Prof Crawley and the Amazing Shrinking Fleabane

Last month, Prof Mick Crawley (Imperial College) led two well-attended BSBI field meetings in Herts. and South Essex to look at urban aliens. These are plants which have reached our shores in recent centuries and have gone on to become naturalised on our city streets and in patches of 'waste ground' - like the Oxford Ragwort which a few hundred years ago famously 'jumped the fence' of the Oxford Botanic Garden and made itself at home in the new habitat of clinker beds on railway lines, which provided similar conditions to the plant's original home on volcanic slopes in Sicily. Check out where Oxford Ragwort grows now, thanks largely to the railway system. 

Mick and botanists peer at alien plants in Herts.
Image: I. Denholm 

Mick was especially keen to show people how to identify three now common species of Fleabane and share his fascinating observation that one of them is shrinking! Mick's knowledge and understanding of the alien plants of the UK is on a par with Clive Stace's - in fact the two of them have just co-authored a volume on Alien Plants in the highly-respected 'New Naturalist' series and it's due out this month. 

Clive was our keynote speaker at last year's BSBI Exhibition Meeting, where he talked about hybrids. We are delighted that Mick will be giving our keynote presentation at this year's Meeting, on the subject of - you guessed it! - alien plants.
   

Mick wrote up a full report about his two urban field meetings for the BSBI Yearbook and has kindly agreed to share part of it here. Compare Mick's account below with yesterday's report of the field meeting in Co. Laois and you get an idea of the scope of BSBI field meetings - they take place in a range of habitats across Britain and Ireland, with some meetings focused more on training, some on recording and some aiming to provide a general introduction for newcomers and/or less experienced botanists. 

Over to Mick:

Jersey Cudweed
Image: M. Crawley

"Becontree (between Barking and Dagenham, South Essex) was constructed between the two World Wars as the largest council housing estate in the world, providing 26,000 ‘homes fit for heroes’ as part of the programme of slum clearance in the East End of London. In contrast Welwyn Garden City (Herts.) was founded by Sir Ebenezer Howard in the 1920s, following his previous experiment in Letchworth Garden City with the idea of combining the benefits of city and countryside, while avoiding the disadvantages of both. 

"Nowadays, both these towns contain extensive tracts of derelict industrial land, which makes them ideal for urban botanising. At Welwyn Garden City there is the site of the former Shredded Wheat factory, and at Becontree there is waste ground associated with the former May & Baker chemical works and the previously much larger Ford Motor Works. The over-riding impression in both places was of the extraordinary abundance of Senecio inaequidens (Narrow-leaved Ragwort). This is a plant that started out as a rare wool alien, but separate invasion(s) in East London have led to a dramatic increase in distribution and abundance in the capital. It is still missing from large areas of London, but these two sites are real hot spots for it.

"At Becontree, the principal crowd-pleaser was Gnaphalium luteoalbum (Jersey Cudweed) which reaches very high densities on some of the front gardens that have been covered with brick paviors to provide off-street parking. The striking pattern is that it is only paving of a certain age that is affected, while both younger and older forecourts are cudweed-free. This suggests that the operator who created the paving was using a batch of sand for bedding-in the paviors that contained a substantial seed-bank of Jersey Cudweed, and that spread by seed away from infected front gardens to both older and younger patios is relatively ineffective. At Welwyn Garden City, the main crowd-pleaser was Bunias orientalis (Warty- cabbage), a plant that has basal leaves which look much more like Asteraceae than Brassicaceae, and these can be very confusing when the plant is not in flower.

Mick and stand of Conyza sumatrensis, 1995
Image courtesy of M. Crawley

"The main training purpose of both meetings was to familiarise people with the distinctions between the three now-common Conyza (Fleabane) species: in both places, Conyza sumatrensis (Guernsey Fleabane) with its hairy involucral bracts was the most abundant species, while the long-established Conyza canadensis (Canadian Fleabane) with its hairless involucral bracts was found to be much less common than it used to be. 

"The most recently arrived species, Conyza floribunda (Bilbao Fleabane), also has hairless involucral bracts, but is distinguished by the hairs on its leaf-margins: these turn abruptly to point towards the leaf tip (as they do in C. sumatrensis) whereas in C. canadensis the marginal hairs stand out straight from the leaf edge. Conyza floribunda was substantially more common at Welwyn Garden City than at Becontree. In other parts of south-east England, C. floribunda has increased in abundance to such an extent over the last 15 years that it has caused a marked decline in the abundance of C. sumatrensis, reducing it from dominance to rarity over substantial areas of waste ground.

Mick and stand of Conyza sumatrensis, 2015
Image courtesy of M. Crawley

"It is interesting to note how the maximum size of C. sumatrensis has declined as the plant accumulates pathogens and herbivores (principally root-feeding nematodes and fungi, I’d guess). The two pictures on this page are from 1995 and 2015 respectively. When it first arrived, the plant often grew well above head height; now it is seldom taller than chest height".

Have you noticed Conyza sumatrensis getting shorter in your local patch? If so - or if you disagree with Mick - please leave a comment below and let us know.

Mick's full report then goes on to list all the other alien species found at both sites, and those found at one site but not the other, and suggests reasons for this. BSBI members will be able to read the full report (and much more!) in the 2015 Yearbook, now in preparation. If you are not (yet) a BSBI member, join today and your subscription runs until the end of next year and your membership welcome pack will contain Yearbooks for 2014 and 2015, while the 2016 Yearbook will be posted to you in January. 

Mick concluded "Both meetings were well attended and both had good weather. The success of the training element is illustrated by the fact that several of the attendees confessed to having been converted to the joys of urban botanising".

Thanks to Mick for the report and images and to BSBI President Ian Denholm, who attended the Herts. meeting and contributed the image of the group above. Ian will also be speaking at the AEM (about orchids on Guernsey), so here's that link to the AEM webpage again, where you will find a flyer about the talks - let us know if you plan to join us on 28th November.