Birch Fruits Galore

October 18th 2021

Birch fruits galore

In September and October any surface beneath Assynt’s abundant downy birches Betula pubescens becomes speckled with their tiny winged fruits. These are shed by the plump hanging female catkins, as these fall apart.   

I was prompted to take a closer look at these fruits after David Haines had sent me one of his excellent close-ups, from the north end of Gleann Leireag (NC1431) on 12th September 2021 (photo 1).  I collected quite a small catkin (15mm long) from a tree in my garden at Nedd (NC1331) and stripped off the catkin scales and fruits to count them.  Most scales bear two fruits tucked into their concave side (photo 2), occasionally three, so 70 scales yielded some 155 fruits.  

How many?

This led me to wonder, idly, how many fruits Assynt’s downy birches might produce in an average season.  Fortunately, more than 40 years ago, two keen researchers had counted the number of fallen fruits per square metre under birch canopy not far away, on the Inverpolly National Nature Reserve (Miles and Kinnaird, 1979).  Over a 6-year range it varied between 3,800 to 43,300 fruits, say 20,000 as a round number.

About 3% of Assynt’s overall area of 475 sq. km is woodland, mainly dominated by downy birch, our commonest tree, i.e. 15 sq. km.  So, if my arithmetic is correct (literally worked out on the back of an envelope), downy birches in Assynt could yield 300 billion fruits (300,000,000,000) each year, a truly astronomical number.

Since the average weight of a fruit is 0.175 mg. (Kinnaird, unpublished data), the total weight of this annual crop is some 50 metric tonnes.  

What feeds on birch fruits?

I wonder what happens to all this biomass?   Many fruits are consumed by seed-eating birds, such as goldfinches, redpolls and siskins, but other organisms are also involved.  Nearly 10% of my sample were fatter than usual, with vestigial wings and curious ‘windows’ on one surface (photo 3, right).  They had been galled by a cecidomyiid fly, Semudobia tarda, one of three members of this genus that gall birch fruits.

One fruit had a tiny black excrescence extending from the top (photo 3, left).  This may well be the stromatal stage in the life cycle of a minute cup fungus Ciboria betulae, the only species found on birch fruits, but that will require expert confirmation.  Unsurprisingly, given its small size, it is not often recorded, with only two of 27 records on NBN hailing from Scotland.  By sheer coincidence, one of these two is by my one-time neighbour and friend, the late John Blunt, who recorded it at Nedd (NC1331), on 12th May 1987.   

Ian M. Evans


Miles, J. and Kinnaird, J.W. (1979). The establishment and regeneration of birch, juniper and Scots pine in the Scottish Highlands. Scott.For. 33, 102-119.  Cited in the very useful Report of the Symposium on Birches, 1982.   Proc.Roy.Soc.Edin., Sect.B (Biological Sciences), 1984.

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