Potential Pest and Disease Impacts

Animal pests and microbial pathogens are a serious threat to urban forests and society, causing direct economic costs from damage, and impacting on ecosystem service provision. It is likely that climate change will result in the introduction of pests and diseases not yet present in the UK. The changing climate of the UK is predicted to increase growth or spore release of root pathogens and to make trees more susceptible to infection. Further temperature changes are likely to affect the geographical range, development rate and seasonal timing of life-cycle events of insects, and will have an impact on their host plants and predators.

The potential damage from pests and diseases varies according to a wide variety of factors such as tree health, local tree management and the weather. In addition, a tree community that is dominated by a few species is more vulnerable to a significant impact from a particular disease than a population which has a wider variety of tree species present.

Risk matrices were devised for determining the potential impact of a pest or pathogen, should it become established within Birmingham, based on whether it affected a single tree genus shown in Table 12, or multiple genera in Table 13.

This informed Table 14 which gives an overview of the existing and emerging risks to Birmingham's urban forest, including the predicted proportion of the tree community that would be affected and the associated amenity value of those trees across the study area. The UK plant risk register contains 1,240 entries, and is multifaceted, considering the current extent of a disease, the likelihood of its spread, the severity of its damage and the ability to mitigate it. Here, emphasis has been given to a subset of pests and pathogens which severely impact trees or pose human health risks. The matrix emphasizes causative agents, which are damaging, would affect >0.01% of the area’s trees.

The pest which could potentially have the greatest estimated impact across Birmingham’s urban forest is the Asian Longhorn Beetle (though this is not currently present in the UK), which could affect 57% of its trees - worth £489 million. The greatest risk, which is already present in the UK are threats to the Oak population from Acute Oak Decline and Oak Processionary Moth which threaten 11% of the total tree population valued at £117 million.

Some pests and diseases only affect a small proportion of the population, for example Dothistroma Needle Blight only threatens 2% of the species in Birmingham. However, these seemingly low-risk pests and diseases can be widespread and are therefore one of the greater threats. The population at risk from Dothistroma Needle Blight is valued at £7.45 million.

Pest/Pathogen	Major tree hosts affected	Prevalence in UK	Replacement cost of trees	Tree Population at risk (%)
Acute Oak Decline	Oak species	Central and SE England, Welsh borders and SE Wales	£117,000,000	10%
Asian Longhorn Beetle	Many broadleaf species	None (previous outbreaks contained)	£489,000,000	57%
Beech Leaf Disease	Mainly American Beech species but also others	None	£66,800,000	3%
Bronze Birch Borer	All Birch species	None	£55,100,000	12%
Ash Dieback	Many Ash species	Occurs in most parts of the UK	£55,000,000	8%
Citrus Longhorn Beetle	Many broadleaf species	None	£396,000,000	45%
Dothistroma Needle Blight	Many Pine species	Widespread	£7,450,000	2%
Elm Zigzag Saw Fly	Some Elm species	Present in SE England and East Midlands	£1,900,000	0%
Emerald Ash Borer	Common Ash and Narrow-Leaved Ash	None	£55,000,000	8%
Great Spruce Bark Beetle	Spruce species	Present	£1,140,000	0%
Horse Chestnut Leaf Miner	Horse Chestnut	Present in all parts of GB	£7,360	1%
Mountain Ash Ringspot	Rowan	Widespread through Scotland and the North. Likely present across the whole UK.	£29,500	3%
Oak Lace Bug	Oak species	None	£117,000,000	10%
Oak Processionary Moth	Oak species	Established in Greater London and some surrounding counties	£117,000,000	10%
Oriental Chestnut Gall Wasp	Sweet Chestnut	Around London and the South East	£4,420	0%
Phytopthora austrocedri	Juniperus spp, Chamaecyparis lawsonia, Chamaecyparis nootkkatensis	Scotland and England only	£49,100,000	2%
Phytophthora lateralis	Chamaecyparis formosensis, Chamaecyparis lawsoniana, Chamaecyparis obtusa, Chamaecyparis pisifera, Rhododendron spp., Thuja plicata, Thuja occidentalis, Pseudotsuga menziesii, Taxus brevifolia	Occurs across the whole of the UK	£65,900,000	4%
Pine Processionary Moth	Pinus nigra, Pinus sylvestris, Pinus pinea, Pinus halepensis, Pinus pinaster, Pinus contorta, Pinus radiata, Pinus canariensis, Cedrus atlantica, Larix decidua, Pseudotsuga menziesii	None	£7,580,000	3%
Plane Lace Bug	Plane species	None	£72,100,000	1%
Plane Wilt	Plane species	None	£72,100,000	1%
Rednecked Long-horn Beetle	Cherry species	None	£15,200,000	6%
Sirococcus tsugae	Cedar and Hemlock species	Yes	£2,320,000	1%
Sweet Chestnut Blight	Chestnut species	Yes but uncommon	£199,000	0%

Ash Dieback

Ash Dieback is a vascular wilt fungus which causes the dieback and death of Ash trees. It has had a major impact upon the Ash population across Europe. Since Ash Dieback was first recorded in the UK in 2012, the rate of infection has increased at a steady rate and is now considered endemic, causing significant damage across the country.

Whilst initially occurring predominantly in Ash populations that had been recently planted, by the summer of 2014 infected trees were being found within established trees in the wider environment. Ash is the fish most populous tree species in Birmingham and provides 8.3% of the total leaf area. Therefore the implications of losing Ash trees cannot be understated.

The effects of Ash Dieback in the UK have already been significant, with many woodlands, hedgerows and landscapes losing a significant proportion of their Ash trees, which compromises social wellbeing and environmental health. To address the impact of Ash Dieback in England and Wales, the Ash Dieback Action Plan Toolkit was developed. The Toolkit is an evolving document being updated with best practice for local authorities in tackling Ash Dieback. Using this toolkit, local authorities can produce their own tailored Action Plans with aims to mitigate the effects of the disease on both the natural environment and the local economy. Support is a provided by a range of organisations, including the Tree Council, the Forestry Commission, Natural England, the Woodland Trust and local authorities.

Tree condition

One of the most important factors when dealing with any potential pest or disease impact is to consider the health of the tree. Tree condition was measured as part of the survey and Figure 19 shows the health of the ten most common trees in Birmingham. Overall, tree health in Birmingham is very positive, with 52.8% rated as excellent condition and a further 32.5% rated good or fair. 14.7% of trees were rated as poor or worse. Approximately 2.9% are dying or already dead.

63.1% of Ash trees in Birmingham are in an excellent or good condition. However, Ash is the second most common species in the West Midlands and the resulting loss from an increased impact of Ash Dieback, as seen in other parts of the country, remains a high risk. Improving the diversity of species, and particularly the evenness of species across the population, will increase the resilience of the urban forest as a whole.

It will be important to tackle Ash Dieback and prepare to replace the trees which will inevitably be lost. Selecting species which are suitable replacements for Ash is key to replacing the lost canopy cover and replacement species should have roughly the same potential for ecosystem service provision as those which are lost.

Holly may be a species of serious concern as they makeup a large portion of the overall population, with over 50% of trees being recorded as poor or worse.

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Birmingham i-Tree Report