How science is supporting a 1,000-year-old oak

The Major Oak is one of the nation’s most iconic trees.

At an estimated 1,100 years old, it is believed to be among the oldest and largest oaks to be found anywhere in these isles. Despite its fragile state, it is also one of the few of Britain’s great old oaks to retain a full canopy and traditional ‘tree shape’ profile.

It is one of the primary reasons that many thousands of people from all over the world make the trip to Sherwood Forest each year, as, of course, is its long and distinguished association with our national folklore as the home to the legend of Robin Hood.

But it is more than a tree of phenomenal size, age and mystery.

The Major Oak is a living timeline of how trees have been cared for over the last few centuries. Work focused on the Major Oak has helped pioneer and shape Ancient tree management in particular, and it continues to inform how we work here at Sherwood.

With careful management, this diorama of science, engineering and amazing micro-habitats can continue for centuries more.

History

For well over 100 years, visitors have been drawn to this natural wonder which has been so closely associated with the legend of Robin Hood.

It’s sheer size (around 11 metres in circumference, with a canopy of some 28 metres), and particularly its voluminous cavity, caused by the natural decay of its heartwood, has intrigued people to get up close and personal, and even to venture inside.

We often hear stories of people who have photos of themselves in the ‘doorway’ to the south of the tree. There are hundreds of photos of the tree over its lifetime, often showing people in or around it, as well as the significant changes to the surrounding landscape.

To mitigate against the impact of natural events, such as high winds, lightning strikes or heavy snowfall, as well as damage caused by people, there have been lots of attempts to protect the tree.

Bracing chains, props, fibreglass (formerly lead) sheets, concrete infilling and fire retardant paint can all still clearly be seen by visitors.

These historic interventions began in the Victorian era when people’s love of this tree really began to be documented.

They have continued up to the early 21st century, with methods that were pioneering and well-intended at the time, but would not be repeated now, with all of the scientific information we have about trees today.

Almost invisible though is one of the greatest threats to the Major Oak’s health and longevity: soil compaction caused by decades of human activity around the base of the tree.

This is currently one of our main foci when managing the health of this green monument, based on years of research and monitoring.

Over the coming years, you will see lots of activity and changes around the Major Oak area, all in a bid to restore vitality to it.

What is compaction?

Compaction is the process of increasing the density of the soil by applying downward pressure on it. This has the effect of reducing the volume of air and water within the soil and limits the amount of moisture and nutrients that can reach the tree’s roots.  

The natural sandstone soils of Sherwood have in the vicinity of the Major Oak, almost the consistency of concrete. The result is that there is very little microbial activity within the soil, a lack of oxygen and moisture, with no flexibility under the ground for the Major Oak’s roots to expand into.

This impedes root growth and interactions between microbes and mycorrhizae (how the tree interacts with fungi), along with other natural life cycles within the earth, affecting the health of the tree. Without these natural processes within the soils, the tree’s complex root system will be under enormous pressure to find enough resource to sustain it.

The result of all of this means that we now think it is much reduced, shrinking back towards the tree to conserve energy.

A tree of this age and size needs huge amounts of water and nutrients to function healthily, which it currently cannot get from the surrounding soils.

We know this from the chemical testing we have carried out both in the soil and the leaf tissue. Not only that, but the historic interventions within the branches have resulted in really compromised sap flow, meaning that what little nutrients and water the tree can take in have an arduous journey to make to reach the leaves.

The Major’s great age, the impact of compaction and the historic interventions, plus the stress to the tree caused by changes in our climate, such as periods of drought and intense rainfall in recent years, are among the reasons why it has looked quite fragile for some time.

What are we doing about it?

One thing is certain; if we do nothing, the tree’s health will continue to deteriorate.

So, we are taking steps to restore vitality to it using the best data and scientific evidence available to us.

Three recent independent studies have recommended urgent action to tackle soil compaction and nutrient depletion in the soil around the roots of the tree.

The RSPB is now working with the UK’s best soil and tree health experts to test new ways in which oxygen, nutrient and moisture intake can be increased.

And we’re continually expanding our knowledge and data of the tree’s health by working with experts in different fields, exploring all the ways in which we can ensure its longevity.

Allowing natural growth

During 2023, we have followed advice to purposely leave the vegetation around the tree to grow, rather than trimming it back frequently, as had been done in previous years. This decision to leave the area to grow naturally – this is a nature reserve, after all – is intended to retain more nutrient quality and moisture in the ground. It allows natural cycles to be completed and root systems to develop which will naturally break up the compacted soils. Not only that, but all of the insects that form part of the forest ecosystem will have a resource for food and breeding throughout the growing season;

Renutrifying the soil

With approval from Natural England, the government’s environmental advisory agency, we are now taking a more radical approach to reducing soil density.

The methods that we are applying over the next 5 years have taken over a year to develop with our external expert consultants.

In November, our staff and volunteers began excavating wedge-shaped trenches radiating from around the tree in a bid to locate and improve the soils adjacent to the existing root system.

The holes were back filled to maintain the original soil horizons (the strata of soil types beneath the surface), but in the lower layers where the holes were deeper, the spoil was mixed with a natural mulch obtained from forest floor close to the Major Oak.

The process will aerate and re-nutrify the soil, re-invigorating the microbial activity that will bring the ground back to life. This should hopefully enable the tree’s roots to extend further to support its huge structure for many more years of its already incredible lifespan.

Watch the video below to hear more about this work.


Who is doing the work?

The work to create the trenches is being carried out by staff and our amazing volunteers, overseen by Simon Parfey of SoilBioLab, the UK’s leading soil health and microbiology laboratories.

Working alongside them and the RSPB are expert arboriculturists Reg Harris, from Urban Forestry (Bury St Edmunds), and Luke Steer, of Treescapes Consultancy, who have worked in Sherwood Forest frequently over many years and have a detailed knowledge of its ancient oaks.

What will this achieve?

Over the next 5 years, it is anticipated that all the soils within the Major Oak enclosure will benefit from aeration and feeding. The aspiration is that this work will be a catalyst for natural life cycles, meaning we won’t need to intervene any longer than necessary.

We already have baseline data to show the state of the soils and tree as it is now. To see if this work is making the difference we need it to, we will carry out annual testing in various ways to monitor if and how the tree responds.

We will take foliar samples from high up in the canopy to assess the colour, chemical content, and chlorophyll levels of the leaves in the summer. This will begin to tell us if the uptake of nutrients and water is increasing as we hope.

We will also be able to test the infiltration levels within the soil by comparing percolation rates and soil moisture levels within treated, untreated and control locations close by in the forest.

We will continue to sample the soils and monitor the chemical contents and microbial activity as well, and ‘ground-truth’ where the Major Oak’s root system is active and healthy.

The results of the monitoring and new data we collect, such as sap flow analysis and transpiration rates, will inform how the work progresses.

As a science-based organisation, we work on the evidence that the testing shows us, adapting the way we work to achieve optimal results with the least detrimental effects and using the best resources available.

The future

We have to accept that each of our current ancient oaks will eventually succumb to a natural end, succeeded by a new generation of Sherwood ancients.

But the Major Oak is different because it is so large in size and emblematic of Sherwood and its unique heritage.

The hope is that this work means that the Major Oak can continue its incredible life, and continue to be an educational tool in tree management for future countryside custodians

Already having been here for around a millennium, the right level of care can sustain it for centuries more.

Not only that, it is hoped that its canopy will be fuller and greener than it has been in recent years as it continues to enjoy its pride of place in Sherwood Forest and at the heart of the UK’s few remaining ancient woodland territories.   


Getting technical

Making the best use of the latest technology and the finest expertise available is vital to understand how our work at the Major Oak is making a difference.

One of the key elements that will tell us how successful this project is is the rate at which water will flow into the soil around the Major Oak.

We know that the compacted sandy soil limits the infiltration of rainwater, and there are also unexpectedly small amounts of vital organic matter in the soil. This can limit the amount of water and soil fertility, meaning that the roots of the Major Oak are not receiving the hydration and nutrition this vast tree really needs to thrive.

With climate change causing more extreme droughts as well as floods, it is really important to understand what is happening in the soil underneath the tree.

In May, hydrological testing was carried out around the tree for the first time since work began on the project, assessing the rate at which water flowed into different points in its enclosure.

Dr Jon Hillman, of local consultancy SWCM, conducted tests in the area that had been aerated and mulched in the autumn, to compare the findings with the rate of flow in an untreated spot.

Water is released from 25 litre canisters at a controlled rate into two metal rings which are placed in the ground to direct it downwards into the soil.

Regular measurements are recorded over time to calculate the rate of flow.

Infiltration tanks used to carry out hydrological testing at the Major Oak on May 2024.
Water is directed into the treated area around the Major Oak to test the rate at which it infiltrates the soil.

Although the composition of the soil and other natural factors mean that it is difficult to take an absolutely identical sample, a ‘control’ tree was selected nearby to measure the water infiltration rate in that location for further comparison.

The results will be compared to assess whether the decompaction work is making it easier for rainwater to enter the soil. If the water flows more quickly into the treated area than in the untreated, it should mean that the aeration is having a positive effect on infiltration.

Testing in this way will take place over the lifetime of the project, examining each of the treated areas created around the base of the tree.

But we are also able to check the volume of water in the soil at any given time.

Beneath the ground are digital ‘data loggers’ installed to record this data in real time, sending the information to our team, so they know precisely how much water is available for the tree’s enormous root system.

The devices also measure soil temperature at surface level and one metre below ground, as tree root development is strongly linked to soil temperature.

Site manager Paul Cook checks a data logger out in the forest to assess water content and soil temperature.
Site Manager Paul Cook checks a data logger in Sherwood Forest to assess water content and soil temperature.

If the roots are too cold, they go dormant, but as the temperature beneath the surface rises they become more active.

Warmer soil also encourages fungi and other micro-organisms to grow, fostering symbiotic relationships that are of a mutual benefit to the tree and a variety of other flora and fauna that depend on it.

Understanding how environmental factors impact upon the rate of growth also tells us much about the health of the tree.

Even at over 1,000 years old, the Major Oak is still growing, but some factors, such as drought, can affect that process.

Relying on the physical appearance of the tree and taking measurements can provide a momentary picture of its health.

However, technology is also playing a part in refining this process and ensuring we can always know how a tree is responding to a range of impacts, such as rainfall and periods of dry weather.

A piece of equipment called a dendrometer can be attached to a tree to measure its growth and shrinkage. It will assess a range of activity, such as growth rates, to provide a highly accurate status report on how the tree is doing.

Thanks to funding from the Woodland Trust, the Miner2Major Landscape Partnership Scheme and the RSPB a number of dendrometer devices have been installed on carefully selected trees in Sherwood Forest – including the Major Oak.

Academic research is also playing a role in this process. Dr Andy Hirons, Senior Lecturer in Arboriculture at University Centre Myerscough in Lancashire, has installed the dendrometers at Sherwood and also a high-definition time-lapse camera, funded through the College’s research programme.  

Dr Andy Hirons prepares to install a dendrometer to the Major Oak in May 2024.

The camera will monitor changes to the Major Oak’s leaves, enabling us to review how their health is affected through the seasons by different climatic or other events over a 12-month period.

It will enable us to compare what we see happening with the Major Oak’s canopy and the data logged by the dendrometer – hopefully providing useful insight between what we see as part of visual assessment of tree health compared to what is happening at a more cellular level.

Site Manager Paul Cook analyses data from dendrometers installed at the Major Oak.

All of this vital information will provide the evidence we need not only to demonstrate the effectiveness of the interventions we are making, but also to inform how we should respond to changes in conditions that could be detrimental to a tree’s health.

It is this data which can also be used not just here in Sherwood, but in other woodland areas to make positive contributions to tree populations and the wildlife communities that each tree supports.


Unlocking genetic secrets

When a tree has reached 1,000 years old, how easy is it to find out about its origins all those centuries ago?

One of the ways the Major Oak is advancing knowledge about trees is through the sequencing of its DNA.

Academics from the University of Cambridge are looking at how the Major Oak has evolved. More specifically, how its genome – the set of chromosomes that make up a living organism – has developed over its phenomenal lifespan and what this can tell us about ancient oaks more widely.

In 2023, Professor Ian Henderson, Professor of Genetics and Epigenetics in the University’s Department of Plant Sciences, took some samples from two diverged branches of the oak to assess the DNA of each.

The samples were used by Dr Matthew Naish back in Cambridge to extract DNA and perform sequencing.

Their initial findings are that the DNA sequence of the two branches were very different from one another. 

Indeed, it has often been considered a possibility that the Major Oak is so vast and ancient because it is the result of a fusion of more than one tree, with that process starting perhaps at the time of its germination from an acorn.

However, what is undeniable is that, whether one tree or more than one, its resilience and its longevity make it a natural wonder. Now the team are collecting further samples and sequencing DNA from multiple points on the tree to find the truth.

If the Major Oak is a single tree, their results indicate that genome evolution within trees can be substantial. However, if the oak in fact represents two trees, then it will still be interesting to study genome evolution within each of the constituent trees and ascertain the extent to which the branches have developed independently of the others?

To further the study, Masters student Laura Cowan, who is working with Professor Henderson, is studying genome evolution in ancient oak trees. Laura came to Sherwood Forest in April to take samples from branches and sub-branches on different sides of the tree.

Laura Cowan takes samples from the Major Oak in April 2024.
One of the samples which will be taken to the University of Cambridge for DNA extraction.

The samples are then ground to a fine powder using liquid nitrogen, and the DNA of the samples will be extracted.

Her plan is to complete further DNA sequencing and ask questions about evolution of genetic resistance to disease along the ancient branches.

It’s exciting to think that this iconic oak is helping us to learn so much more about how ancient trees evolve.