Pollen Dispersal, Deposition, and Persistence

To get pollen from one individual to another by wind, a plant must produce astronomical quantities. A single birch catkin releases around five million grains; a large birch tree in full flower can release hundreds of billions in a season. The grains are small (15-40 micrometres for most temperate trees), lightweight, and aerodynamically efficient. They can travel hundreds or thousands of kilometres in upper-level airstreams and have been recovered from open ocean air samples far from land.

For forensic interpretation this high production and long dispersal creates a challenge. Finding birch or grass pollen on a suspect's clothing says very little about where the person has been during pollen season, because background airborne deposition means virtually everyone carries it on exposed surfaces. The evidential weight of anemophilous pollen comes from its concentration, its assemblage context (what it is combined with), and any unusual taxon within an otherwise typical regional mix, not from the presence of common wind-pollinated species alone.

Insect-pollinated plants have no need for mass production. They make smaller quantities of sticky pollen designed to adhere to an insect and be carried directly to another flower. This pollen is typically large (30-100 micrometres for many species), heavily ornamented, often coated in pollenkitt (a lipid and protein mixture), and spiny or verrucate in sculpture to maximise grip on insect bodies. It does not ride the wind effectively.

The forensic consequence is that entomophilous pollen is largely absent from the background airborne signal except very close to flowering plants. Finding pollen from a specific insect-pollinated cultivar on clothing is therefore much harder to explain as chance deposition than finding birch or grass pollen. Patricia Wiltshire's casework in the UK routinely exploits this property: garden shrubs, unusual ornamental plants, and rare wild flowers can all contribute highly specific grains that tie a person or vehicle to a precise location.

When pollen settles on a surface it accumulates as a mixture of grains from different sources and distances. Palynologists distinguish three components of the pollen rain on any exhibit:

• Regional component: reflects the broad landscape, carried in by upper-level air movement over tens to hundreds of kilometres. Dominated by the abundant wind-pollinated trees of the region. Varies by climate zone but is relatively predictable for a given season.
• Extra-local component: pollen from vegetation within a few kilometres, over-represented relative to its abundance by proximity. A small grove of alder on a nearby stream will contribute more alder to a downwind sample than its proportional cover would suggest.
• Local component: pollen from plants immediately at or near the sampling point. The strongest signal for placing a person or vehicle at a specific location. Includes entomophilous types and any unusual taxa not found widely in the region.

Forensic provenance depends on the local component being detectable above the regional noise. This is easier for soils (where local pollen accumulates over time) than for clothing (where a brief visit may add only a small local signal onto a pre-existing regional background). Comparing a sample from an exhibit against control samples from the suspected location and from the suspect's everyday environments is the analytical strategy that makes the local signal interpretable.

Sporopollenin is not just chemically resistant; it is also physically durable. Pollen grains embedded in the interstices of a wool or cotton fabric can survive months of storage, light handling, and even a single wash cycle if lodged in seams or pockets. Experimental studies have repeatedly confirmed that forensically interpretable pollen assemblages can be recovered from clothing seized weeks or months after an incident.

Grain morphology affects persistence too. Spiny echinate grains hook onto fabric fibres more effectively than smooth or reticulate grains. Large grains may be trapped in pocket corners or seam stitching more reliably than tiny ones. An analyst reporting a clothing sample should note the fabric type and any known cleaning history, because these factors affect what the assemblage can say about how long ago and how strongly the person was exposed to a given environment.

Sporopollenin is a complex polymer cross-linked into a three-dimensional matrix that resists enzymatic attack, hydrolysis, oxidation under normal atmospheric conditions, and most acids. Older literature described it as a polymer of carotenoids and carotenoid esters, but current research identifies phenylpropanoid derivatives and long-chain fatty acids as the core structural components. The exact chemical structure is still debated in the biochemistry literature, but its resistance profile is well characterised for forensic purposes: pollen will outlast soft tissue, most DNA, and almost every other organic residue in the same sample.

The conditions that do degrade sporopollenin over forensic time scales are worth knowing. Strong oxidising agents, including household bleach (sodium hypochlorite) and concentrated hydrogen peroxide at high temperatures, will etch and eventually destroy the exine. Very high temperatures from fire will carbonise pollen; calcined bone from a cremation contains no usable pollen. Very strongly alkaline soils (pH above 9) cause hydrolysis of the exine over months. Peat bogs, by contrast, are both acidic and anoxic, which is why bog-body contexts yield superb pollen preservation thousands of years after burial.

A forensic palynologist uses dispersal and persistence knowledge to answer the question that the court actually needs: does this assemblage associate this exhibit with this location, and is there an innocent alternative explanation for what was found? Both dispersal and persistence bear on that question.

High-abundance anemophilous pollen requires a cautious interpretation. Finding oak and grass pollen on a jacket in June in western Europe tells you almost nothing about where the jacket has been, because it would appear on almost anyone's jacket through ambient deposition. Finding fifty grains of Galium (bedstraw, a low-growing woodland-floor plant that is insect-pollinated and produces minimal background airborne pollen) is a different matter entirely.

Persistence multiplied by dispersal distance gives a rough sense of the specificity of a pollen observation. The combination of low-background, low-dispersal pollen type plus an assemblage that matches a specific location and not the suspect's everyday locations is the strongest evidential configuration. The combination of common, high-background, wide-dispersal pollen type on a frequently worn garment with no location-control samples for comparison is the weakest.