Why do we favour certain tree species over others?

Why do we favour certain tree species over others?

During these strange circumstances I’ve found myself with a lot of computer time – a rarity!  I will aim to blog every week whilst I’m stuck at the desk and will try to pick a topic to delve into in a little more detail rather than a generalised update of everything. I shall at least try to present any research in an approachable form, but I do also want to provide you all with a little food for thought!

If anyone has any comments (good or bad, I have a thick skin), questions or topics they would like me to write about then please do drop me an email, [email protected] First on the blog agenda…

Why do we favour certain tree species over others?

It all starts with a simple question and a not so simple answer…what trees support the most wildlife?  More often than not, people like to refer to the native vs-non-native argument and then reel out an influential study carried out in 19611 that listed the number of foliage feeding insects for certain native and non-native trees. This study highlighted that native trees predominantly supported more leaf eating insects than non-native trees. However, does that mean we should remove all non-native trees? In short, no. As you can imagine, there’s a little more to trees than just the number of leaf feeding insects they support, though this is an important factor.

Tree Creeper (Certhia familiaris) on the commons Pedunculate oak trees. Photo Credit: Andrew Wilson

Trees can support (and are supported by) an array of other organisms such as; fungi (both soil dwelling ‘mycorrhizal’ fungi and wood decaying fungi), cavity nesting birds and bats, fruits and seeds for fauna, blossom for pollen and nectar, wood decaying insects as well as numerous epiphytes (lichens, mosses, algae)  and, in turn, all the species that depend on them!

What’s interesting (to me, at least) is that certain non-native trees can partially support species that would otherwise rely on our native trees. For example, several invertebrates found on native field maple (Acer campestre; very small population on the common) can also be found on the non-native Sycamore (Acer pseudoplatanus) which, due to its ability to spread rapidly and reseed, is plentiful across the common. Sycamore also support a large number of aphids, which are important for insects and birds. Similarly, the decay of the heartwood (dead central area of a tree) in the non-native Sweet Chestnut (Castanea sativa) and False-acacia (Robinia pseudoacacia) is similar to that of our native oaks, attracting some of the invertebrates associated with decaying wood in oak trees2.

Ok, so now we know that we shouldn’t be too hasty removing all non-native trees when should we remove some? Let’s look at one of the species mentioned above, False-acacia, which can be found in several areas across the common forming dense stands such as here and here. As we’ve established, this is a non-native tree. However, it also has a few other characteristics that can make it very undesirable and mean it’s also classed as an invasive non-native species (INN’s). It’s rapid growth rate and ability to spread via an extensive root network which throw up shoots (known as suckers) means that it out competes our native flora and causes an overall reduction in the amount of wildlife that is supported, in the area’s that it is present3.

Shaggy Scalycap (Pholiota squarrosa) in a silver birch tree. Photo credit: Andrew Wilson

It is for these reasons that we have been removing some of the False-acacia across the common. We will only remove a selection of (smaller) trees per year as to allow native wildlife to adjust gradually to any change over several years. Secondly, we will not remove mature trees, as they can harbour invertebrate species as well as having cavities that are suitable for birds and bats. Such habitat is in short supply on the common until our young trees age and gain suitable habitat features.  The removal of these dense stands of False-acacia also allows more light to penetrate the woodland floor and provide a more open woodland structure. Trees that can grow to their full (open structure) potential with plenty of space between each tree, will support more wildlife than densely packed closed canopy

The same evidence-based approach is taken to other species across the common. The previously mentioned Sycamorecan readily colonise secondary woodland and shade out native species. Another example is the Turkey oak (Quercus cerris), a rapid coloniser across the common, with a large spreading crown. It hybridises (cross pollinates) readily with our native oaks and supports very little wildlife. At certain times and areas of the common we may remove some of these species, so they do not dominate the flora. However, when removing smaller specimens and where safe and possible the trunks will be left standing to support deadwood invertebrates.

I hope this goes some way to explaining the processes as to why we remove certain species across the common and how we reach our decisions. It is no surprise that native trees do, as a whole, support more wildlife than non-native trees as native wildlife has had thousands of years to associate with our native flora. However, this does not mean we should group all of the non-native trees across the common as ‘bad’ for wildlife, only those with an invasive nature reduce the level of biodiversity when they dominate an area. This is also not to say we should be planting non-native trees.  We will always opt for natural regeneration of our native trees and provide areas where they can flourish!

(well done if you made it this far)

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  1. Southwood, T.R.E., 1961. The number of species of insect associated with various trees. The Journal of Animal Ecology, pp.1-8.
  2. Alexander, K., Butler, J. and Green, T., 2006. The value of different tree and shrub species to wildlife. British Wildlife18(1), p.18.
  3. Lazzaro, L., Mazza, G., d’Errico, G., Fabiani, A., Giuliani, C., Inghilesi, A.F., Lagomarsino, A., Landi, S., Lastrucci, L., Pastorelli, R. and Roversi, P.F., 2018. How ecosystems change following invasion by Robinia pseudoacacia: Insights from soil chemical properties and soil microbial, nematode, microarthropod and plant communities. Science of the Total Environment622, pp.1509-1518.

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