Seeing the Trees for the Forest: Wytham Woods Completes its Fifth Tree Census

A small team of researchers worked together for nine weeks to complete the census. (Image Credit: Andrew Bailey) 

Over nine weeks among the trees of Wytham Woods, a field team led by Dr Cecilia Dahlsjö has completed the fifth census of one of the UK’s most intensively studied patches of woodland. The team recorded around 22,000 trees across an 18-hectare research plot, adding the latest measurements to a long-term record of Wytham Woods. 

The census comes at a particularly interesting moment for Wytham. Ash dieback is increasingly reshaping the woodland, while an exceptionally dry summer has offered a striking reminder of the climatic pressures forests are likely to face more frequently in the future. These changes bring the importance of long-term ecological research into sharp clarity. 

A living laboratory 

Wytham Woods is the University of Oxford’s research woodland, a Site of Special Scientific Interest, and one of the longest continuously studied woodlands in the world. Ecological research has been conducted here since the 1940s. 

Generations of researchers have studied its birds, mammals, trees and wider forest dynamics, creating a record that stretches across decades. 

Using consistent measurements across each census allows changes in tree growth and forest biomass to be tracked over time. (Image Credit: Andrew Bailey) 

The tree census, conducted every five years, takes place in an 18-hectare permanent forest plot established in 2008. It forms part of the Smithsonian Institution’s Forest Global Earth Observatory (ForestGEO), a worldwide network using standardised research protocols to track long-term changes in forests around the world. 

Six scientists, 22,000 trees 

Over nine weeks, the census team worked through the plot tree by tree, cataloguing around 22,000 individuals. They recorded which previously measured trees were alive or dead, identified new stems and trees large enough to enter the census, and measured how much surviving trees had grown since the previous census five years ago. 

A key measurement is diameter at breast height (DBH), normally taken at 1.3 metres above the ground. This requires precision: tape must sit evenly around the trunk, moss may need to be moved aside, and each stem of multi-stemmed trees must be recorded separately. 

A woodland reshaped by ash dieback 

One of the clearest changes during this year’s census was among Wytham’s ash trees. 

Ash dieback was first detected in the woods in 2017. During the previous census in 2021, mortality was concentrated among younger trees and saplings. This summer, the team encountered many more dead or dying ash trees, demonstrating why long-term monitoring is essential for tracking the unfolding impacts of diseases like ash dieback. 

These shifts matter for both the future composition and functioning of the woodland. Previous measurements have shown Wytham acting as a carbon sink, absorbing more carbon than it releases. The new census will help researchers determine whether it continues to accumulate carbon or whether pressures such as tree mortality are beginning to change that balance.

Ash dieback has become increasingly visible across Wytham Woods, with this year’s census recording many more dead and dying ash trees than in previous surveys. (Image Credit: Andrew Bailey) 

Long ecology in a changing climate 

The conditions during this year’s census made the value of long-term monitoring particularly clear. Wytham Woods experienced five near-consecutive heatwaves over the census period alone, with the record-breaking conditions leaving vegetation visibly wilted and the forest exceptionally dry. Census takers observed moisture-loving goat willow trees to be dead or dying and even recorded diameter shrinkage in trees whose bark had contracted from water loss. 

The effects of a historically dry summer may take years to emerge. Some trees recover quickly, while impacts on growth or mortality in others may appear much later. Drought, disease and competition can also interact in ways that are difficult to understand through short-term observations alone. 

Researchers complete the tree census at Wytham Woods during one of the UK’s hottest and driest summers on record, providing a first-hand view of the challenging conditions facing the woodland. (Image Credit: Andrew Bailey) 

This is where Wytham’s long research history becomes especially valuable. Models can help scientists anticipate how forests might respond to climate change, but permanent plots allow researchers to return to the same trees repeatedly, observe changes and refine models accordingly. 

Over time, these observations can reveal gradual shifts that might otherwise go unnoticed and provide early warning of larger ecosystem changes. 

Seeing the trees for the forest 

Nine weeks spent measuring a woodland one stem at a time reveals how dynamic a seemingly stable forest really is. Trees grow and die, new individuals enter the census, diseases spread and species respond differently to changing conditions. 

The fifth Wytham census captures an important moment in a dynamic space, yet its real value comes from the longer record around it. Analysed alongside previous censuses, the new measurements will help reveal how ash dieback, changing species composition and climate are reshaping Wytham — while its place in the ForestGEO network contributes to a much wider understanding of how forests around the world are changing. 

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