Saturday, 19 March 2016

In defence of non-native planted trees

In my younger days I spent many weeks/months/years clearing Sycamore from local sites. At the time the mantra was that Sycamore was an undesirable and invasive alien. Many of the allegations against sycamore have some foundation; it is undoubtedly capable of taking over sites where it can readily gain a foothold. Such sites are often secondary woodland and decaying urban open space. My local site, Mitcham Common, had a significant issue with Sycamore: many of the youngest woodlands were being over-run by young trees. So, we cleared the mature trees (I spent a lot of time gaining valuable chainsaw skills on these trees) and had our teams pulling the seedlings. Today, sycamore is nothing like the problem it once was. But, were we right to take such drastic action?

Some years ago, Ted Green published a think piece in British Wildlife in which he described Sycamore as 'Northern Plane'. At the time, I thought 'I wonder if Ted has struck a more sensible note?' Certainly, if one travels north, Sycamore is a very important component of our landscape, and does not appear to be the problem that I had encountered further south. Indeed, I think we could almost say it is a fundamental part of the ecosystem on northern England and Scotland. For example, I recall one evening in Perthshire, around 9 pm, when we came across a huge Sycamore in flower that was literally buzzing. Closer inspection revealed that this tree was the focus of huge numbers of bumblebees; hence the buzzing sound. Obviously it was important in a local context and I wonder whether it plays a bigger role than we think as a nectar source of bumblebees (across the British Isles)?

After many years investigating Sycamore I have reluctantly concluded that it actually plays quite an important role in the woodland ecosystem too. True, it is fast-growing, shades out other trees and perhaps vernal flush species, and does not fit neatly into the perceived wisdom about woodland ecology. Yet, its wood and bark has many of the important characteristics demanded by epiphytic lichens, and the rot processes lead to excellent rot holes for Diptera and to very nice sap runs that support many of the species that native Elm once did. In parkland situations, old Sycamores can be immensely important habitat for saproxylic Diptera.

Ancient Sycamore at Burghley Park. The rot hole is of exceptional size and is indicative of the tree's value for Saproxylic invertebrates.
Not only is Sycamore good for saproxylics, it is also a fantastic tree to monitor when looking for hoverflies. If I visit a woodland and the ride lacks flowers I am more than happy to monitor the sunlit leaves of Sycamore, especially in the early spring. These leaves are ideal for leaf baskers and are a standard place for finding Brachyopa adults. The flowers are also great for hoverflies and for solitary bees. Meanwhile, the leaves are often infested with aphids that are favoured by a wide range of hoverfly larvae. Unfortunately, the numbers of Lepidoptera utilising Sycamore are low, and the biomass they provide for birds is correspondingly low. Nevertheless, in a controlled situation Sycamore is not the threat it is perceived to be.

Thinking more broadly, Sycamore coppice has many strengths: it is fast-growing and produces good biomass; the timber is relatively light and is readily transported; and the root system develops like other coppice stools and generates excellent decaying wood. There is therefore something of a case for thinking about Sycamore as a possible way of facilitating rapid carbon capture and use in sustainable fuel provision.

Let us not get too carried away by its strengths. Sycamore can be very invasive, its leaves support a very limited invertebrate fauna, and it does produce an awful lot of shade and leaf litter.

Thinking in a broader context, I have the good fortune to live in Stamford and to have the wonderful Burghley Park on my doorstep. I visit the park almost daily and spend a lot of time gazing at the wonderful hybrid Limes. These magnificent giants are, again, hardly native; but they have fantastic saproxylic features. The older examples are substantially hollow and support big Ganoderma fruiting bodies. Some also have fantastic sap runs, so beloved by a wide range of Diptera and Coleoptera; and, again, they can be great for aphids and species that are predacious upon aphids. In some ways they rank higher than Sycamore in the pantheon of most favoured trees by saproxylic ecologists (I recall the immense outcry amongst entomologists when it was proposed to fell the ancient Limes of Queen Anne's Drive at Windsor Great Park).
Keith Alexander recording the saproxylic features of an ancient Lime at Burghley Park in March 2016
Close-up details of sap run and young fruiting bodies of Ganoderma on the same ancient Lime

We then  move to Horse Chestnut, a southern European species that is so favoured as a parkland species. Dense foliage and tough leaves give the impression that this is not a species to be loved by the ecologist. BUT, like Sycamore, it develops fantastic rot holes and sap runs. Its decay processes lead to excellent subterranean decaying timber, and it grows quickly; thereby creating new habitat for some species where other longer-lived trees may fail to deliver in time.

And, finally, what about Sweet Chestnut? Now this is one that I have never been fond of. But, I am a Dipterist. Were I to be a Coleopterist, I might think differently, as Sweet Chestnut supports many of the important heart rot fungi of Oak, and as such it also harbours many of the noteworthy beetles that like dry red heart rot.

Thus, I hear the calls - why are you an apologist for the 'Foreign Invader'? Well, I am not! But, in today's World I think we have to start to think in broader terms. We have already lost the Elms that supported many important Diptera. Thank goodness for Sycamore and Horse Chestnut that have maintained the necessary habitat. Various Oak diseases threaten our iconic ancient trees and thus the beetles and other invertebrates that utilise decaying timber. Thank goodness for Sweet Chestnut!

And then there is the stately hybrid lime! A magnificent feature tree, fast growing and capable of supporting a wide range of saproxylic invertebrates.

Now, I am not advocating replanting the landscape with non-natives; but I think in the current climate where new pathogens threaten out native trees such as Ash, we have got to look seriously at the possible alternatives that will allow our native fauna to survive!

1 comment:

  1. Nicely balanced Roger. This has all the makings of an interesting article for British Wildlife!