Acute Oak Decline

This case looks at a different form of tree defect that has become a recent hazard. It concerns a pedunculate oak tree in Stratton Strawless, near Norwich. The oak has been suffering from Acute Oak Decline (AOD) which has has rapidly affected the trees health over just 2 years. Within falling distance of the oak is a highway and immediately beneath is a small BBC Radio Norfolk radio station (not Alan Partridge) which can be seen at the right of the trees base in Fig. 5.1 and Fig 5.3 

Fig 5.1 Oak suffering from AOD. The main target, a Radio Station, can be seen just to the right of the main stem at ground level.

In this case, extreme decline can be seen throughout the whole canopy with a significant amount of dead wood. Acute oak decline is a relatively new phenomenon, being diagnosed in the UK within the past 35 years. Other than dieback within the canopy, primary symptoms are as follows; weeping fissures that release a black fluid known as stem bleeds (see Fig 5.2.), lesions of dead tissue and a network of galleries in close proximity to dead tissue that are caused by the beetle Agrilus biguttatus. Researchers have found various species of bacteria in such lesions in conjunction with the beetle Agrilus biguttatus and it is thought that the combined presence of the beetle and the bacteria are primarily responsible for AOD (Forest Research, 2018).

Fig 5.2. Close up view of lesion as a result of AOD.

Recent advice indicates that retrenchment pruning can be a beneficial treatment for a veteran tree in a state of decline (Fay, 2016; Lonsdale, 2013). However, AOD is a particularly severe form of decline, where trees have been known to die completely within 5 years from onset (Forest Research, 2018). In other cases they may continue to survive in a weakened state. Bearing in mind the potentially fatal condition, it would be unsurprising if the owner of this oak tree had requested for the tree to be felled immediately, to eliminate all forms of future risk. However both the owner and the arboricultural consultant in this case were keen to retain the tree in some form because of it’s ecological and heritage value so two main options were explored in order to minimise risk to the highway and radio station: leaving a monolith of dying/ dead habitat at a low height, or retrenchment pruning as an attempt to maintain life, with a schedule for further monitoring. In making such an assessment it is necessary to be aware of vitality and vigour within the tree in relation to it’s potential for survival (Lonsdale, 2013). Retrenchment pruning was the agreed remedial measure taken as can be seen in figure 5.3.

Fig 5.3. Retrenchment pruning in practice upon the affected oak specimen. The remaining canopy was cut directly between the remaining leaf growth and the base of the MEWP bucket in this image in an attempt to mimic natural decline within the tree (Fay, 2008). This sharp reduction is more severe than the periodic treatment Fay (2008) or Lonsdale (2013) would recommend, but it could be argued that in the case of severe decline a more severe approach is needed.
Fig 5.4. (Fay 2008)

In conclusion, it is apparent that specific guidance cannot always be based upon arboricultural norms. For instance a non standard approach to retrenchment pruning (Fay, 2016; Lonsdale, 2013) was necessary in this case due to the extreme nature of decline. A periodic treatment staggered in 6 stages over 30 years or even 4 stages over 12 years would be inappropriate (see Fig 5.4), only removing a portion of dead wood, so pruning that mirrors the actual state of decline is much more applicable. This may turn out to be three stages over 10 years dependent on future surveys of the tree.