Continued from Part 1 Ecology
Written by Francesco Benvenuti, University of Edinburgh
UK Government management response
Preview to the discovery of the ash dieback disease in the UK, many governmental agencies had, in 2011, jointly published the “UK Action Plan for tree health and plant biosecurity”. Besides recognising the biosecurity threats associated to the poorly regulated global trade of plants and trees, the action plan called for the implementation of stricter national control measures on imported sapling. However, despite the many objectives and preventive actions promoted in the plan (Table 1), ash dieback eventually entered and spread into the UK.
A mapping of the pathogen distribution was urgently commissioned by the UK government to numerous environmental agencies and plants’ disease specialists. Additionally, the UK parliament also rapidly banned the imports of any ash saplings or seeds and imposed strict controls on the movements of domestically produced wood and saplings from infected and susceptible sites. However, as some ash species do not show clear sign of contagion, the detection of the disease remains difficult and poses a major challenge for understanding the pathogen distribution[9,13].
Implications for Scotland and recommendations for disease management
As shown in figure 2, confirmed cases of ash dieback have recently been reported across Scotland. Although no cases have yet been documented in North and North-West Scotland, it is predicted that the spreading of the disease, if not already present but undetected, might be inevitable in the future. Hence, as uneducated management reactions might undermine the future of ash in the UK[12,15], land managers should be provided with the relevant scientific knowledge to make informed decision.
Lesson learned: prevention of uncontrolled and extensive logging
As over time the pathogen penetrates deeper into the tree’s stem, increasingly affecting wood quality and its marketable values, many UK landowners feared a potential drop in their timber revenue and immediately reacted by logging and selling extensive areas planted with ash. Namely, in England and East-Scotland, 58.000 and 30.000 trees have been felled respectively. However, in the future such practice should be strongly discouraged among managers for two main reasons. Firstly, trees removal could potentially lead to economically adverse outcome by flooding the market with excessive ash wood and causing a price-crush.
Secondly, extensive logging could result in severe ecological consequences by narrowing the genetic diversity of ash populations and potentially removing any in-built resistance to the pathogen. Indeed, differently from elm which is usually propagated from suckers and that was in the past decimated by the Dutch elm disease due to its narrow genetic bank, ash is grown from seeds and is a very genetically diverse species [9,16]. Accordingly, evidence from continental Europe suggests that ash trees differ in their susceptibility to the disease and that up to 20% of individuals may display some degree of resistance. Relatively healthy trees hence represent an important genetic bank for pathogen resistant genes and could aid the future creation of resistant ash populations by enabling natural selection processes.
Genetic resistance selection
It is consequently suggested that the management of newly discovered infected ash tree populations should differ according to tree age as well as on the disease development stage within populations. In Denmark and Sweden for example, two different silvicultural management strategies have been adopted. Namely, apparently immune saplings in young stands were identified, protected and monitored. For older stands instead, trees were felled only when the wood quality begun to be badly affected by the disease. On the other hand, it could be argued that, as resistance genes are apparently highly inheritable, natural regeneration from relatively healthy trees should be promoted and felling prevented. Selective breeding of resistant trees could indeed be achieved by planting seeds from healthy trees in highly infected areas and subsequently selecting resistant phenotypes.
Economic Vs. Ecological arguments
Over the past months, two main conflicting arguments for the disease management have emerged. While under a purely economic perspective failing to harvest infected trees and leaving them to die might be a complete waste of timber revenue, in contrast both standing and decomposing deadwood habitats, especially if characterized by old dead trees, are ecologically valuable in terms of ecosystem health and biodiversity. Additionally, although upper branches might die, infected old trees can survive for a relatively long time (Table 2) and thus provide a whole range of organisms with a variety of valuable habitat niches . Hence, the vision of ash trees solely as a commodity should be discouraged and their value for biodiversity better promoted.
The role of UK tree nurseries
The fact that the ash dieback disease entered the UK by importing infected saplings highlights the current inefficiency of bio-security regulations associated to international plants trade. Indeed, despite the pathogen was known to be present in various European countries since 2006, saplings’ import from such countries remained high (Table 3). It can also be recognised that the current disease spread may have reduced landowners’ confidence in planting trees and could consequently impact on domestic tree nurseries’ businesses in the near future. It is however suggested that UK tree nurseries will likely play a fundamental role for the future of UK woodlands and should be supported.
Indeed, producing the totality of tree saplings planted in the UK internally, would effectively minimize the risks of future non-native pathogens outbreaks. Domestic production would also encourage landowners to start substituting ash trees with other UK native species of local provenance rather than stop planting altogether.
Secondly, as a result of the disease establishment into the natural environment, new plantings should be rapidly promoted to compensate for the unavoidable future tree losses, especially in mixed broadleaved woodlands. Accordingly, it is recognised that replenishing broadleaved forests can be most effectively achieved by allowing natural regeneration and supporting it through by planting additional saplings of local provenance[20,21]. As such saplings are most often produced only by local tree nurseries it is suggested that, for the future benefit of national woodlands, the UK government should increase the support for domestic sapling production.
As emerging knowledge suggests that ash dieback may already be established in the UK, a long-term management strategy is required for mitigating its long term impact on national forests. Such strategy will have to primarily aim on better educating land managers on the ecological and genetic consequences of their management decisions. Particularly, in locations such as North-West Scotland where the disease has not yet been found but it will likely be in the near future, it is important to engage early with land managers to prevent extensive and indiscriminate logging of infected plantations. It is indeed argued that the future of ash in the UK as a valuable forestry species will likely depend on the harvesting strategies which will be deployed in the near future. Namely, although it might be economically valuable logging some of the infected trees for revenue before the wood quality is affected, it is also crucial not to remove healthy trees and young saplings. Indeed, they might represent a valuable capital of pathogen-resistant genes which could aid the future re-establishment of resistant ash populations. Moreover, the removal of old infected ash trees should be discouraged and their conservation value for biodiversity and potential for natural regeneration better promoted. Finally, the implementation of more effective international trade regulations is also urgently needed from the UK government. It is suggested that, besides aiming to better implement the UK Action Plan for tree health and plant biosecurity, incentives and support should be provided to the domestic tree nursery sector. This will not only mitigate the risk of future non-native pathogen entering the UK by stimulating the local production of native tree saplings, but will also promote landowners’ confidence in the UK forestry sector.
NB Part of In Class Assessment for Edinburgh University’s 4th Year Ecological Science Field Course, November 2012
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