Plant Growth on Remains as PMI Indicator

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 (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.

Lichenometry was developed by the Icelandic geographer 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.

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.

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.

• Annuals germinating in season: Constrain the deposit to at least the current growing season. If the annual has set seed and died, at least one full growing cycle has passed.
• Biennial stem base: A biennial with a rosette in its first year and a flowering stem in its second year indicates at least two growing seasons of surface stability.
• Perennial crown increments: Some perennials add a measurable increment to their crown each year. Counting these gives a minimum establishment age for the plant and a minimum surface-stability period.
• Root development into soil or bone: Higher-plant roots growing through or around bone integrate with the root-dating methods discussed in a companion topic, providing corroborating PMI evidence.

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.

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.

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.

1. Document all visible plant growth on and immediately around remains before any disturbance, with macro photographs and a scale bar at each location.
2. Collect calibration samples from nearby surfaces of known age (dated headstones, concrete structures with known pour date) for lichen growth rate estimation.
3. Record microclimate data: shade/sun aspect, estimated moisture availability, proximity to water, altitude. These contextualise growth-rate adjustments.
4. In the laboratory, identify organisms to genus and species level using appropriate keys. State the identification confidence and the reference used.
5. In the report, frame all PMI estimates as minimums derived from observed growth and calibrated rates, state the uncertainty range, and note any conditions that might have delayed colonisation.