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Change for the worse?

By Maria Grant, Sam Oxley, Kate Ahern and Nick James
Widespread tree disease could have a dramatic effect on the character of our landscape. Landscape professionals may be able to mitigate the effects and even come up with solutions that improve biodiversity.

Ash dieback has been prominent in news reports in recent years, although many other British tree species are under threat from a wide variety of tree pests and diseases. These can take many forms (the primary causes are fungi, bacteria/viruses or insects) and include horse chestnut canker, acute oak decline and Phytophthora pathogens which infect a wide variety of plants, including British native species. Recently there has been an increased emergence of the oak processionary moth (Thaumetopoea processionea) in south east England, which can also have a damaging effect on human health. 

The late 20th century saw the loss of 25 million trees from the British landscape as a result of Dutch elm disease, and the subsequent effect of these losses, particularly on lowland English landscape character, is well documented. Ash trees make up around a third of Britain’s woodland resource (approximately 80 million trees), and are found across most of our landscapes. Therefore a decline in ash trees is likely to have a more profound effect than Dutch elm disease in landscape terms. 

The implications of these emerging threats have the potential to be severe, widespread and to damage some of Britain’s most iconic landscapes. This article aims to help readers visualise these potential landscape scale impacts. 

Visualising and illustrating the effects of tree loss on landscape
The threatened species occur across a wide variety of the UK’s landscapes, for example: 

The limestone pavements and escarpments of the Yorkshire Dales and Cumbria are characterised by stunted and wind-sculpted ash trees, the loss of which could drastically change landscape character by exacerbating the woodland losses that have arisen from grazing pressures. 
The remnant wildernesses of Scotland’s Caledonian pinewoods could be threatened by red band needle blight. These unique woodland landscapes support a number of species of junipers, birches, willows, rowan and aspens, some of which are found nowhere else in the British Isles. 
Juniper woodlands are a relatively rare yet highly valued habitat concentrated at locations in Scotland and Teesside, and are susceptible to Phytophthora rot. The loss of juniper could also threaten gin making in the UK, an element of our cultural landscape.

Forested valleys and plateaux 
The extensive larch plantations in the coal mining valleys of Wales have been a characteristic feature of this landscape since the mining era, when the timber was used for pit props. Six million infected trees are to be felled due to widespread infection by Phytophthora ramorum. As well as the visual changes in the landscape, the large-scale felling will also will have an impact on recreational usage of the affected areas. For example, Cwmcarn Forest is popular for its mountain biking trails and the seven-mile Forest Drive is likely to be influenced by the extensive movement of heavy forestry machinery. Phytophthora ramorum is also affecting larch within forestry plantations on the strath-sides in Scotland, for example Strath Glass, where the landscape is simultaneously suffering the loss of riverside alder trees through Phytophthora alni. The general impression is of an unhealthy and changing landscape. 

Quintessential trees of the British countryside are found in lowland areas including oak, elm, ash and horse chestnut which are characteristic hedgerow and in-field trees of the classic English landscape, and are also important within broadleaved woodland communities. The loss of these as well as of majestic individual ‘champion’ trees will have a widespread effect on landscape character. 

Riverine landscapes
Alder trees are a common feature along river corridors as a pioneer species which has an important role in stabilising river banks, flood control and moderating the temperature and nutrients of water bodies. Alder trees are threatened by Phytophthora alni which results in the dieback and death of alder trees and can also have a negative impact on adjacent water bodies. Implications for landscape character therefore go beyond tree loss, with bare eroding river banks giving the impression of a degraded landscape.

Designed landscapes and street trees
Avenues composed of a single species are likely to be particularly vulnerable to disease from both a biological and landscape perspective. One prime example comes from the Royal Parks of London which often have single-species avenues, such as lime, beech or horse chestnut. The latter is vulnerable to bleeding canker, commonly caused by the bacterial pathogen Pseudomonas syringae pv aesculi.

Parkland trees within historic estates are often characterised by veteran specimens, whose loss will be particularly noticeable within the landscape since these trees are, to a degree, irreplaceable. Some of Britain’s most famous individual trees which are well known within parkland landscapes and are important in a historical and cultural context, for example, the Major Oak in Sherwood Forest. 

Plane trees are under threat from massaria disease, which can cause damage to the major boughs of the tree. Although they are a cultivated species, they are a popular choice in parks and urban areas because of their high tolerance of pollution and water shortages. Due to their presence in the public realm, trees infected with massaria disease, caused by Splanchnonema platani, can pose a hazard to public safety because of the risk of branch drop. There are implications for the townscape character and parks of London for example. 

Wider implications of climate change and compound or secondary effects
The resilience of trees to these threats is likely to be affected by additional pressures resulting from a changing climate and a shift in land uses. Climate change is suspected to have a role in the spread of pests and disease, with warmer weather in particular facilitating the spread of pest species from warmer climes. The emerald ash borer is a beetle of Asian origin which has had devastating impacts on ash trees in the US and Europe and is expected to arrive in the UK in the coming years.  

Additional pressures on trees may involve but are not limited to increased wind, storms, rainfall and flooding events, as a result of climate change.

Knock-on effects include:
- The reduction or loss of many of the ecosystem services provided by trees, including air cooling, the provision of shelter or creation of micro-climates, carbon attenuation and improving water quality.

- Changes to soil structure which may occur through lack of stabilisation by tree roots, leading to soil erosion and potential slope collapse, notably along river banks where tree loss can compound the effects of erosion due to increased flood events. 

These pressures may also interact with each other, exacerbating the problems that trees face and altering other aspects of the landscape. Changes to the physical aspects of the landscape are likely to have cascading impacts, eventually culminating in altering the composition of local ecosystems, as well as vast change to the visual qualities and character of our landscape. 

The role of landscape professionals and a positive outlook
Whilst the primary weapon against these potentially devastating changes to landscape character lies in preventing the spread of these diseases, including vigilant screening of plants, biological control by predators and the development of genetically resistant cultivars and hybrids, landscape professionals can play a number of roles by helping land owners and managers to plan for and adapt to change.

Since the advent of Dutch elm disease, biological understanding and techniques have advanced significantly and there are projects aimed at increasing elm numbers – the Great British Elm experiment (in collaboration with the Conservation Foundation) has planted thousands of genetically resistant specimens and periodically gathers data on their condition, which may prove useful in the battle against these emergent diseases. Most elm trees are genetically identical which creates a high susceptibility to disease. That is why Dutch elm disease was catastrophic. Biologists from John Innes Centre in Norfolk have made progress in identifying trees which could be genetically resistant to ash dieback, which it is hoped can be propagated. Research programmes into a number of tree pests and diseases are being undertaken by the Forestry Commission and the Woodland Trust. 

The role of landscape planners, designers and managers in preventing a lasting adverse impact on landscapes must not be understated. Landscape designers can help restore and improve landscapes that have suffered from tree loss by selecting resilient species, with an appropriate landscape and ecological fit. Designers and managers can also recommend and plant a diverse range of trees in both urban and rural environments to ‘future-proof’ our landscapes of trees against diseases, planning perhaps 100 years or more ahead, for changes to our avenue and veteran trees of the future. Planting a diverse range of species from a variety of families to ensure greater genetic diversity can also increase the resilience of tree communities to pests and disease. Forward thinking and planning plays a key role in managing the process of change, avoiding dramatic change and providing a phased approach based on selective replacement and advanced planting. However, this approach is only effective if it is put into practice before large-scale tree losses are apparent in the landscape. 

The extensive larch felling occurring in Wales opens up an opportunity to increase the cover of native broadleaved woodland in these areas and also to make the landscape less homogenous and more diverse, by reinstating open areas within the landscape. This process will be assisted through the planting of native species including birch, rowan and oak with the goal of producing a richer semi-natural habitat, which also has the added benefit of being less vulnerable to future disease outbreaks. Increased diversity in tree communities has a positive effect on resilience, biodiversity and landscape, although the selection replacement species in these communities is key for an appropriate landscape and ecological fit. 

Landscape character is by no means a static entity, as is recognised by the European Landscape Convention, and threats to valued landscape features such as trees are part and parcel of the planning and management of landscape change. Whilst these issues have the potential to be devastating, with appropriate planning and action being taken early, these events can also create opportunities to enhance and diversify landscape character for the future.

The authors all work for LUC 

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Posted by JOHNMILNER - November 9th, 2017

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