RBGE tree expert outlines benefits and challenges around agroforestry

There are concerns about the impact of climate change on agroforestry - can you talk us through some of the challenges?

The increased incidence of extreme weather events is having an impact on the health of trees. Trees that are stressed by, for example, a significant period of drought, will be more vulnerable to pests and diseases that they may otherwise have been able to cope with. This is particularly the case if extreme weather conditions follow each other quickly, for example waterlogging, then a long period of drought, followed by more waterlogging, then a big freeze. Of course, trees are able to adapt to changing conditions, but this process can take time whereas the climate is changing very quickly.

Climatic conditions are also becoming more conducive to pests and diseases that would not ordinarily be able to survive and spread in the UK. The Asian Longhorn Beetle (Anoplophora glabripennis) has shown it can survive and spread after an outbreak in Kent in 2012. This was managed but if the pest is introduced again, it could become more widespread and would impact many deciduous tree species.

Cedrus Libani needle blight (Image: Web)

Another example is the bacterial plant pathogen Xylella fastidiosa which has caused a devastating disease outbreak on ancient olive trees in Italy. Xylella has a very wide host range which includes many trees and plants commonly found in Scotland such as cherry (Prunus species), oak (Quercus species) and maples (Acer species). However, some strains of Xylella are restricted in their distribution because they cannot survive cold conditions. It therefore stands to reason that warmer conditions in the UK would mean that this pathogen could have an impact here should it be inadvertently introduced.

Are people right to be concerned about the use of non-native species such as Sitka? Are there any benefits of these?

Forestry is very important to the Scottish economy and Sitka spruce plays a significant role as an excellent timber species. Outside of timber production, biodiversity (the variety of animal and plant life in a particular habitat) is what is important. Therefore, a relatively small number of ‘escaped’ Sitka spruce are unlikely to have a significant impact on biodiversity if they are managed properly and are prevented from colonising habitats at the expense of native tree species. This is particularly important for sensitive environments such as Scotland’s blanket bogs.

Does agroforestry bring additional benefits to neighbouring agri land?

Yes, absolutely. Even individual trees are a hot spot of biodiversity and have a positive influence on the surrounding environment. For example, oak trees are known to host 2300 species, including 38 bird species, 229 bryophytes, 108 fungi, 1178 invertebrates, 716 lichens, and 31 mammals. Of these, 326 species were found to be obligate associates eg they are only found on two native oak species, Quercus petraea, and Q. robur.

Canker on the trunk of Toxicodendron verniciflura (Image: Web)

In return, the oak tree provides a farmer with shade and shelter for livestock, improvements to soil health, and improved water carrying capacity of the land, which is important for reducing ground saturation and associated flooding. In an arable setting, tree roots gather nutrients and water from deep in the soil which benefits surrounding crops, and the tree canopy protects the harvest from weather extremes. In addition, trees attract insects which move around the landscape pollinating crops and wild plants as they go.

Can more be done to ensure the right trees are in the right places and not encroaching on crop production?

Yes, it can. Perhaps it is helpful to think of the two elements as mutually compatible rather than in competition with each other for space. For example, in a three-dimensional farming system, trees become part of the cropping process, using their height for a crop that does not impact on ground space. For example, single rows of apple trees planted north to south to minimise shading with an alley between them for cereal production. Wildflower species are planted beneath them to attract vital pollinators. This approach could also provide another income-earning crop (i.e., from the fruit trees) and improve business resilience.

The obscure Curreya pithyophila fungus has now been spotted in Scotland. Do you anticipate that this will spread to Scots pines throughout the country and can anything be done to prevent its spread?

Investigations into the extent and potential impact of this fungus on Scots pine in Scotland are ongoing. What is understood so far is that C. pithyophila is in a symbiotic relationship with a small sapsucking insect known as an adelgid, an association which has been noted on several conifer species since the 1800s. What is different now, is that the fungus has seemingly become very widespread across Scotland very quickly. The reason for this is yet to be understood so there is currently no information on how to manage this disease.

Curreya pithyophila is now in Scotland (Image: Web)

It is not clear whether this disease will have an impact on the growth or survival of Scots pine. Some individual trees have been found with extensive dieback, but for the most part, trees exhibit some thinning of the crown and dieback of shoots and branches in the lower crown.