Early autumn in Strathfarrar

Blog edited on 27th October to add this video footage of the dance of the aspen leaves in autumn:

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It had been several weeks since my last photo trip out to the Caledonian Forest, because of a recurrence of the sciatica in my leg, so on October 5th I was very pleased to get out again. I headed to Glen Strathfarrar with my partner Pupak, planning to have an easy day that wouldn’t stress my still-recovering leg too much. My intuition was that we’d find somewhere in the forest there, close to the road, where there would be plenty of interesting subjects for photography, and that indeed turned out to be the case.

Driving into the glen, we’d gone less than a kilometre when I spotted the brilliant yellow foliage of an aspen tree (Populus tremula) through a group of Scots pines (Pinus sylvestris) at a section of the river that I hadn’t explored before. I had driven past this particular place countless times, but hadn’t noticed the aspens previously, most likely because my visits hadn’t coincided with the peak time for their autumn colour change. Today, though, the aspens stood out like an unmistakeable welcoming flag, so we stopped the car and decided to explore the area.

As soon as I stepped out of the car, and before I’d taken more than half a dozen steps towards the aspen trees, I spotted something of interest –  a bright orangish-red mushroom on the forest floor. I recognised it as a waxcap fungus in the genus Hygrocybe – these are a distinctive group of fungi, often characterised by bright red, orange or yellow colours. I suspect this one may be the honey waxcap (Hygrocybe reidii), but I’ll have to get that confirmed by an expert.

While I was photographing the waxcap, Pupak found another mushroom nearby – one which the cap had been broken off from its stem. When she brought it to me I recognised it as being the orange birch bolete (Leccinum versipelle), a common species in the Caledonian Forest at this time of year. As its name suggests, it grows in mycorrhizal association with birch trees (Betula pendula and Betula pubescens). In this mutually-beneficial symbiotic relationship, the hyphae of the fungus wrap around the roots of a birch tree, providing nutrients to the tree that the fungus accesses in the soil, and in return receiving sugars and carbohydrates that the tree manufactures by photosynthesis.

Turning the cap of the fungus over, to look at the tubes or pores on the underside, I noticed a couple of small invertebrates crawling over the surface of the pores. Looking through my hand lens, I recognised them as being springtails – a class of invertebrates that are distinct from insects, although they appear insect-like to a casual observer. Scientifically known as Collembola, springtails derive their common name from a tail-like appendage that is held under tension beneath the body and when released, enables them to jump, to escape danger.

Springtails are some of the most abundant invertebrates in the world, occurring in soils and mosses all over the world. Their density has been estimated at up to 100,000 per square metre in some locations! We had a survey for springtails done at Dundreggan in 2013, so I collected these specimens after photographing them and sent them to Peter Shaw, who did the survey for us. From looking at the specimens, he replied that they are a common species (Pogonognathellus flavescens), which he’d found in abundance during his survey at Dundreggan.

As is so often the case, once I started looking closely at the vegetation around me on the forest floor, I noticed other things of interest amongst the rich diversity of plants, lichens and mosses that flourish in the understorey of Caledonian Forest remnants like this one in Glen Strathfarrar. A patch of reindeer lichen (Cladonia sp.) caught my eye, with its fractal, multiply-bifurcated shape contrasting with the frond of a common polypody fern (Polypodium vulgare) growing beside it.

A couple of steps away, another juxtaposition of a lichen and moss also drew my attention. In this case, it was a patch of common haircap moss (Polytrichum commune), whose individual shoots look like miniature pine seedlings, that was growing next to a clump of a different, but related, lichen (Cladonia sp.). Mosses and lichens can often be overlooked by people, but for me they are some of the most interesting and photogenic subjects in the forest!

Walking on a little more, I came to a slightly wetter area of ground.

Beside the fallen trunk of a small birch tree, there was a patch of one of the distinctive sphagnum mosses in the forest – five-ranked bog-moss (Sphagnum quinquefarium). The orange-red and pale yellow leaves of this species make it stand out from greener mosses such as the common haircap. Just beside the sphagnum, another plant that grows in damp areas was flourishing – bog myrtle (Myrica gale).

This is an aromatic shrub which can grow to about waist height in good conditions, but the plants here were about half that size, most likely because of being kept in check by the grazing pressure of red deer (Cervus elaphus), which find the plant highly-palatable. Bog myrtle is deciduous, and some of the leaves had already changed to shades of yellow and orange. On another plant, the buds that would produce next spring’s flowers were clearly visible, amongst the leaves.

A few metres away, I was attracted to a moss-covered boulder, as there are often things of interest on rocks like that – in this instance, another common polypody fern. This one was larger than the other I’d seen a few minutes previously, and had more fronds. It was also growing at about shoulder level on the rock, which meant that I was able to look at the underside of the fronds quite easily, without having to lie flat on the ground to do so! The reason for my interest there is that the common polypody has distinctive clusters of sporangia, which are called sori (or sorus in the singular), on the bottom of its fronds.

The sori are the structures that release the fern’s spores, and on the common polypody they are larger and more visible than on many of the other ferns in the Caledonian Forest. On this particular polypody, there were some sori at an early stage of development, where they looked like translucent, pale cream insect eggs, and others at a mature stage, where they were brown and had begun to open out, so that the spores could be discharged into the air, in order to facilitate the reproduction of the fern.

By this time, well over an hour had gone by, and we still hadn’t reached the aspen trees, although they were only about 100 metres from where we’d parked the car! It had been quite windstill so far, and as I like that condition for photographing aspens, without their leaves blowing all over the place, I stopped looking at the details on the forest floor, and we made our way to the grove, to take advantage of the stillness while it lasted.

The bright yellow colour of the aspens were in effect a delightful trailer for the autumnal colour display to come in the other deciduous trees of the forest during the rest of the month.

While I was photographing the aspen trees from various different angles, Pupak had spotted some lichens on the ground with bright red apothecia on them, so I took the opportunity to grab a few images of them too.

The apothecia are the part of the lichen that release the spores of the fungal partner in the symbiotic partnership between an alga and a fungus that makes up the lichen.

By this time, we’ed eaten our lunch and it was mid-afternoon, and, as is often the case, the wind had picked up. However, it wasn’t blowing constantly, but rather was coming in strong gusts for a few minutes, followed by a brief period of stillness again. This provided me with an ideal opportunity to take some contrasting images of the same scene – some using a long exposure when the wind was blowing, to give a sense of the motion of the aspen leaves, and others when it was perfectly still.

Aspen is unusual in that the leaf stems or petioles are not round, as with most trees, but are flattened where they join the twig or stem they grow out from. It is this flattening that causes the leaves of aspen to tremble or shake in the slightest breeze, and, in stronger winds, to dance in massed synchronous movement. While this can be observed at any time of the year, it is more pronounced somehow in the autumn as the unique colour of the aspen leaves adds to the conspicuousness of their dance with the wind.

In North America, the closely related aspen species (Populus tremuloides) is called the quaking aspen, because of the similar movement caused by the flattened petioles. Between them, these two very similar aspen species have possible the widest geographic distribution of any trees in the world, with their shimmering foliage gracing the boreal forests all across northern Europe, Russia, Siberia and the northern part of North America.

There is no widely accepted reason for why aspen has evolved these flattened petioles. However, one theory is that by having leaves that flutter in the slightest breeze, the movement makes it harder for leaf-eating insects to stay on the foliage, thereby lessening the amount of damage the tree sustains from their herbivory. However, I think this is rather dubious myself, as I’ve seen plenty of caterpillars and beetle larvae feeding on aspen in the Highlands, where it is generally quite windy – some of those aspen-feeding insects have featured in my blogs such as this one and another one here.

The dance of the autumn aspen leaves is one of my favourite events during the year in the Caledonian Forest. It also brings to an end the relatively brief period when aspens are in leaf. They are one of the last trees to gain their leaves in spring, with them sometimes only opening fully in the north of Scotland in early June. Then, as this tree here demonstrated, they also lose their leaves early than most trees, in this case at the beginning of October.

The consequent short period of just over 4 months doesn’t provide a lot of time for  aspen to photosynthesise and utilise the sun’s energy for its own growth. However, perhaps to compensate for this reduced access to the sun’s energy through its leaves (compared to other tree species), aspen is able to photosynthesise through the presence of chlorophyll in its bark, which can be seen as the green coloration in this image here.

Perhaps this too is an evolutionary adaptation, like the flattened petioles, to reduce the impact of insect herbivory on the tree’s ability to harvest energy from the sun, as its bark is not subject to the same levels of insect attack as its leaves, or those of other trees. These are some of the questions I ponder when I’m out in the forest, spending time with trees like these, and deepening with connection with, and understanding of, the woodland ecosystem and the role that everything plays within it…