Analysis of the Canopy Effect and Its Application to Stonehenge Cremations

(It's not often I get to use my MA (Oxon) in "Agricultural and Forest Sciences" for Stonehenge research so I am please to be able to share this.)

Corroboration of Snoeck et al.’s References by Recent Research

The landmark study by Snoeck et al. (2018) in Scientific Reports used strontium and carbon isotope analysis to interpret the origins of individuals and the wood used in their cremation pyres at Stonehenge. Their interpretations relied on foundational research into the “canopy effect”—the phenomenon by which plants in dense forests exhibit depleted δ¹³C values compared to those in open environments. Seminal studies such as van der Merwe & Medina (1991) and Drucker et al. (2008) established that this effect is due to a combination of reduced light intensity and the recycling of ^13C-depleted CO₂ from soil respiration, with these depleted signatures transferring up the food chain, Vogel (1978).

Recent research has directly measured the canopy effect in woody tissues, addressing earlier limitations where extrapolations were made primarily from grasses. For example:

·         van der Sleen et al. (2014) analyzed tree rings of Peltogyne cf. heterophylla in a Bolivian moist forest, finding that Δ¹³C values decreased by 1.5–2.5‰ after gap formation due to increased light, confirming the predominant role of light in the canopy effect.

·         Brienen et al. (2022) studied Cedrela trees across three tropical forests, observing Δ¹³C reductions of 4–6‰ from understory (24–25‰) to canopy (17–18‰), with tree height as the main driver (–0.15 to –0.41‰ per meter).

·         Starkovich et al. (2024) demonstrated that hazelnut shells from denser canopies had δ¹³C values up to 5‰ lower than those from open settings.

These studies provide direct, quantitative evidence of the canopy effect in woody tissues, validating the isotopic principles applied by Snoeck et al. (2018) to infer the origins of pyre wood at Stonehenge.

In-Depth Explanation of the Canopy Effect

The canopy effect in isotope ecology refers to the systematic depletion of δ¹³C values in plants growing under dense woodland canopies. This is primarily due to:

Reduced light intensity: Limits photosynthesis, increasing the ratio of intercellular to ambient CO₂ (C_i/C_a), and thus enhancing discrimination against ¹³C.

Recycling of ¹³C-depleted CO₂: Soil respiration under forest cover releases CO₂ with δ¹³C around –27‰, further depleting plant isotopic signatures.

The δ¹³C value, expressed in per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB) standard, is governed by carbon isotope discrimination (Δ¹³C) during photosynthesis in C3 plants (which dominate UK woodlands). The standard model, after Farquhar et al. (1982), is:

Δ¹³C = a + (b - a) ⋅ C_i/C_a

where:

• a ≈ 4.4‰ (fractionation during diffusion),
• b ≈ 27‰ (fractionation during carboxylation),
• C_i/C_a is the ratio of intercellular to ambient CO₂ concentration.

In dense canopies, low light raises C_i/C_a (e.g., 0.7–0.9), leading to greater discrimination and lower δ¹³C values (–30‰ to –32‰). In open landscapes, higher light reduces C_i/C_a (e.g., 0.5–0.7), resulting in higher δ¹³C values (–25‰ to –27‰).

Bonafini et al. (2013) quantified this in Wytham Wood, UK, finding up to 5‰ δ¹³C depletion in grasses under closed canopies, primarily due to shading. Recent timber studies, such as Brienen et al. (2022), confirm similar depletions in tree rings, with Δ¹³C shifts reflecting canopy density. However, water stress can also influence δ¹³C in drier sites, potentially confounding purely light-driven effects.

Archaeological Application

In cremation contexts, the canopy effect is preserved in bone apatite, as carbon from pyre wood is incorporated during high-temperature burning. Lower δ¹³C values in cremated remains suggest wood from dense woodlands; higher values indicate more open landscapes. This isotopic signature, combined with strontium isotope analysis, enables reconstruction of both human mobility and the environmental context of cremation practices.

Link to Snoeck et al.’s Stonehenge Work

Snoeck et al. (2018) analyzed strontium (^87Sr/86Sr) and carbon (δ¹³C) isotopes in 25 cremated human remains from Stonehenge. Strontium isotopes in tooth enamel indicated that 10 individuals had ratios consistent with west Wales (e.g., Preseli Hills, the source of Stonehenge’s bluestones), while others matched the local chalk geology of Salisbury Plain. δ¹³C analysis of cremated bone apatite revealed that individuals from Wales had lower δ¹³C values, suggesting cremation with wood from denser woodlands, while those from Wessex had higher values, indicating wood from open downlands.

Recent studies directly corroborate this interpretation. Starkovich et al. (2024) showed that woody tissues from dense canopies have δ¹³C values up to 5‰ lower, matching the lower δ¹³C in some Stonehenge remains. Brienen et al. (2022) confirmed that tree rings in shaded understories exhibit significant δ¹³C depletion, supporting the idea that Welsh woodlands produced the wood for some pyres. In contrast, the open Wessex downlands, with higher δ¹³C values, align with the isotopic signatures of local cremations.

Archaeological Context: Transport of Cremated Remains

The physical context of the burials reinforces this interpretation. Excavations by Hawley and later researchers found that many cremation deposits at Stonehenge were clustered in the Aubrey Holes and were often contained within circular margins, suggesting they had been placed in organic containers—most likely leather bags—before burial. These organic containers have long since decayed, but their impressions remain, supporting the hypothesis that the cremated remains were transported as discrete packages.

As summarized by recent overviews and the Stonehenge Riverside Project, Willis, C. et al. (2016), this practice fits with the idea that Stonehenge was a ceremonial centre where people from distant regions—including west Wales—brought their dead for burial. The movement of both the bluestones and people from the Preseli region underscores the monument’s role as a focal point for inter-regional connections during the Neolithic.

Synthesis: Cremation in Wales, Burial at Stonehenge

Given the combined strontium and carbon isotope evidence, and the archaeological context of the cremation deposits, the most parsimonious explanation is that the individuals with Welsh isotopic signatures were cremated in west Wales using local woodland fuel. Their remains were then carefully collected—likely in leather bags or similar containers—and transported to Stonehenge for burial. This scenario is supported by the absence of local pyre debris at Stonehenge, the preservation of distinct isotopic signatures, and the physical evidence for organic containers.

This interpretation is now widely favoured over the alternative hypothesis that non-local wood was transported to Stonehenge for use in cremations. It also fits with the broader pattern of Neolithic mobility and ritual, where both people and materials—including the bluestones—were moved over considerable distances.

References

·         Bonafini, M., Pellegrini, M., Ditchfield, P., & Pollard, A. M. (2013). Investigation of the ‘canopy effect’ in the isotope ecology of temperate woodlands. Journal of Archaeological Science, 40, 3926–3935.

·         Brienen, R. J. W., Schöngart, J., Zuidema, P. A., et al. (2022). Paired analysis of tree ring width and carbon isotopes indicates when controls on tropical tree growth change from light to water limitations. Tree Physiology, 42(6), 1137–1150.

·         Drucker, D. G., Bridault, A., Hobson, K. A., et al. (2008). Can carbon-13 in large herbivores provide an insight into palaeoenvironmental conditions? Palaeogeography, Palaeoclimatology, Palaeoecology, 266(3–4), 183–191.

·         Snoeck, C., Pouncett, J., Claeys, P., et al. (2018). Strontium isotope analysis on cremated human remains from Stonehenge support links with west Wales. Scientific Reports, 8, 10790.

·         Starkovich, B. M., Krauß, R., & Britton, K. (2024). Carbon isotope values of hazelnut shells: a new proxy for canopy density. Frontiers in Environmental Archaeology, 3, 1351411.

·         van der Merwe, N. J., & Medina, E. (1991). The canopy effect, carbon isotope ratios and foodwebs in Amazonia. Journal of Archaeological Science, 18(3), 249–259.

·         van der Sleen, P., Groenendijk, P., Vlam, M., et al. (2014). Understanding causes of tree growth response to gap formation: Δ¹³C-values in tree rings reveal a predominant effect of light. Trees, 29, 439–448.

·         Vogel, J. C. (1978). Recycling of carbon dioxide in a forest environment. Oecologia Plantarum, 13, 89–94.

·         Willis, C. et al. (2016) ‘The dead of Stonehenge’, Antiquity, 90(350), pp. 337–356. doi:10.15184/aqy.2016.26.

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