MMM #18: Welcome Guest or Unwanted Invader?
A maintenance visit to fix a missing #17 presented an ideal opportunity to plug a gap at the end of the MMM series . . .
The cache, a small camo-taped screw-capped sample tube, is hidden next to a wooden gate in the ivy-covered wall, overlooked by a mature invader tree - a sycamore.
Many trees in the English countryside, such as Ash, Oak, Beech and Lime are loved and admired, but not often the Sycamore, which may be regarded by some as an arboreal ‘bad guy’.
The case for the prosecution is well made. It is not part of our native flora, although exactly when it was introduced is still debated. Having arrived, it has spread rapidly, and is now the most widespread tree species in the UK. It also doesn't seem to promote biodiversity, supporting only 15 species of herbivorous insects, compared to the oak's or birch’s 300+ species.
Honeydew (see below) drips from the tree coating everything beneath in a sticky layer. The copious leaves shed in autumn decompose to a slippy, mucilaginous mess on pavements, in gardens (killing or at least inhibiting growth of any plants beneath) and on rail lines - causing the infamous annual engine wheel slippage and disruption of train journeys.
However, this is not the whole story. Although indeed non-native, looking at British flora overall, some 50% of the species have been introduced and are now naturalised in the countryside.
Although it may invade various types of woodlands including birch, ash, alder and oak woods, and many disturbed sites, the rate of invasion is usually slow because the conditions suitable for sycamore regeneration only occur at irregular intervals despite regular and abundant seed production.
Rootlet penetration after is prevented by compacted and dry soils. Young trees cannot tolerate grass competition, are liable to fungal attack and grazing by slugs and/or small rodents, whilst saplings are grazed by large herbivores.
The relative importance of these controlling factors is affected by light availability, soil nutrients and weather (chiefly rainfall). So, sycamore is actually a gap species, which becomes established in treefall gaps, heavily disturbed areas, or under the canopy of low shade-casting species such as birch, sea buckthorn and alder.
Sycamores produce a copious flowers in spring, providing both nectar and pollen for bees and other foraging insects, and in winter, the seeds are food for small rodents.
Aphids are particularly abundant on sycamore, with an estimated 2.25M on a mature tree in late summer, providing a great food source for birds, and insects like ladybirds and hoverflies. If you park under a sycamore in summer your car will soon become be annoyingly covered in a sticky substance called honeydew.
The aphids feed on tree sap - their food source and mainly just a sugar solution, with only 1-2% protein. So, they must ingest lots of sap to get enough protein, and what is excreted is similar to what went in – minus a few absorbed amino acids. Honeydew - is therefore just aphid pooh. Ants often ‘milk’ the aphids, and bees and wasps may feed on honeydew in late summer when there are fewer flowering plants for nectar.
Sycamores, like plane trees, also have a flaky bark, under which it is home for several invertebrates.
So, although it would be a good idea to remove sycamores from our ancient semi-natural woodlands in order to prevent them rapidly spreading and taking over, elsewhere we may need to be more tolerant and appreciative of their positive impact.
See here for a fascinating article on the sycamore in Britain and how it may be effectively controlled through the retention of male flowering trees which do not produce viable seeds which are the main means of spread and subsequent invasion.
See here for an illustrated blog on sycamore through the seasons and here for another excellent, well illustrated blog on The most unpopular tree in Britain?
See here for a great set of sycamore botanical illustrations.
Following is Zoe Mitchell's wonderful poem on the Sycamore Gap tree by Hadrian's Wall in Northumberland. It was featured as Poem of the Week in the Guardian of 27/1/20 which describes the 'fierce standoff between a wall and a tree rich with allegorical resonance'.
Sycamore Gap
You’re history, said the tree to the wall;
the last crumbling remains of empire.
You are the invader, replied the wall.
I am the conqueror, said the tree to the wall;
sending platoons of seeds across my territory.
I stand alone, replied the wall.
I chose this valley, said the tree to the wall;
stretching my roots under your scored foundations.
I belong here, replied the wall.
I am growing taller, said the tree to the wall;
you’re a lonely stone sentry outstripped by a sapling.
I remain, replied the wall.
I am a survivor, said the tree to the wall;
I host the resurrection of each turning season.
I endure, replied the wall.
You’re the one they blame, said the tree to the wall;
insensate barrier, stone-deaf to the rough bark of liberty.
You cannot know, replied the wall.
The tree appeared in the 1991 film Robin Hood: Prince of Thieves, so it is known as the Robin Hood Tree. It is one of the most photographed trees in the UK and in 2016 was crowned English Tree of the Year in the Woodland Trust awards.