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Algae, mosses, lichens, and higher plants colonise surface-deposited human remains in a measurable sequence, providing post-mortem interval estimates that complement entomological and chemical methods.
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A human body deposited on the surface of the ground does not stay bare for long. Within days, if moisture is present, microscopic algae begin forming a thin green film on exposed bone and clothing. Within weeks, moss spores arrive and begin to germinate in sheltered crevices. Over months and years, lichens establish, grass seeds sprout, and the remains become increasingly embedded in a carpet of plant growth that records the passage of time in centimetres of thallus diameter and growing seasons of root development.
This plant colonisation is not random. It follows a predictable succession shaped by substrate availability, climate, and the nutritional changes created by decomposition beneath. Each phase of colonisation has a measurable rate, and those rates can be calibrated against local climate data to generate post-mortem interval estimates that are independent of insect succession, soil chemistry, or soft-tissue condition.
This topic covers algal and moss colonisation as early PMI indicators, lichenometry as the primary tool for longer PMI ranges, seed germination patterns and their interpretation, and the practical challenge of combining these botanical signals with entomological and pathological evidence to bracket the time since death as tightly as the evidence allows.
The first plant to arrive on exposed bone does so within days. That speed is its value.
Green algae and cyanobacteria are cosmopolitan organisms with airborne propagules that reach virtually any exposed damp surface within days. On newly exposed bone, particularly in humid environments or following rainfall, a thin photosynthetic film develops as algal cells adhere to the calcium carbonate and organic matrix of the cortical bone surface. This film is visible to the naked eye as a pale green or grey-green discolouration that is quite different from the yellowing of dry oxidised bone.
The diagnostic value of algal colonisation is primarily temporal in the short term. Absence of algal film on bone that has been exposed in a humid environment suggests very recent deposition, potentially within the previous few weeks. Presence of an established, dense crust suggests at minimum several weeks to months of exposure. The limitation is sensitivity to microclimate: a bone in deep shade retains moisture and algalises rapidly, while one in direct sun on a dry, south-facing slope may remain uncolonised for months.
Cyanobacterial communities (blue-green algae) are particularly useful because they include nitrogen-fixing species that begin to modify the substrate surface chemistry. Their presence, along with the identifiable genera (Pleurocapsales, Oscillatoriales) recorded by microscopy, can sometimes be compared against regional records of algal communities to confirm the habitat type of the deposition site, contributing to scene reconstruction beyond pure PMI estimation.
Mosses take months to years to establish. Their presence tells a mid-range story.
Mosses (division Bryophyta) are among the first macroscopic plants to colonise disturbed or bare surfaces, particularly in humid temperate environments. Their spores are widely dispersed by wind, and germination can occur on a range of substrates including bare soil, decaying wood, stone, and bone. On skeletal remains, moss colonies typically establish within a few months to two years of initial surface exposure, depending on climate and shade.
The forensic value of mosses comes from two sources. First, presence of established moss colonies with rhizoid penetration into the bone surface indicates a minimum exposure period of several months to over a year: moss takes time to germinate, establish a protonema (the filamentous juvenile stage), and develop a leafy gametophyte visible to the eye. Second, certain mosses have measurable annual growth increments, and in some cases the number of shoot segments corresponding to growing seasons can be counted, giving a rough minimum colonisation age.
Collecting and identifying mosses on remains requires care. Species identification is done under dissecting and compound microscopy using leaf morphology, cell shape, and reproductive structure. The botanical literature provides growth-rate data for common temperate species (for example, Funaria hygrometrica, a pioneer moss frequently found on disturbed damp ground, can establish a visible colony within two to three months under favourable conditions). Comparing the growth state of the moss on the remains against local background growth rates provides a calibrated estimate.
Slow is useful when time is measured in years.
Lichenometry was developed by the Icelandic botanist Sigurdur Thorarinsson in the 1950s for dating glacial moraines and archaeological structures, using the maximum diameter of crustose lichen thalli on rock surfaces as a proxy for surface age. The principle is simple: a lichen that grows at a known radial rate will reach a diameter proportional to the time since the surface was first exposed. Applied to bone, the same logic holds: the largest lichen thallus on a bone surface has been growing since at least the time of first colonisation.
In forensic contexts the most commonly used species belong to genera with well-documented growth rates: Rhizocarpon geographicum (map lichen, typically 0.3-1 mm per year), Xanthoria parietina (orange maritime lichen, faster at 1-3 mm per year in maritime climates), and Lecanora spp. (variable, but widely studied). Site-specific growth rates must be calibrated from surfaces of known age in the same environment, such as headstones or dated masonry in the vicinity.
| Lichen genus | Typical growth rate (radial, mm/year) | Substrate preference | PMI range suited to |
|---|---|---|---|
| Rhizocarpon geographicum | 0.3-1.0 | Siliceous rock surfaces | Decades to centuries |
| Xanthoria parietina | 1.0-3.0 | Calcareous stone, bone | Years to decades |
| Lecanora campestris | 0.5-2.0 | Calcareous stone, concrete | Years to decades |
| Physcia adscendens | 0.5-1.5 | Bark, calcareous stone | Years to decades |
| Parmelia sulcata | 1.0-4.0 | Bark, rock | Years to decades, faster warm climates |
Measuring thallus diameter is done under a hand lens or dissecting microscope with a calibrated eyepiece graticule or a digital calliper applied gently to the thallus. The largest thallus on a given bone is the one that colonised first and thus gives the maximum minimum age. Multiple measurements and photographs are taken and retained as exhibits.
When an annual plant germinates on human remains, the body has been there at least one growing season.
Seeds carried by wind, water, or animals land on and around surface-deposited remains continuously. If conditions allow germination, the resulting plants incorporate information about minimum exposure time into their age and growth state. Annual plants that complete a full life cycle in one year are the most straightforward: a first-year annual growing from a seed embedded in clothing or soil around the remains means the remains have been present for at least the current growing season.
Perennial plants tell a longer story. A clump of grass with a root-base showing three distinct annual growing seasons means the grass has been established for at least three years, and the surface beneath it has been stable for at least that long. For tussock-forming species such as Deschampsia caespitosa or Molinia caerulea, tussock height and stem-count have been used as rough proxies for establishment age in temperate bog habitats.
Each discipline covers a different part of the timeline; overlapping ranges is where confidence grows.
Botanical PMI estimation is most powerful when it corroborates or refines estimates from other disciplines. Forensic entomology covers the early PMI window well, typically from days to several months, using insect succession and development rates. Pathological evidence (decomposition stage, adipocere formation, mummification) provides a broad qualitative bracket. Botanical colonisation fills the gap from weeks onward and extends through years to decades in ways that insect and soft-tissue methods cannot.
A well-constructed PMI report for a surface deposit of skeletal remains might state: entomological evidence indicates a minimum PMI of 18 months based on insect void patterns and absence of recent colonisers; algal and moss colonisation on the cranium is consistent with at least 18-24 months of surface exposure; the largest lichen thallus on the parietal bone measures 8 mm in diameter and, using a calibrated growth rate of 1.2 mm per year from local headstone comparison, indicates a minimum colonisation age of approximately 6 years, suggesting the remains may have been present longer than the insect evidence alone implies.
That kind of convergent reasoning, in which multiple independent methods produce overlapping ranges that narrow around a plausible window, is the standard for PMI evidence in serious casework. Where the botanical estimate extends beyond the insect estimate, it raises a hypothesis worth testing: was the body moved? Was it initially covered and only recently exposed? These are investigative questions that the botanical evidence cannot answer alone but can legitimately raise.
The scientific value of botanical PMI evidence depends on how it was collected and how the uncertainty is framed.
Botanical PMI evidence on remains requires careful sampling to preserve the organisms for laboratory examination without destroying the contact-zone context. Algal crusts are sampled by sterile swab or by gently lifting a section of surface film with a scalpel onto a microscope slide. Moss patches are photographed in situ with a scale bar, then removed with their substrate if possible, or sampled with a small fragment for identification. Lichen thalli are measured and photographed in place; if removal is necessary for species identification, the point of measurement is GPS-tagged and photographed.
A lichen thallus on a cranial bone measures 18 mm. The local calibrated growth rate is 1.5 mm per year. What does this give the forensic botanist?
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