The Cause

Poor form at the Dalkeith site could be attributed to a whole variety of environmental factors (we can rule out genetic factors as virtually every tree is bad!). Identifying the cause on any particular site requires logical detective work to narrow down the suspects.

Oak bud galls caused by Arnoldia quercus were observed at the Dalkeith site in 2008, which can cause bud death and poor form in oak. However no sign of the insect was found in 2009, ruling it out as the source for the continued poor form. So further investigation was required;

In 2010 Rob Sykes and Stephen Hendry visited the site at Dalkeith to assess the site and look into possible causes of the poor form. An ecological site classification analysis and assessment of other site conditions (including soil fertility) were carried out. Although conditions were far from optimal for tree growth the site was found to be suitable for Quercus robur. Root excavation showed that even trees with very poor form possessed healthy, relatively well-developed rooting systems.

Symptoms displayed by trees across the entire site were highly variable and included slight chlorosis of the foliage, miniaturisation of foliage, failure of lateral buds on the leading shoots, and dieback of the current year’s extension growth. There was no sign that particular symptoms were clustered in specific areas. Furthermore, no signs of either insect or pathogen activity were found to be associated with any of these abnormalities in growth.

The current ground vegetation consists almost exclusively of Rosebay willowherb (Chamerion angustifolium), which would suggest heavy application of weed control. Great care should be exercised when using a systemic herbicide such as glyphosate during the period when active tree growth is occurring and particularly in the spring when new tissues are both expanding and being formed. Sub-lethal damage by a systemic herbicide could account for the majority of the symptoms which are currently evident at the site.

Salvaging the Situation

So the site at Dalkeith is suitable for growing oak and there is no evidence to suggest that site conditions per se have had an adverse impact on growth. Furthermore, there is no current indication that a disease or insect pest is present on the site which would compromise future attempts to manage the trees with a view to improving their form.

Therefore, it has been proposed that the trees of poor form could be stumped. This is a traditional management practise where young trees are cut back to a low stump a few years after planting and allowed to reshoot. It sounds and looks excessive but often the cut stumps will regenerate healthy shoots with good vigour and form. (please see ‘Stumping Oak’).

Extreme care should be exercised in controlling the vegetation in any areas where stumping back is carried out, and it may be preferable to employ a soil-acting residual herbicide or use hand weeding to control subsequent growth of ground vegetation during the period of active tree growth.

Always Experimenting

The rooting systems of the trees at Dalkeith are well-established and strong regrowth might therefore be expected after stumping back. However, given that there is circumstantial evidence to suggest that use of glyphosate may have had an adverse effect on past growth, there is a possibility that regrowth from dormant buds may not initially be as vigorous as expected.

So it is proposed that only a proportion of the trees are cut back to stump, as an investigative trail. 25% of the area will be stumped, starting with the worst affected trees. If re-growth from these trees is good quality, then other areas of the site can be treated. Furthermore the operation presents an opportunity to gather data of the response by individual tree families to stumping back, should the worst happen (even to an improved seed source!).