Oak Decline

oak decline

Oak decline

Oak decline is a condition that many people may have already heard of. It is a loosely defined ‘disease complex’, caused by the effects of a range of biotic and abiotic factors that lead to decreased tree resilience and a general, visually apparent, gradual deterioration in physiological condition and root health, often eventually leading to tree death. Oak decline has been noted to affect most species of oak across most of the varied habitat types the genus occupies across the world. In the UK both our native species of oak, English oak (Quercus robur) and sessile oak (Quercus petraea), are affected.

Watch Geoff’s ‘Hypothesis of Oak Decline’ webinar

In general, on a landscape scale, the vitality of most oaks is synchronised and episodic in nature, with periods of either reduced vitality or decline, interspersed with periods of recovery. These widespread episodes are thought to be linked to wider patterns of rainfall and hence water availability over the preceding growth period of several years. The most recent widespread decline episode started c. 2006, with subsequent recovery becoming evident since 2016/2017. Despite these limited periods of recovery, the overall trajectory in decline affected trees is usually of gradual deterioration over time. Further localised episodes of decline have been noted since 2017 in the southeast of the UK and it seems likely the succession of serious droughts over recent years may well precipitate another widespread decline episode.

Decline affected trees are also more likely to be sent into rapid decline or even killed by one off, extreme or combination events, such as severe drought or infestation with oak mildew (Erisyphe alphitoides). Oak mildew is an imported pathogen that exacerbates drought stress and adversely affects tree energetics, it is thought to be an important biotic factor in oak decline. Some manifestations of this sort of rapid decline have come to be known as ‘acute oak decline’ (AOD).

Most of the academic research undertaken in the UK so far has been concentrated downstream in the disease complex, focusing on a bacterial species complex that often cause profuse stem bleeding in AOD affected trees. In some trees this may hasten their eventual demise. However, it's important to recognise that AOD affected trees are already highly stressed trees.

Since 2006 researchers in the UK have tried to label ‘chronic oak decline’ and AOD as separate conditions. Our position has always been that these are two manifestations of the same condition, with common underlying causes, representing different points on a continuum of oak decline. More recent research has now shown that a long term depression in growth, sometimes over several decades, usually precedes the development of AOD symptoms, thus validating our position (Reed et al, 2020).

In 2021 our consultant Geoff Monck proposed the first full 'working' hypothesis of oak decline in a webinar given for the Ancient Tree Forum (ATF). Researching and working out how oak decline works has been a long-term project over more than a decade for Geoff and his co-researcher. This has involved long term experimental studies looking at possible therapeutic treatments, lots of detailed expert observation in the field, ongoing literature review and an awful lot of thinking time!

Geoff's presentation about this journey can be viewed here:

The journey and the understanding of oak decline gained has enabled Treecosystems to develop and offer Regenerative Arboriculture treatments that have been demonstrated to reverse oak decline when caught early enough. Treatments improve root and soil ecosystem health and appear to be effective in controlling oak mildew.

Prior to proposing the hypothesis, Geoff was the first to publicly challenge the position of UK academic researchers on AOD in presentations given at the 2016 National Tree Officers Conference, the 2017 ATF Summer Forum and the Arboricultural Association’s 2017 International Amenity Tree Conference. Many of his new insights were subsequently proven to be statistically valid in the academic literature (Brown et al, 2018). Geoff’s original presentation slides can be seen here:


The two models Geoff developed to summarise his hypotheses are shown below:

A general deterioration in physiological condition of several tree species (in addition to oak)

A general deterioration in physiological condition of several tree species (in addition to oak)

Many of the underlying causes of oak decline such as soil pollution inputs, a compromised tree/soil microbiome, environmental extremes due to climate change and the effects of various sublethal pests and pathogens, also affect most other tree species to varying degrees. Observations suggest there may be a general deterioration in the physiological condition of several other tree species over the last 10-15 years, most likely as a result of these cumulative pressures.

As with oak decline, this is generally most apparent in the southeast and east of the UK, where rainfall levels are much lower, with the southeast being officially classified as ‘semi-arid’. Together with the effects of climate change, this places trees in this region under much greater water stress, with knock on effects upon tree physiological condition. Species that increasingly appear to be affected to some degree include:

  • Sweet chestnut (Castanea sativa)
  • Yew (Taxus baccata)
  • Horse chestnut (Aesculus hippocastanum)
  • Sycamore (Acer pseudoplatanus)
  • Alder (Alnus glutinosa)
  • Birch (Betula pendula and Betula pubescens)

Research into the apparent widespread deterioration in physiological condition of yew trees in the southeast of England (and possibly further afield)

Research into the apparent widespread deterioration in physiological condition of yew trees in the southeast of England (and possibly further afield)

Treecosystems has been instrumental in setting up and leading a research consortium looking into the apparent serious deterioration of yew trees of all age groups over the last 10-15 years at many sites in the southeast, up into the midlands, and possibly also further afield.

The consortium represents a collaboration between the owners of the three most significant populations of AVT yews in the UK: Newlands Corner/Merrow Downs in Guildford (Albury Estate/Guildford Borough Council), Great Yews near Salisbury (Longford Estate) and Kingley Vale in West Sussex (Natural England), and their associated yew expert advisors. Other consortium members include academic lead Myerscough University College, the Arboricultural Association and other members of the Ancient Yew Group, including expert dendrochronologist Toby Hindson.

The consortium is currently carrying out a pilot project at Newlands Corner/Merrow Downs. This pilot is trialling novel, state of the art equipment for continuous measurement of tree stress and growth rates over time, whilst at the same time amassing a large dataset for subsequent evaluation.

Next the group plans to set up some ‘Case Funded’ PhD’s in collaboration with an industry partner, to build upon the work of the pilot project and investigate further the causes of yew tree stress and, crucially, what treatments can best be used to improve the prognosis for affected trees and improve yew tree and soil vitality.

See also: Declining Yew Trees Feature

The Yew Tree Project – Newlands Corner, Merrow Downs, Surrey

The Yew Tree Project - Newlands Corner, Merrow Downs, Surrey

Treecosystems played a key role in initiating, designing and participating in the execution of a project to manage visitor pressure and the resulting soil compaction damage around the key ancient yew tree population at Newlands Corner. The project was a collaboration between Treecosystems, the Albury Estate [the landowner], the Surrey Hills AONB and Surrey County Council, with several other project partners including Natural England, The Tree Council, the Ancient Yew Group, Guildford Borough Council, Surrey Choices, The Surrey Coalition of Disabled People and the Forest Bathing Institute.

The project involved installing measures such as ‘dead hedging’ and educational interpretation designed to encourage people to stick to the main paths, as well as a boardwalk providing sustainable access to three of the oldest yew trees at the site. The project has been generally very well received locally, and so far early indications are that the measures put in place appear to be working well. We will continue to monitor the success of the scheme in improving tree and soil health through soil testing and site and tree observations.

Treecosystems’ Director Geoff Monck gave a presentation at the 2022 Arboricultural Association International Amenity Tree Conference

Treecosystems’ Director Geoff Monck gave a presentation at the 2022 Arboricultural Association International Amenity Tree Conference

Geoff gave a presentation on Regenerative Arboriculture and the importance of the tree/soil microbiome to tree health and tree declines at the 2022 Arboricultural Association International Amenity Tree Conference. This can be viewed here: Link Coming soon.

Guildford Environmental Forum

Treecosystems’ Director Geoff Monck will be delivering a presentation for the Guildford Environmental Forum at Zero Carbon Guildford on the evening of Wed 28th June 2023. This is open to all, so please book your free place here:


Geoff will also be running a walk and talk session down at the yew trees at Newlands Corner on Wed 12th July, book your place here:


Treecosystems attended a workshop at National Trust Sheffield Park

The workshop was organised by the National Trust’s Trees Advisor for the southeast Tom Hill. A group of leading conservation arborists and consultants operating in the southeast, as well as National Trust staff and the ATF’s Technical Officer Claire Harkin, were invited to attend to share their knowledge and experience. The aim was to promote a collaborative professional network to further the positive impact on ancient and veteran tree (AVT) conservation of the group.

It was a great day with all members of the group sharing their experiences openly in the true spirit of putting the conservation of AVTs before any competitive and commercial interests.

The world of AVT management is a rare industry where most of those involved place their primary focus on achieving the best outcomes for AVTs, far ahead of personal and commercial interests. Those involved in the industry know they are never going to get rich saving trees, but that is not why we do it! Conserving vitally important AVT trees and their associated biodiversity, thus making a hugely important contribution to the conservation of our planet’s remaining ecosystems, is, across the board, a primary shared goal.

Treecosystems organised a site visit to Kingley Vale to look at the condition of the trees in the expansive yew woodland and its notorious grove of large ancient yew trees

Kingley Vale nature reserve is a SSSI and a SAC for its yew woodland and is owned and managed by Natural England. A site visit was set up with the Kingley Vale team and a small, focused group of tree experts including yew specialists Peter Norton (Ancient Yew Group) and Hugh Milner (Head Forester), along with Dr David Lonsdale (former Lead Researcher at Forest Research), Ted Green (OBE – co-founder of ATF), Caroline Davis (Ancient Tree Forum), J-P Berry (Guildford Council Senior Trees Officer), Jon Stokes (The Tree Council) and Treecosystems’ Geoff Monck.

Similar signs of increased tree stress were identified in many of the yews at Kingley Vale, as at other yew sites in the south of England. It was very useful to discuss these amongst the group out in the field. 

After the site visit, a number of outcomes came out of the round table discussion:

    1. An expanded research consortium would be formed called the Yew Health Working Group. The group will further investigate apparent increased tree stress and possible deterioration in the physiological condition of yews across a number of sites in the south of England, and possibly further afield. The consortium represents an alliance between the teams and landowners behind three key ancient and veteran yew populations in England – Kingley Vale (Natural England), Newlands Corner/Merrow Downs (Albury Estate and Guildford Borough Council) and Great Yews (Longford Estate), along with the academic lead Myerscough University College. Geoff Monck from Treecosystems is the overall group lead.
    1. The consortium will continue with and possibly expand the current pilot research project underway at Newlands Corner.
    1. In the meantime, the consortium will seek to set up some case funded PhDs in conjunction with an industry partner and an additional academic partner.
    1. Discussion of the various indicators of increased yew tree stress and relevant site conditions observed during the day helped the group to formulate hypotheses to be tested by the research. These build upon the work and observations already done in this area by Geoff Monck in relation to both declining yews and oak decline.
    1. Treecosystems will continue to support managers of all three sites with identifying and addressing tree stress factors that can be managed. At Newlands Corner and Kingley Vale, visitor pressure and the resulting soil compaction damage are key threats requiring urgent intervention.

See also:

The Yew Tree Project

Research into the apparent widespread deterioration in physiological condition of yew trees in the southeast of England (and possibly further afield)