Root Growth and Grave Dating

A buried body does not decompose in isolation. As soft tissues break down, they release ammonia, nitrates, phosphates, and a range of organic acids into the surrounding soil. These compounds diffuse outward in a concentration gradient. Plant roots detect gradients like these through differential ion uptake along their tips and respond by growing toward higher-concentration zones, a behaviour called chemoautotrophism or, more specifically where nutrients are involved, rhizotrophism.

The effect is measurable. In controlled burial experiments, grass-root density within 10 cm of a buried animal carcass was several times higher than in adjacent undisturbed soil within the first six months. For larger tree roots, which move more slowly, the response takes longer but is more permanent: lateral roots deflect toward a grave cut, and fine feeder roots proliferate inside the cavity created by the disappearing soft tissue.

Soil texture and drainage modify the picture. Sandy, well-drained soils let the nutrient plume diffuse quickly, spreading roots across a wider area but at lower local concentrations. Clay soils hold the plume tightly around the burial, producing a more concentrated root response directly above and around the remains. Cold soils slow root growth and decomposition alike, which means the correlation between root size and burial duration can hold up better in cold climates where conditions are more consistent year to year.

In temperate regions with distinct growing seasons, woody roots produce annual growth rings just as tree trunks do. Early-wood cells form in spring when growth is rapid and water is plentiful: they are large-celled and appear pale in cross-section. Late-wood cells form in the slower summer and early-autumn period: they are smaller, denser-walled, and appear darker. The boundary between one late-wood band and the next year's early-wood is sharp enough to count under 40x magnification.

1. Locate a root with a deflection or injury
   In the field, identify roots that curve sharply around a bone or show a branching scar at a contact point. These are the candidates for sectioning. Document their orientation, depth, and relationship to the remains with photographs and scale markers.
2. Section and stain
   Cut a thin cross-section (30-60 micrometres) with a sliding microtome or a razor blade for larger roots. Stain with safranin-fast green or toluidine blue to differentiate early-wood (green) from late-wood (red/dark). This contrast is what makes counting possible.
3. Count from the injury outward
   Start at the innermost ring adjacent to the injury or deflection scar and count each dark late-wood band to the outermost ring. This count gives the number of growing seasons that have elapsed since the root first contacted the barrier.
4. Apply the minimum-interval logic
   The ring count is a minimum. The root may have taken several seasons to reach the burial depth before it contacted the remains, so the actual burial predates the ring count by whatever time growth took to penetrate the overlying soil. The botanist must estimate this pre-contact growth period separately, usually from species-specific penetration-rate literature and soil temperature records.

Forensic application of root dating requires published or experimentally measured penetration rates for the species in question. Without them, a ring count is a relative count only. A growing body of literature provides these rates under controlled conditions, though field application requires adjusting for local soil temperature and moisture.

Soil temperature is the dominant control. Root growth in a temperate deciduous forest essentially stops below about 5°C (41°F) and has a thermal optimum around 20-25°C. Using soil temperature loggers placed during the investigation, or retrospective weather-station records, a forensic botanist can reconstruct the growing-degree-day accumulation since burial and cross-check it against ring counts. Where the two estimates converge, confidence in the burial interval increases.

Beyond ring counting, the physical relationship between roots and bone tells its own story. As a lateral root thickens year by year, if it has grown alongside or partly around a bone it will begin to compress and eventually deform softer parts of the cortex. In older burials the root may have an impression of the bone surface burned into its outer wood, a cast-like contact zone that is sometimes called root encasement.

Roots can also displace bones vertically and horizontally over time. A fine feeder root entering a foramen (natural opening in bone) grows inside the marrow cavity and, as it thickens, can crack the bone from within. This is not trauma. Forensic anthropologists and botanists work together to distinguish root-induced cracking from peri-mortem injury: root damage follows the path of least resistance through nutrient-rich marrow spaces, produces characteristic smooth-walled channels without radiating fractures, and often shows root-fibre impressions under magnification.

Where bones have been displaced, a careful excavation using the single-context recording method can document their original articulation and the direction of movement. A bone that has been pushed 20 cm from the spine of a skeleton by a tree root of known age bracket gives the investigator a physical minimum: the burial is at least as old as the root that moved the bone, plus the time the root needed to grow to that diameter.

Forensic entomology uses insect succession to estimate time since death for surface or shallow deposits, with the best precision in the first weeks to months. Root-growth evidence comes into its own later, from roughly six months to decades, a window where insect succession data is largely exhausted. The two methods are therefore complementary rather than competing.

• Entomological PMI: Insect succession gives best resolution for recent deaths (days to months). Recovered pupal cases, cast skins, and void patterns in soil help establish the minimum time since death at the surface before burial.
• Root ring dating: Gives a minimum burial interval from first root contact onward. Most useful for intervals of one growing season to several decades. Precision is plus-or-minus one growing season in best cases.
• Soil chemistry (cadaver decomposition island): Elevated nitrogen, fatty acid markers, and altered pH persist in soil for years after the body has gone. Sampling profiles around the remains provide corroborating evidence of decomposition duration.
• Pollen stratigraphy: If pollen has accumulated in grave fill, a profile through the deposit may show seasonal or year-on-year changes. Pollen from within the grave that matches a known seasonal assemblage can bracket the deposit to a specific time of year.

In a well-documented case from the United Kingdom, convergent evidence from root ring counts (minimum three growing seasons), a cadaver decomposition island chemistry profile, and a lack of insect evidence consistent with fresh burial all pointed to a burial interval of three to five years. This bracket narrowed the suspect's known travel history to a manageable period and was accepted by the court as reliable scientific opinion.

Root evidence is easily destroyed during excavation. A shovel through a root-bone contact zone severs the contact relationship on which dating depends. For this reason, forensic archaeology protocols include botanist involvement from the point at which a grave is first suspected, so that root sampling is completed before skeletal removal.

1. Document all visible roots in the grave fill and at the interface with the remains using scale photography and hand-drawn plans before any disturbance.
2. Sample roots in direct contact with bone first. Photograph the contact zone, label the root in situ, and remove it with a sharp scalpel rather than scissors or pulling.
3. Bag roots individually in sealed paper bags (not plastic, which encourages mould) with unique exhibit numbers, GPS coordinates, depth from surface, and species identification if possible.
4. Collect control root samples from outside the grave cut at the same depth and from the same plant species to establish local growth rate norms.
5. Record soil temperature at the burial depth if possible, and request retrospective temperature records from the nearest meteorological station for the period of interest.