A week after my visit to the large aspen stand in the River Cannich gorge, I spent a day in Glen Strathfarrar. As it turned out, it was another day in which most of my time and attention was taken up by aspen trees (Populus tremula). It was still the first half of October, and as aspen leaves change colour in autumn before most other trees, they were the most brilliant and spectacular trees on this day in Strathfarrar.
I know where quite a number of aspen stands are in the glen, and some of those featured in a blog I wrote in autumn 2011. However, on my way into the glen now I spotted a stand beside the road that I hadn’t seen before, although it is recorded on our aspen database. Autumn is the best time to pick out aspen stands, as the distinctive bright yellow of their leaves contrasts sharply with the green of the surrounding birches and Scots pines.
Where the aspens are growing closely together, the intensity of colour of their massed autumn leaves is quite dramatic. This is particularly so in a breeze or wind, as they all flutter, and it can appear as though they are dancing in synchrony with each other, to the tune or rhythm of the wind.
The fluttering of the leaves is due to the fact that the leaf stalks, or petioles, of aspen are flattened (whereas on other trees they are round in cross-section) and very flexible near the leaf blade, causing them to tremble in even a mild breeze. This characteristic also gives rise to the second part of the binomial scientific name for aspen – tremula. This morning, though, it was quite wind-still, so I was able to get some photographs without the leaves being blurred in them, due to movement in the wind.
Because these aspens were growing in an area where rocks rose steeply on either side the river, it was a rich mossy environment – a little pocket of temperate rainforest in the glen. This was confirmed by the presence of tree lungwort (Lobaria pulmonaria), a mainly-arboreal lichen that is indicative of temperate rainforests. One fallen aspen trunk was completely covered in the lungwort, forming the largest single patch of this lichen that I know of.
In amongst the lobes of the lungwort, a few fallen aspen leaves had settled, and the juxtaposition of the rounded leaves amongst the complex fractal shapes of the lungwort lobes made for some interesting visual contrasts. Like all lichens, tree lungwort is a composite organism, in this case consisting of three partners – a fungus, an alga and a cyanobacterium – that form a mutualistic symbiosis (a partnership in which all the partners benefit from each other).
Some patches of the lungwort had brown discs on them. These are the fruiting bodies of the fungal partner in the lichen symbiosis, and are called apothecia. Tree lungwort also reproduces asexually, via structures called isidia and soredia that occur on the lobes and contain both algal cells and fungal hyphae. The isidia and soredia break off at their base and are dispersed by wind and rain. If they reach a suitable substrate, such as the bark of a tree or a rock surface, they will then grow on to form a new lichen.
On the trunk of one of the standing aspen trees nearby, a white fungus was growing out of an old wound that had partially healed over. This was the angel wings fungus (Pleurocybella porrigens), a distinctive species that fruits on a variety of trees.
It’s been quite a poor year for fungi in general in our part of the Highlands, with far fewer fruiting than in 2011. However, on a nearby aspen I saw some puffballs, beside another patch of tree lungwort. I’ll need to get the species of puffball confirmed by an expert, but I could see that each fruiting body had a hole in it, which clearly showed the results of exploding outwards from within. This indicated that the fungal spores had been released – in puffballs this is triggered by raindrops hitting the ripe fruiting bodies, which burst open explosively, so that the spores are scattered and carried off by the wind.
Down on the ground a flash of pinkish-purple colour caught my eye, and on looking closely I saw that it was Herb-Robert (Geranium robertianum), a member of the cranesbill group of wildflowers. It seemed quite late in the season for it to still be in blossom in mid-October, but this has been an odd year for the timing of many phenomena in Nature.
There were quite a lot of aspen leaves on the ground already, and they must have been blown off the trees by a strong wind as they were still at their peak colour – usually they will fall from the trees when their colour has started to fade. Some of them had landed amongst blaeberry plants (Vaccinium myrtillus), and their yellow colour contrasted vividly with the bright red of the last leaves of the blaeberry, which is a deciduous species.
The closely-related cowberry (Vaccinium vitis-idaea) is evergreen, and another aspen leaf had landed in a mixed cluster of blaeberry and cowberry, which together with a piece of fallen beard lichen (Usnea sp.) made a colourful visual combination on the forest floor.
Some of the aspen leaves had distinctive wedges of green colour in them, and these blotches are caused by the larvae of the aspen green island leaf miner moth (Ectoedemia argyropeza). The feeding of the larvae near the base of the leaves (which can be seen in the photograph as a small brown area at the point of the green wedge in the left-hand leaf), prevents the withdrawal of chlorophyll from the leaf, thereby creating the ‘green island’ in the otherwise yellow leaf.
Going a little further west in the glen, I came to another aspen stand, also beside the river. This was a stand I was familiar with from previous visits, and the trees here were also at the peak of their autumn colours. One of the trees is particularly interesting as it has multiple trunks and they are leaning out over the river. This growth is possibly a result of grazing pressure from red deer (Cervus elaphus), which meant that the only way the tree could grow was out over the water, where the deer couldn’t reach it.
By this time I had spent all day at these three different aspen stands, but there are many others in Glen Strathfarrar, and my colleague Mick Drury has just found two more that we didn’t know about previously. There’s certainly enough aspen interest in the glen to fill a few more trips, so I’ll be writing some more aspen blogs from Strathfarrar in future! In the meantime, to finish for just now here’s a compilation of some video footage I shot during the day:
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