Assynt Aspens in flower
Aspen Populus tremula is one of our most attractive native trees in Assynt, scattered widely across the parish ‘in rocky woodland, gorges and on cliffs’ (Flora of Assynt, 2002, p.91). It suckers freely, establishing patches of clones, individuals with the same genetic make-up. The Flora continues, ‘Although it does not flower or fruit freely, it must sometimes set seed successfully, as the majority of its more isolated sites could not have been colonised in any other way.’
2005 was the last definite report of local flowering. In the previous year Bill Ritchie had reported catkins on trees at Braigh-linne, south of the road to Achmelvich (NC068245). On 3rd June 2005 Bill showed Pat and Ian a tree in that area with two branches bearing female catkins, and on 26th June members of the Field Club found a tree bearing female catkins at the southern edge of the ‘reedy field’ (NC05982451).
Recently (on 26th April 2022), Bill reported that he had just seen catkins on a tree just off the road to Ardroe, not far away (NC0724, photo 1). We were not able to visit the site until 2nd May, when we soon located a large, fallen, but still vigorous tree above the road (NC07312439, photo 2), whose branches were, indeed, covered with catkins (photos 3-4). These were about 6cm long and on examination proved to be female.
After photographing this tree, we took a closer look at an even larger, upright, tree about 15m to the north (NC07312440, photo 5). There were a few catkins on a low branch on the western side of this tree, shorter, darker and somewhat twisted (photo 6).
A closer look at these revealed clusters of purplish-red anthers, showing that this tree is male. We might have assumed that trees so close together were from the same clone, but this is obviously not the case. There were more aspens nearby, but without catkins we could not tell what sex they might be. So, there is potential for the production of fruits on the female tree, and Bill is keeping his eye on it.
As you will have gathered, aspens, like other poplars, and the closely related willows, bear female and male flowers on separate trees. The fine illustration in Stella Ross-Craig’s Drawings of British Plants (1948-1973, volume XXVII, plate 39) is reproduced here (photo 7). It shows female catkins and flowers at B and G, and male catkins and flowers at A, E and F.
What appeared to be ripe fruit, with the abundant fluffy hairs that aid dispersal (photo 7, C, H and I), has been seen on local aspens in the past, at Achmelvich and Badnaban. However, in both cases the female flowers seem not to have been pollinated and the actual seeds had not developed.
We mentioned aspen flowering at a meeting of the Highland Biological Recording Group at Drumnadrochit on 7th May and were referred to Stewart Taylor, an experienced Speyside naturalist. He later emailed that the ‘mass flowering of aspens we had locally in 2019 followed the very hot, dry summer of 2018…and this was the first time since 1996…which again followed a hot dry summer’. Weather conditions differ across Scotland, but 2021 was certainly a warm dry summer in the north-west, which may have primed our local aspens. Climate change may result in more frequent flowerings in the future.
Stewart also asked us to look out for a rare fungus on female aspen catkins, Taphrina johansonii, which galls fruits, causing them to swell and turn golden-yellow. It had already been seen this year at Strathpeffer. What is not apparent, incidentally, is where the fungus holes up in the often long gaps between flowering years.
More catkins and fungal galls
David Haines visited and photographed the Ardroe trees on 4th May, and on 7th May he found a further stand of trees on the Achmelvich road (NC07932481) with abundant female catkins (photo 8). On 12th May we noted female catkins on planted trees in the grounds of Inver House on the Baddidarach road (NC09202306).
A couple of days later, on 14th May, we had a look at the trees that David had found on the Achmelvich road. There were at least eight older trees bearing female catkins between the crash barrier and the loch, and on several the developing fruits were liberally interspersed with the striking yellow galls of T. johansonii (photos 9-10).
On the morning of 15th May Gwen found trees at Badnaban (NC074212), with a few female catkins. On the afternoon of the same day Ian had a look at the extensive stands of aspens on the northern side of the saltmarsh at Pollachapuill, (photo 11; NC105331/2). He found five trees bearing a few female catkins, mostly on high branches. Flowers on a catkin on the one branch low enough to reach appeared not to have been pollinated, which may well be the fate of many isolated trees.
The Ardroe site was re-visited on 19th May and we found to our surprise that the female tree had ‘acquired’ a sprinkling of the yellow galls of Taphrina johansonii (photo 12), of which we had seen no sign on 2nd. We must have overlooked them on that occasion, since they were visible, albeit smaller, in David’s photos taken on 4th.
We had another look at the male tree, and found that it was still producing ripe catkins. Pollen is therefore produced and spread by the wind over quite an extended period, which should improve the possibility of fertile fruit on the female trees. The trunk on this male tree is, incidentally, some 90cm in diameter, one of the largest that we have ever seen in Assynt.
This site sprang yet another surprise on that date, since four more large aspens just to the north of the male tree (at NC07312443) were adorned with the woody galls of a mite Aceria populi (photo 13). They can grow to fist-sized or even larger, and the mite may be responsible for even larger ‘burrs’ on the trunks of aspens, such as one photographed at Pollachapuill (photo 14). These galls have been seen just once before in Assynt, at Meallard, Ardroe (NC15723366) on 19th February 2001, and once elsewhere in West Sutherland, at Traigh Allt Chailgeag, east of Durness (NC44176555) on 25th July 2007.
The next event in this saga is the production, hopefully, of fruits on female trees that have been successfully pollinated. Seeds are enveloped by long silky hairs, so fruiting catkins should be really fluffy (which is why poplars are called cottonwoods in North America). We do not know when to expect fruiting, and since we have only located one male tree, at Ardroe, it may not be very widespread. Bill is monitoring the Ardroe trees and we will check the others. Watch this space!
Ian M. Evans and Gwen Richards