By its very nature, footwear may pick up
palynomorphs from more than one place. However, experience
has shown that unless the offender walks on soil, vegetation, or
on organic debris, shoes tend to pick up small (often
insignificant) amounts of pollen [16] during general wear.
Footwear is such an interesting and valuable resource in
forensic palynology that it warrants further consideration
elsewhere.
6. Final comments
At the beginning of this paper, the areas where palynology
has been of help in criminal investigation were listed. No
detailed information on how to carry out these investigations
has been given but some of the problems that the forensic
palynologist must keep in mind at all times have been outlined.
Forensic palynology has become a well-established part of
criminal investigation in the United Kingdom and the discipline
has been boosted by the support and enthusiasm of the British
Police. Every case brings novelty—problems not previously
encountered, and questions not previously posed. The future
looks bright for the discipline in the United Kingdom and it is
hoped that information presented here will help palynologists in
other parts of the world to be brave enough to apply their skills
to criminal investigation.
Acknowledgements
I am very grateful to Ms. Sandra Bond, Dr. Judith Webb,
Dr. John Daniell, Mr. Peter Murphy, Ms. Rebecca Owers, and
Ms. Anna Wesley, for their expert and generous help in many
of the criminal cases quoted in the text. I am also grateful for
the enthusiastic support of police officers and civilians
working in many of the British Police Forces. All these people
have played an incalculable role in the development of
forensic palynology in the British Isles. Thanks are also due to
Professor K.J. Edwards for his useful comments on my
original text.
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