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: