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Questioning ancient woodland indicators

At the University of Gloucestershire, we believe specialist ancient woodland plants can be more resilient than we thought. Some species are able to come back from interruptions in woodland cover or survive long after a wood has been cleared. This is great news, but it changes how we consider plants when identifying ancient woodland.

Ancient oak woodland (photo: Anne Goodenough)
Ancient oak woodland (photo: Anne Goodenough)

Ancient woodland and the current ancient woodland indicator (AWI) system

Ancient woodland in England and Wales is defined as an area that has been continuously wooded since 1600AD, and in Scotland since 1750AD. Judging whether or not a wood is ancient takes a lot of detective work. Centuries old maps are needed as well as site assessments to look for archaeological and cultural remains, and biological indicators such as woodland specialist plants.

Those plants found more commonly in ancient woodland than elsewhere are known as ancient woodland indicators, and are used as part of the assessment to determine whether a wood might be ancient. What makes a plant a good indicator is described in a separate blog if you’d like to learn more.

Small leaved lime pollen under a microscope (0.045mm in diameter) (photo: Julia Webb)
Small leaved lime pollen under a microscope (0.045mm in diameter) (photo: Julia Webb)

Testing the AWI system

Technology provides a different way of assessing the 'ancientness' of a wood – by analysing fossilised pollen in soil samples and radiocarbon dating. Prof. Anne Goodenough and I applied these techniques to a sample of nine woods from across the UK and Ireland. These woods were also surveyed for AWI plants to see if the two different methods told the same story regarding ancient woodland status.

All nine sites had 12 or more AWIs present, which would lead to classification as ancient woodland if other evidence such as remnant features were present and historical maps showed the area as wooded. However, the pollen analysis told a different story. Only two of the sites had been continuously wooded, while four had been cleared of trees at some point in time but were now wooded. The other three sites had long been cleared of trees but still held a rich AWI assemblage.

Some AWIs were only found on wooded sites, including greater stitchwort and valerian, as opposed to ramsons and bluebells which were often found outside woods.

Lesser celandine and wood anemone (photo: Anne Goodenough)
Lesser celandine and wood anemone (photo: Anne Goodenough)

Building on the AWI system

Based on these results, there are various ways indicator species could be used with greater reliability. These suggestions include:

  • Weighting AWI species according to the degree of their association with woodland – the stronger their association, the higher the weighting.
  • Considering the wood size and/or total number of species present when determining the minimum number of AWIs needed for classification as ancient woodland.
  • Incorporating ‘reverse indicators’ of ancientness – those that are very tolerant to disturbance such as foxglove.
  • Incorporating other taxonomic groups such as lichen, fungi and beetles to make a ‘super-system’ with higher precision.

If these measures were incorporated in ancient woodland assessments, the hope is that an improved AWI system will help in preserving ancient woods and their unique biodiversity, of which the plants are just a part.

Learn more about woodland plants and why they’re so special

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