Managing sites with ash dieback to conserve functional traits

Adopting a technique generally used in the social sciences but rarely in ecology, Louise Hill (University of Oxford) et al. provide a new summary for land managers looking to predict and manage the effects of ash dieback. Their work was recently published in Journal of Applied EcologyMaintaining ecosystem properties after loss of ash in Great Britain.

Ash dieback, an invasive disease of ash trees, is a serious conservation concern across Britain, and there are many questions to be answered regarding the best approaches for managing the disease. It is starting to cause serious damage to populations of the common ash tree, the third most common broadleaved tree in Britain. As ash trees die, significant changes are likely to occur across woodland and non-woodland ecosystems for which ash trees are important components.

Hill photo 1
‘Unhappy’ ash trees by Louise Hill

We developed a new approach that allows us to both predict some of the changes ecosystems may experience with ash loss, and recommend management actions to limit them. Our approach used the distributions and functional traits of tree species to map the changes in traits that may occur across Britain. Functional traits are the characteristics of organisms that can influence ecosystem functioning (such as height or fruit type), and where there are large changes in the functional traits present in an ecosystem there are likely to also be changes to other important aspects of ecosystems. For instance, the provision of ecosystem services and the suitability of an ecosystem for habitation by other species are both closely linked to the functional traits of the ecosystem.

Our method also borrowed a technique from the social sciences, called the Analytic Hierarchy Process, which allowed us to give weights to different traits based on their importance for different ecosystem services. This technique has rarely been used in ecology but has great promise in improving the use of functional traits in ecological applications where some traits are known to be more important than others for a particular purpose.

When we removed ash trees and remapped the traits present in ecosystems, we found that in the worst-hit areas of Britain, more than 50% of ash-associated traits could be lost. This represents a major ecosystem change – especially when you consider that many ash-associated traits can also be provided by other tree species. However, the combination of traits that ash provides is highly unusual and no other tree species has a similar complete collection of traits to ash trees. This means that in some areas, certain traits can be provided by other tree species while other traits may be lost entirely. Some areas (particularly the Yorkshire Wolds) and ecosystem types appear to be particularly vulnerable to ash loss. However, we have produced three key management recommendations which could, according to our estimates, greatly reduce this impact.

Firstly, we have identified appropriate mixtures of alternative tree species to encourage through planting or management, to compensate for loss of ash. By identifying the functional traits that are most at risk in different locations, we could select tree species mixtures for these areas to provide the specific traits needed to fill these ecological gaps.

Secondly, our results highlighted the importance of ash in non-woodland ecosystems. Ash is thought to be the most common species of tree outside of woodlands in Britain, so its loss in these situations may lead to profound change. Non-woodland trees have huge ecological value and provide many ecosystem services, but often their importance is poorly recognised. We recommend that lost non-woodland ash trees should be replaced as a priority with suitable mixtures of trees.

And thirdly, our study showed the importance of compensatory growth by other tree species as the major way by which ecosystems can remain resilient following ash loss. We recommend that conservation practitioners focus on relieving pressures on woodlands and trees that might limit natural tree regeneration, such as overgrazing by deer. If there are high levels of regrowth of other species following ash loss, the long-term impacts of ash dieback could be far less severe.

We have produced a one-page summary document for land managers, outlining our recommendations for management of sites with ash dieback, which is available here: AD Management recommendations. This also includes our recommended species mixtures to plant or encourage in different areas and habitat types. We hope that this will make it easier for land managers to develop management plans that will help to maintain ecosystem functioning after ash dieback.

Read the full paper, Maintaining ecosystem properties after loss of ash in Great Britain in Journal of Applied Ecology.

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