Citizen science and tree health in the UK, Andy Gordon, Observatree Volunteer

The first use of volunteers for monitoring tree health in the UK occurred in 1993, when a project was established to track the condition of amenity trees at several sites across five regions in England alone. Before that time no formal monitoring of the health of amenity trees took place in the UK.

The project, which was funded by various government departments, was seen as an extremely valuable means of collecting information on the condition of as many tree species as possible. It became more focussed over the years, and the last phase, initiated in 1999, involved the collection of more quantitative data, concentrating purely on 16 of the most common tree species in amenity plantings within England.

In total, 106 different survey sites were chosen. Plots of up to 100 young and mature trees in 12 different species were selected. Pre-existing plots were sometimes used and 30 National Trust properties got involved.  

A report was prepared in 2006 (Tubby, 2006) even though monitoring in some of the locations continued until 2009. Some of the findings of these pre-2006 surveys include:

  • The year-by-year northward spread of the horse chestnut leaf blotch (Guinardia aesculi) from southern England
  • The expansion of the exotic insect pest the horse chestnut leaf miner (Cameraria ohridella) since its first sighting in London.
  • The annual variation in the prevalence of diseases such as anthracnose on plane trees
  • The first reports of ash dieback (10 years before it was officially named!)
  • The increasing profile of oak decline
  •  The first appearance of bleeding canker (Pseudomonas syringae pathovar aesculi) once again on horse chestnut.

Through lack of funding the whole survey was brought to a close in 2009, but one of the sites still continuing to monitor the diseases was Attingham Park, the National Trust property near Shrewsbury, where I am based. In 2008 I agreed to take over the monitoring of the Attingham survey from another volunteer Dr Steven Reynolds. In September of that year I accompanied Steve on his survey. 

I joined the Observatree volunteers from the beginning in 2013 and once I heard about the Sentinel Tree project I thought that the earlier volunteers’ survey might be of interest to Observatree. So, I made contact with Steve Reynolds to see if he, by any chance, had kept his records.  Like all good scientists, he had not thrown all his records away and was happy to pass those for 2007 and 2008 on to me. I have since carried out surveys as per Katherine Tubby’s 2008 instructions in 2017 and 2018.

When writing this blog I contacted Katherine to see if any of the records from other years and sites were available.  Unfortunately data was collected using old software that is no longer accessible and there are no originals remaining. However, it has been interesting looking at the more recent data to see what changes, if any, have taken place since 2008.

The results

Of the 16 species of trees studied in the original survey, five oak, beech, lime, London plane, horse chestnut and ash were chosen at Attingham. The first four species were all located in the wood pasture grazed by cattle astride the entrance drive to the park, while the last two were in semi-woodland close to the visitor reception area. By 2017 the horse chestnuts had been completely removed to make way for a new reception building and three of the ash close to a nearby access road had been felled as they had been infected by honey fungus.

Steve Reynolds kept the individual survey sheets for 2007 and 2008 but also had tables of the diameters of all the species at 1.3m every two years from 2002, which has allowed the growth rates to be calculated.

Oak

Only four of the five trees were measured in 2017 and 2018 as the fifth turned out to be pin oak (Quercus palustris).  These have shown an average increase in girth of 7%, which is about the norm for this species. Although the trees will have benefited from constant fertilisation from the cattle, the compaction of the soil around the trunks will have had a negative effect. In 2017 one of the trees had a very severe attack of knopper gall, with a small number of uninfected acorns remaining on the tree. The other trees carried very few acorns and even fewer knopper galls. It is interesting to note that less than 300m from the four oaks is a 250-year-old turkey oak, a species required by the gall wasp to complete its life cycle. 

However, all suffered to a greater or lesser extent from the circular midge galls on the under surface of their leaves, as shown in the photo. Three trees appeared to be generally healthy but thinner in the crown than in 2008 – this might have been due to competition from other trees. One tree suffered from a dead branch in the mid-crown and three had some dead lower branches that were not recorded in 2008. In 2018 there was no acorn crop and very few circular midge galls.  On the whole, the trees appeared healthy with good leaf colour.

Lime

The five trees are in a group of 11 without labels, so it was important to choose the same trees to measure and examine.  The suckers at the base of the stems which have been eaten by the cattle suggest that the trees are Tilia xeuropaea rather than T. cordata or T. platyphyllus. Assuming that the same trees had been measured in both 2008 and 2017/2018, the average increment in girth was just under 18%, which is greater than  expected. Some crown thinning had taken place on one of the trees, but this could be due to its closeness to another smaller tree or to a multiple Phytophthora-like lesion on its south-west side. This tree also showed some lower branch dieback which was difficult to distinguish from cattle browsing damage or dieback after pruning. Aphids were conspicuous by their absence and no leaf galls were spotted.  In 2018, two of the trees had a reasonably heavy crop of fruits with, unusually, a proportion of mature seeds. Without sufficient warm sunny days in August, limes do not set fertile seed.

London plane

Three of the trees were in a group and two were single trees, one in the middle of the front lawn and one in the corner of the field. The ground under both of these suffered more from compaction by cattle than the other three. One of the three trees lost a major limb some years ago and had been pruned. Despite this, the overall increase in girth diameter between 2008 and 2017 was 18%, which is to be expected for this species. In 2008 all trees suffered from dark blotches on the leaves with crown yellowing or mild to moderate crown thinning, which is believed to have been widespread in the UK that year. In contrast, in 2017 and 2018, other than the tree which had lost a limb, all trees showed a full and vigorous crown with excellent leaf colour. In 2017 one tree had a minor infection of Pulvinaria scale, which was absent in 2018, and one showed a small area of exposed bark.

Beech

In 2008 one of the five trees was reported as missing, leaving four to measure. These showed a 13% increase in girth between 2008 and 2017, which is on the high side of expectations. In 2008 two trees showed some leaf blotches, whereas in 2017 only one (different) tree showed these leaf blotches. In 2018 they were absent. In 2008, three trees were reported to have perennial cankers but none was spotted in 2017 or 2018. No aphids were spotted  on the lower accessible branches either. One tree had a moderate mast and one a light mast crop in 2018. Generally, the trees looked healthy and did not seem to be suffering from the ground compaction by cattle.

Ash

In 2008, both of the two trees which were still surviving in 2017 and 2018 showed some thinning of the crown. One of the trees that has been felled showed significant crown thinning, but it is not known if that was an early sign of failure or whether this and the other two trees were removed for safety reasons. These trees formed part of a belt of mixed species which had been planted too close to each other, resulting in the ash being rather tall and thin for their age, with small crowns. Ivy was reported to be growing around the trees in 2008 and one of the surviving trees had significant ivy growth, which would have added 5cm to the circumference. Without making any allowance for this ivy, and using the girths obtained in 2008 and 2017, the increase in girth was 9% – about what was to be expected. 

Conclusions

 The 2017 and 2018 surveys have shown that, with the exception of one lime tree with a number of Phtophthora-like lesions, the health of the trees has changed only marginally over the 10-year period and that in the case of the London planes has improved markedly.  The rate of growth as assessed by the increase in girths over this period tended to be on the high side of what would be expected for these species, which confirms that the trees seem to be in good health. The full and vigorous crown on the London planes was particularly noteworthy.

Dr Andy Gordon

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