One of my favourite events of the year is that time in spring when the new leaves of the deciduous trees re-appear after the dormancy of winter. It always feels to me like there is a tremendous sense of joy and celebration in Nature when the bright green foliage of the trees bursts out from the buds, transforming the forest as it does so. I always refer to this part of the spring season as ‘The return of the leaves’, and I wrote a blog about that, with numerous photos of the new leaves of various different trees opening out from their buds, back in 2015.
The exact timing of when the new leaves appear varies from year to year, depending on factors such as the daily temperatures, rainfall patterns and the persistence of snow or frosts. In some years, the first trees to get their leaves – usually the birches – do so in mid-April, but in other years it can be early May before the buds burst. This year, I was away in Chile from the 22nd of March until the 24th of April, so I was hoping it would be a late leafing year, and that I wouldn’t miss too much of it by having been abroad.
As it turned out, it was quite cold in northern Scotland while I was away, with snow falling for several days in early April, so leaf burst was later than in many other years. After my return from Chile I went to Ireland for 3 days in early May, to speak at a native woodland conference there, so it was only on the 13th of May that I was able to get out into the Caledonian Forest. I went out to Glen Affric then, and as I drove up into the glen I was greeted by the bright green colours of newly-opened leaves on the birch trees.
I stopped just above Badger Falls, where the most accessible oak tree (Quercus petraea) in the glen is growing, right beside the bridge. This is a favourite place of mine, as the branches of the oak tree reach across the parapet of the bridge, and I’ve often found (and photographed) interesting insects feeding on the tree’s leaves. This day, however, it was the new leaves themselves that caught my attention, as they were just bursting from their buds, and the catkins were also unfurling.
Less than 10 metres from this oak, there’s a young oak tree about 3 metres tall that has obviously grown from an acorn that came from its elder neighbour. The buds on this young one hadn’t opened yet, and as I looked at them I noticed an unusual gall that I didn’t recognise at the tip of one of the twigs. While I was photographing it I came across several more galls, also at the tips of twigs. When I contacted Margaret Redfern, an expert on galls who helps me with their identification, she said it was called the cola-nut gall, and is induced by a wasp (Andricus lignicolus).
When I then did some research on this species, I discovered that these galls are induced by the agamic generation of the wasp, ie the stage in its life cycle when the females can reproduce without being fertilised by males. What surprised me though was the fact that the sexual generation of Andricus lignicolus induces galls in Turkey oak (Quercus cerris), a non-native species that has been introduced to the UK. I’m not aware of any of those oaks in the vicinity of Glen Affric, but there must be some not too far away, to have enabled this wasp to spread to the area and induce these galls in our native oaks just beyond the entrance to the glen. While I’m always interested in, and fascinated by, galls on plants, it is rather alarming to discover what is a non-native species making an intrusion into the Caledonian Forest at Glen Affric. While the threats posed by introduced species such as the grey squirrel (Sciurus carolinensis) and rhododendron (Rhododendron ponticum) are well-known and are receiving a lot of conservation action, these more inconspicuous invasions by small invertebrates are occurring under the radar, with consequences that are as yet unknown for our native biological diversity.
Not far from the oak tree I came across a young rowan (Sorbus aucuparia), whose leaves were also just opening out from their buds. For me, there’s a real contrast between how different trees leaf out in spring, and this is perhaps best exemplified by comparing leaf emergence in the oak and the rowan. Whereas the former gives me the impression of a mass of leaves pushing powerfully out of their buds, a bit like rugby players driving a scrum forwards, the new leaves of the rowan always seem to me to emerge more like a ballet dancer – graceful, elegant and with a beautiful visual symmetry. As the leaves stretch up from their bud, still held together, they remind me of a ballerina’s arms, reaching together over her head, and then being lowered slowly to each side, as she dances.
Further west in the glen I came across a rowan where the leaves were already fully open. As a result, it seemed to me that there was quite a variation between how advanced different trees of the same species were, in terms of their leaf emergence.
By this time I had arrived at the Loch Beinn a’ Mheadhoin picnic site, where downy birches (Betula pubescens) are the predominant tree species, and most of them had their leaves out as well. They created a bright green atmosphere in the forest, which was enhanced by the sunshine. I had my lunch while I was there, and also spent some time looking closely at the new leaves. Whilst I always greet their emergence in spring with a lot of joy and enthusiasm, I’m not the only one who eagerly awaits the bursting forth of the new season’s foliage.
On one of the birches where the new leaves were just opening I noticed a small green shape – it was an aphid. I have a special fascination with aphids as a group of organisms, but my feelings in this case were more with the tree. Its new leaves weren’t even fully opened yet, and already aphids were on the scene, ready to suck the sap flowing in them. Many species of aphids time their emergence each spring to coincide with the rising sap in trees and plants, and other insects also hatch out or become active from their overwintering state to take advantage of the lush new growth of foliage in the spring.
The return of the leaves is a major event in the life cycle of many invertebrates, and as I looked at the new leaves of various trees I saw a diverse range of insects on them.
It was not just herbivorous insects that were out and about, as I spotted a beetle (Glischrochilus quadripunctatus) on the leaf of a downy birch that is a predator of European bark beetles. I hadn’t seen this species before, but it is quite distinctive, and I’m very grateful to Sholto Holdsworth of the Natural History Museum in London who identified it for me.
I spotted several stoneflies on leaves and tree trunks, and those adults must have recently emerged from their larval forms in Loch Beinn a’ Mheadhoin, which was just a few metres away. On the trunk of a rowan I came across a cranefly that I didn’t recognise, and although I wasn’t able to catch the specimen, Peter Chandler, an expert in Diptera (or two-winged flies) who helps me with identifications, said it was a species in the genus Ula, but it wasn’t possible to tell which species from the photo alone.
A week later I made another visit to Glen Affric and returned to some of the same places I’d been to 7 days before. Stopping again at the oak tree by the bridge above Badger Falls, the change that had taken place in the interim was very obvious, as the new leaves were now fully opened and bright green in their freshness. The catkins that had just been beginning to appear previously were now unfurled, and some of them had currant galls on them already.
These galls are induced on both the catkins and leaves of oak by the common spangle gall wasp (Neuroterus quercusbaccarum), and the agamic (or nonsexual) generation of these wasps time their emergence to match that of the new spring growth on the tree.
The agamic females are able to produce eggs without mating with a male, and they are laid on the buds of the catkins and leaves.
The currant galls swell as the leaves and catkins develop, with a wasp larva growing inside each one. These larvae are the sexual generation of the wasp, and males and females emerge in early summer, mate, and the females then lay eggs on the underside of the oak’s leaves. These stimulate the oak to produce different galls – the common spangle galls – within which the larvae of the agamic generation of the wasp develop.
The galls remain on the leaves when they fall to the ground in the autumn, and it is from these that the agamic females emerge in the following spring to lay their eggs on the catkin and leaf buds, and so the remarkable alternating cycle of gall production continues. Last year, leaf emergence (and consequently gall development) was several weeks earlier than this year, and I was able to photograph an adult wasp when it was ready to emerge from a currant gall in late May. I also photographed a female laying her eggs on the underside of a leaf, from which the spangle galls develop. The remarkable relationships between species, such as this one between the wasp and the oak tree, never cease to fill me with wonder, delight and awe!
The common spangle gall wasp is of course just one of the species that induces galls in oak. Oak hosts more gall-inducing organisms than any other tree in Britain, with over 70 species stimulating the production of galls on it. Many of those are wasps, and on one leaf I saw both a currant gall and a different gall, at the tip of the leaf. I didn’t recognise the latter so I consulted with Margaret Redfern, who told me it is induced by another wasp (Cynips sp.). There are apparently three different species that could be the causal agent for it, and it is only possible to identify them from the adult wasps, not from the galls themselves.
Moving on from the oak tree, I stopped at an eared willow (Salix aurita) that was just getting its new leaves – on the 20th of May this was surprisingly late. While the leaves were just emerging from their buds, the flowers of this male tree were also opening. Willows are dioecious, meaning that an individual tree is either male or female, unlike oak and many other trees, where male and female flowers occur on the same individual. Willows are insect-pollinated, and the newly-opened male flowers are very attractive – this one had a muscid fly (Phaonia sp.) visiting it.
Oaks and eared willow are not alone in having their flowers emerge at the same time as their leaves. When I reached Dog Falls, there were some blaeberry bushes (Vaccinium myrtillus) right at the falls themselves, resplendent in their brilliant green new foliage and flush with flowers.
There’s an old aspen tree (Populus tremula) growing right at the gorge at Dog Falls, and I was keen to see if it had got its new leaves yet, as it had still been bare when I was there the previous week. Aspen is always one of the last, if not the last, species of trees to get its new leaves in spring, and in some years it’s been the beginning of June before they are fully in leaf. This year, the old tree and the various younger ones growing nearby off the same root system were just leafing out on the 20th of May.
New aspen leaves are a particular favourite of mine, as they are very delicate when they first open, and have a beautiful coppery tone to them, which only lasts for a few days before they turn their more familiar green colour.
Aspen leaves flutter in the slightest breeze, but it was now early evening and quite wind-still, so I was able to take a number of photographs of the emerging leaves. While I was doing so, I noticed some movement on the young aspen’s stem, and when I looked to see what it was I was surprised to discover a pine weevil (Hylobius abietis) climbing up. This distinctive insect is widely regarded as a serious pest of commercial forestry, but in natural forests such as those in Glen Affric it is in balance, as part of a diverse and healthy ecosystem. It breeds in the stumps of conifers, but the adults feed on the living bark of a range of woody species, so I’m not sure what effect it would have on the aspen. However, its presence, like that of the birch aphids and gall wasps that I’d seen earlier, was another reminder that I am just one of many different organisms who eagerly await the rising sap and emergence of the new leaves of the trees each spring!
Dear Alan,
Have just read with great interest, your essay on leaf emergence in May. It was most erudite and educational. One learns something new every day. It has opened my eyes to aspects of trees never before realised. Thank you – you are a great teacher, never stop sharing your knowledge and observations interpreted with such enthusiasm and wisdom.
Your devoted fan
Midge
IOM Woodland trust
hi Midge,
Many thanks for your comment and the appreciation of my blogs. As I write these to share my knowledge of the forest and its species, it’s great to get feedback like yours, which confirms the value and worth of the time I spend on creating the blogs.
With best wishes,
Alan
Another great blog — Early Spring was always a favourite time for my father and I remember on numerous occasions he would remark on the beauty of the wide variety of greens as the new leaves emerged on the different trees. It is a magical time indeed. Thanks Alan for another interesting blog
Many thanks, John – I always appreciate your positive feedback about my blogs!
With best wishes,
Alan