A twinflower day

With the travel restrictions that have been imposed as part of the COVID-19 response, it’s been over four months since I was able to get out to any of my favourite places in the Caledonian Forest, such as Glen Affric. By 10th July, when those limitations were relaxed in Scotland, I was keen to reconnect with the forest and its seasonal phenomena. Top of my list was the opportunity to see twinflower (Linnea borealis), one of the rare plants in the Caledonian Forest, in blossom.

There are only a few tiny patches of twinflower in Glen Affric, and I’ve never seen them flower there, although I’ve visited them at the right time of year for that – late June and early July. However, there are some sites in the Cairngorms National Park where the species is more abundant, so it was one of those, at Upper Port Wood near Grantown-on-Spey, that I was heading for on my first trip out after the lockdown travel restrictions had been eased.

I’d been introduced to that site in 2017 by Andy Scobie, an ecologist who had done a PhD on rare plants in Scotland, including twinflower, and who subsequently was the Project Officer for the Cairngorms Rare Plants Project, which included twinflower as one of its four target species. When I’d visited Upper Port Wood with Andy in late June 2017, I’d been astonished at the abundance of twinflower there.

Ideally I would have liked to return to the site in the last week of June, as that is when the peak of flowering takes place, but the five mile limit on journeys to get out into Nature for exercise was still in force, and it precluded that, as Upper Port Wood is about 30 miles from where I live. However, when I arrived there on 12th July I saw straightaway that there were still plenty of plants in flower, even though it was obviously past the maximum, with shrivelled blossoms visible amongst those that were still fully open.

By carefully focussing on the plants with the freshest-looking flowers I was able to appreciate twinflower’s beauty anew, and get some reasonable photographs.

It was a good day for doing close-up photography, which usually requires long exposures to get an acceptable depth of field. There was a gentle breeze but it was intermittent, with calm spells in between when the delicate flowers stayed motionless for long enough to provide an opportunity to take clear and sharp images.

Some years ago I wrote a Species Profile about twinflower, and that’s available on the Trees for Life website here. The species has a circumboreal distribution, meaning that it occurs all around the world at northern latitudes, in the boreal forest zone.

I’ve seen it flower in great profusion in other northern countries, including Norway and Canada, so it’s always saddened me that it’s so rare in Scotland.

The problem with twinflower is that it is self-incompatible, meaning that one plant cannot pollinate itself – at least two plants are required for cross pollination and seed production to take place. However, in Scotland today twinflower has been reduced to isolated patches, often separated by very large distances, with each patch possibly consisting of a single clonal individual which has spread vegetatively over time, via above-ground runners known as stolons. In this it is somewhat similar to aspen (Populus tremula), which also often occurs as isolated clonal individuals and therefore rarely produces seeds in Scotland. Whereas aspen is wind-pollinated, twinflower relies on flies, hoverflies and bumblebees for pollination, but these insects will not usually travel for the kilometres that may separate different twinflower patches, so seed production is rare.

Twinflower is highly susceptible to ground disturbance, with the stolons easily being damaged, and it is also quickly destroyed by fire. As a result, it has disappeared from most of the Highlands along with its native Caledonian Pinewood habitat, due to forest exploitation, burning and clearance for agriculture. Because of its reliance mainly on vegetative reproduction, it cannot easily recolonise areas from where it has been lost, and this has led to its rarity in Scotland today.

While I was the Executive Director of Trees for Life the charity initiated a project to propagate and plant out some of the rare woodland flowering plants of the Caledonian Forest, including twinflower. Cuttings were taken from the sites where it survives in Glen Affric and propagated at the charity’s tree nursery at Dundreggan, with the intention being to establish some new populations at suitable sites in Affric. Another aim was to enrich the existing populations there by planting out twinflowers in them that had been grown from the other sites, thereby hopefully getting around the self-incompatibility problem that limits natural seed production.

That project is still ongoing and it will hopefully help to boost the twinflower population in Glen Affric, as well as establishing some new areas of twinflower in other nearby pinewood remnants. It would be great if one day the twinflowers in Glen Affric can rival the density and abundance of the species in patches like the one in Upper Port Wood.

While I was looking at some of the twinflowers this week I noticed a few discoloured leaves on a cowberry plant (Vaccinium vitis-idaea) beside them. I recognised the red patches as indicating the presence of the cowberry red leaf fungus (Exobasidium vaccinii), a species which induces the production of unusual, brightly-coloured galls on the leaves (and occasionally the stems) of cowberry. The red blisters visible in this photo are the early stages of the development of the galls.

As the fungus becomes more established, each gall takes on the form of a distorted cup shape, sometimes becoming larger than the rest of the leaf it is growing on. The cups are usually red on their upper surfaces and white underneath, where the fungal spores are released from. Strangely enough, it seemed to me as though the red and white of the fungus complemented (and enhanced) the pink and white flowers of the twinflower plants they were growing beside.

As I looked around, I spotted more and more examples of the cowberry red leaf fungus at various stages of development on different cowberry plants, in amongst the twinflowers.

Like all gall-inducing organisms Exobasidium vaccinii is a parasite, utilising the cowberry plant for its own gain, without giving anything back in return. The exact mechanism by which gall-inducers stimulate their host plants to produce specific growth forms, such as these cup-like shapes, is not clearly understood. Possible explanations that have been proposed include the action of chemicals or hormones secreted by the gall-inducing organism, or the transfer of genetic material from the gall-inducer to the host plant by means of endosymbiotic bacteria. The subject of gall formation is one that is ripe for further research, having not been prioritised in the past by the reductionist approach of science, which has focussed mainly on looking at organisms in isolation from each other. Gall formation is a classic example of an ecological relationship – it is the interaction between the different organisms involved that produces such dramatic results.

Interestingly enough, just a few feet away from the cowberry red leaf galls I spotted another very visible example of plant galls. There was a young downy birch tree (Betula pubescens) growing beside the path, and as I passed it I noticed some dark pink shapes on a few of its leaves. I recognised these as galls that are induced by a mite – I’ve seen (and photographed) them before in Glen Affric.

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The mite is known as the birch gall mite (Acalitus longisetosus) and there are relatively few records of it in the UK, although it is almost certainly more widespread than they indicate. The main cluster of records is in the Scottish Highlands, including the Cairngorms National Park and Glen Affric, with a few others scattered across England and Wales. The pink spherical structures that form the gall are collectively called an erineum, and the mite larvae develop inside them. Some further information about the species is available here.

While I was looking at this young birch tree, I turned over a few of the new leaves at the ends of its twigs, to see if there were any aphids there. I’ve had a longstanding interest in aphids, and in the past three months have taken the opportunity afforded by the COVID-19 lockdown to develop that considerably, mainly by looking for, and finding, lots of different aphids in my garden and on the trees and wild plants near my house.

Almost immediately, I found some aphids on a few of the newly-opened birch leaves, and was able to identify them later (using the excellent Influential Points website as a reference) as being a species called the yellow dark-veined birch aphid (Calaphis flava). I’d not seen this particular species before, so I was very pleased to find them here, especially when I noticed one that was giving birth.

Part of my fascination with aphids is due to their remarkable lifecycle. This includes not only reproduction by laying eggs (as most insects do), but also during part of the year (typically in the spring and early summer) by parthenogenetic means, whereby a female does not need to mate with a male in order to produce young. In this stage, a female gives birth to a live nymph, which is basically a miniature version of herself. Even more remarkably, the newborn nymph will already have the developing embryo of the next generation growing inside her. This phenomenon is called ‘telescoping of generations’ and is the means by which aphid populations can build up very quickly.

As I continued to look around this young birch I spotted another aphid on the stem of one of its leaves. This was a different, but closely-related species – the black-tipped dark-veined birch aphid (Calaphis betulicola). It’s differentiated from its near-namesake by the fact that it has dark flanges or tips to its siphunculi (the upward pointing protuberances near the rear of its body, that are used for excreting alarm pheromones, wax or defensive chemicals when it is threatened).

Although this was a small birch tree, being only two metres or so in height, there was plenty of life on it, including a convoluted mine that I noticed on one of its leaves. This was the work of the larva of a micro-moth (Stigmella lapponica), which had grown from an egg laid on the leaf by a female moth. Once it had hatched out, the larva burrowed its way through the leaf, between the upper and lower cell layers, feeding as it did so. The brown line in the centre of the mine is frass – the waste product excreted by the larva. When it was fully grown the larva would have pupated and the new adult micro-moth would have emerged to fly off and find a mate, leaving this distinctive linear pattern in the leaf as the tell tale sign of its development there. Many different micro-moths make mines in leaves, with five different species of Stigmella alone occurring on birch leaves.

By this time I’d been in Upper Port Wood for about four hours, but had spent all of that time in an area of about 100 square metres, where the twinflower occurs in abundance, because there had been so much of interest there. I decided to walk a little further into the wood for the remaining hour or so that I had available, to see what else I would find. As it turned out, I only went about another 30 metres before my eye was drawn to another young birch tree, this time a silver birch (Betula pendula).

Looking underneath a few leaves led almost immediately to me finding some more aphids – a different species called the small downy birch aphid (Betulaphis quadrituberculata). As their common name indicates, these are very small aphids, with the adults (the largest ones in this photo here) measuring a maximum of two millimetres in body length. They were present on the underside of a few leaves, and in one case I spotted an adult giving birth to a nymph.

Aphids feed by sucking phloem sap from the leaves, stems and twigs of trees and plants. They do this by inserting their hypodermic needle-like rostrum into the plant to reach the sap, and this effectively fixes them in place while they feed. Because of this, they are very vulnerable to predators, which include small birds such as the chiffchaff (Phylloscopus collybita). To reduce the risk of predation, the vast majority of aphids feed on the underside of leaves, to make them less visible to birds, and therefore requiring interested people like me to search for them there! However, aphids have many other predators as well, and an adult syrphid hoverfly had laid an egg beside one group of the aphids, which would provide easy pickings for the hoverfly larva when it hatched out. In situations like this, I’ve observed that the aphids continue feeding as usual, seemingly completely oblivious to the danger they are in.

There were quite a few young silver birches growing together beside the path, and on another one I found a different aphid species again. This was a winged adult aphid, much larger than those I’d seen before, and with an obvious dusting of blue-tinged wax on its antennae and legs. I recognised it immediately as being one of the Euceraphis birch aphids – there are two closely related species, one of which is more commonly found on downy birches and the other on silver birch trees. I guessed this one would most likely be the silver birch aphid (Euceraphis betulae), but it requires examination under a microscope to differentiate between the two species.

All too soon, it was time to head for home again, even though I hadn’t gone more than a few hundred metres into the wood. On my way back, I stopped once more to appreciate the twinflowers, knowing that it would probably be in June next year when I have another chance to enjoy their delicate and graceful blossoms.

(with thanks to Rob Edmunds for his assistance in identifying the micro-moth that made the mine in the downy birch leaf).