Turnover and change in the street flora of Cambridge – a talk given to the BSS Conference on Urban Floras in 2023.
This talk reports on two studies of the urban flora of Cambridge. The first, an annual survey of plants in flower in a transect of eight streets in the city. It began with the 2016 New Year Plant Hunt and aimed to find out what was happening in the other months of the year. The key finding was a huge turnover in plant species from year to year and even month to month. It also showed how drought and the minimum temperature of the preceding month affects the number of flowering species.
The interim results of a study which repeated one first undertaken of 14 streets in Cambridge in 1998/99 were also presented and discussed. The key finding was again the huge turnover in species. Seven species were discussed that had increased greatly and another seven that had decreased greatly.
Dr Chris D. Preston, our speaker, has lived in Cambridge since 1973. He worked in biological recording, editing atlases of bryophytes and vascular plants, and since retirement has developed interests in urban plants and plant parasitic micro-fungi.
Right um thanks Jim um we heard I think yesterday rumors about somewhere called the south of England where some of the uh frequent species uh uh there uh some of the rarer ones in the central belt of Scotland and I’d like to take you there now
Um to to Cambridge which is uh as you’ll see here in uh one of the warmer parts of Britain and certainly in the driest part and Cambridge has a a curious history of growth in that until the early 19th century it was surrounded completely by open agricultural Fields
Uh managed in the medeval way and um development was completely prohibited in this common land and so the Georgian Town became increasingly uh crowded increasingly insanitary and it wasn’t until enclosure of the common fields in the early 19th century allowed residential areas to expand onto the surrounding land and the streets I’m
Talking about are largely those that were developed in the 19th century as part of this residential expansion and um so they’re outside the historic center of the town but only a rather brief walk into it and my Talk’s based on two studies um I’m starting with a survey of the plan
Flowering in the streets around my house which I’ve carried out monthly since January 2016 and then I’m going on to describe the interim results of another survey in which I’ve been repeating a study of another group of streets first carried out by uh my friend Philip Oswald in
1998 1999 and which I’ve been resurveying this year so My First study is a transa of eight streets um in which I started in January 2016 as part of the New Year plant hunt and I’ve continued it monthly ever since because it struck me that we were gathering an awful lot of
Information about what was flowering in New Year but we knew rather little about what was flowering for the rest of the year in any systematic way and I list the plants um flowering in each of these eight streets each Street individually at the start of each month I include
Only the roads and the roadsides the publicly accessible areas and the streets I cover vary from the simple streets at the bottom right um such as Albert Street with uh houses coming directly onto the road to the much more complicated um what I call a roundabout and the council calls a gyratory system
Here and uh with Lawns and flower beds Etc and I do note the habitats as you do for the urban studies in which the flowering plants are found uh Pavements walls uh vires Etc now those of you with a statistical bent May dismiss this exercise as a
Study based on a sample size of one and therefore telling you nothing that can be generalized beyond the eight streets of the survey and you’d have a point and I’m perhaps uh fortunate in retirement that I don’t have to subject my research proposals to critical scrutiny uh but
There are I would say compensations for a study in which you’re walking one transact again and again rather than as so often in modern recording uh visiting singly visits a whole range of different sites and the most obvious of course it’s you can build up a time sequence of
Uh data but there’s also much less tangible benefits of getting to know what’s going on in detail of watching the results of all the factors that are affecting the Flora such as climatic extremes the managements of the streets by the council the very treatment of their frontages by corporate land owners
And individuals and even the longevity and persistence and fate of individual plants and this um well strawberry became something of a landmark for me as it persisted for five years on this wall of modling college until it was officiously rooted out uh one spring to my
Sadness um I don’t want to say much about the phology of the Flora because I’ve um written about this elsewhere but there’s a clear annual sequence the number of flowering species increases rapidly in the spring to a maximum in June or July and then decreases much more gradually to a minimum in February
Or more rarely in March and there’s a clear relationship ship um shown here between the number of plants flowering in the winter months and the temperature here I’ve plotted it against the mean minimum temperature but clearly that’s correlated with all sorts of other factors like the uh absolute minimum temperature for the preceding
Months uh similarly um it’s clear at least to me that in drought years and we’ve had two very severe droughts in in my six-year period in my 8year per period that the number of plants in the high summer is reduced compared to what it is in a normal year
And i’ uh despite John’s warnings yesterday uh would have the temerity to assert that this is causing effect and that we are showing uh a reduction in in the very dry years the third um significant factor which Michael mentioned is wheat killing and until this year the council has
Applied wheat killer to the street streets in late spring or early summer now you realize that a corporate weak killing as you walk along the streets isn’t that efficient uh areas between part behind part bikes which there are a lot in Cambridge are under part cars are ignored walls aren’t sprayed private
Areas leading off from the streets aren’t sprayed but nevertheless the weat killing clearly reduces the number of flowering plants and I think there’s an interaction with the climate in that mature plants which might survive through the drought of the summer are killed uh but don’t then regenerate from
Seed until moist conditions return in the Autumn so in effect the wheat killing is enforcing a more Mediterranean Lifestyle on the species and it’s favoring winter annuals which germinate in the Autumn and flower in the spring or those what I call opportunistic annuals which can germinate whenever conditions are
Suitable and complete their life cycle uh within a few weeks and it’s also uh of course favoring or encouraging the evolution of herbicide resistance which I’ll come back to in a moment now in this spring the council decided not to apply the annual round of weed killer and they also reduce mowing
So that this closely Cut Bank it was left to grow and was had a sort of token trim around the edge and and that’s all and the result of these changes became very obvious by June in my Transit initially by uh the number of poppies that turned up they’re clearly
Just as vulnerable to herbicides on the street as they are in arable fields and they were much more frequent this year and as it was a moist summer uh plants continued to grow and and by now uh there’s a great green ribbon at the bases of the uh walls in the uh less
Heavily trampled streets and the junction between the Pavements and the roads and you may have noticed when I showed the number of flowering plants I’ve recorded that the total this year of the number that’s the number in the of of species time Street the what every species can occur in up
To eight or the number of species itself is considerably higher than it has been in any previous season and uh perhaps more significantly for the long term though I don’t record these as they’re not flowering are the number of seedlings of Woody species that are turning up uh quite frequent
Birches buers Plains here poppers and and Willows on the streets now the new Council policy is controversial and looking at the internet I can see that it’s throughout uh Britain there are similar arguments raging about Council stopping weed killing and in one street um there was a
Complaint and a squad of eight men were sent to to deal with it uh three with hes and a spade uh they were very unhappy about what they had to do they thought it was too hard work um there were two driving the Vans with the rotary rushes there were two driving
Vans behind them which you can’t see to to take away the stuff that they cleared and then there was the statutary man with a clipboard overseeing proceedings and this hasn’t happened anywhere else and I really the the complainant must have been extremely influential that’s all I can
Uh but because of this sort of thing one thing I’ve become aware of in visiting the streets month after month is the high turnover species and if you see an interesting plant you want to photograph or identify you have to do it straight away there’s no point procrastinating it
So often it won’t be there it’ll be here today and gone tomorrow well I’ve summarized the eight years of the transact here uh each of these numbers refers to a species the number of streets uh I’ve recorded it in cumulatively over the years and the number years and um rather crudely uh
The 301 species I should say Al together that I’ve recorded and if you rudely regard the number of streets of the frequency and the number of years as the persistence I’d like to use this structure to try and give you an idea of my impression of the population dynamics
Of the species on our streets and I’ll start in the bottom right hand corner here with the 40 eight species I’ve recorded uh in every year and cumulatively in eight streets and that includes eight species which have been recorded in every street in every year now here they are and mainly there
Are the what I call the opportunistic annuals as are the near misses the species that that almost make that category uh things like ground chickweed um hairy bitterest Etc but the one that really intrigues me is actually one of the few perennials uh pereria uh we haven’t I think heard it mentioned uh
Yet this conference I have a suspicion that it’s much more frequent in Cambridge than it was just a few decades ago um certainly if you go back to 1860 babington’s florer he lists uh 15 sites on Old walls in the county eight uh churches or churchyard walls and only
One one is in Cambridge itself and by 1964 later Flora it was still only occasional on Old walls and buildings throughout the county well it’s now abundant along Pavements as well as occurring slightly lesser extent on Old walls and if you look at my street totals uh you can see it throughout as
Throughout the year don’t believe what the books tell you they won’t tell you this um and there is a little dip in the late winter uh but the really cold weather we had in December uh 2022 uh really hit it back and the flowering shoots were killed it died
Back either to the base or was killed completely although it regenerated from abundant seedlings and this um strengthens my suspicion that it might have been favored by recent milder Winters um that it’s also I suspect herde resistant although I think herbicide resistance is a really unstudied feature of the British Urban
Flora but it is recorded as resistant uh in in other European countries uh there isn’t a great deal of support from the BBA bsbi Atlas data for my theory that it’s increasing recently but but I still think there is evidence in Cambridge that this is the
Case now at the opposite end of my table at the bottom row um founding in eight years but in only one street there are a group of three species and uh another group recorded sorry as a single colony but uh in fewer years and the best example of these
Which shows the remarkable Persistence of some plants to continue as small populations in the same place year after year without spreading is Sedum album which is confined on the transa to just 1.5 M of wall here um and flowers every year quite happily um unfortunately the
Wall has begun to fall down and uh I suspect the sedums uh days are numbered because uh come the Reconstruction of the wall I would think it’s almost certainly going to be eliminated in repointing and although these small populations can generally seem stable over the short term I think that
Probably their high life is I suspect rather short as they’re vulnerable to this sort of accidental extermination this is a more surprising member of the category ertica membresia Mediterranean annual expanding its range in Britain but recorded in Cambridge since 2009 as just two small restrictive populations uh one of them on the transa
Uh and hasn’t spread since then from them there’s another small group of species which are equally persistent um as very small populations but their persistent is mass because some years they don’t escape from the seed bank uh but they do pop up uh more often than not and my
Best example of this is Toad Rush junus bonus restricted just to the edge of one manhole cover and I receed recorded it there six out of the eight years and Ro rip austrias is in the same area not quite so limited uh whereas uh kinin and origanifolium um
This is um just in one small stretch of pavement I’ve seen this in four years of the eight uh and um none of these species are found anywhere else on the transit they’re just these small persistent populations but these recurrent populations coming from the seed bank are not always easy to distinguish from
Species recur because they’re repeatedly introduced from nearby parent plants cultivated nearby and this Mel elima actually sews itself onto the street here from my own garden where it is here now I wouldn’t recommend it it’s a thuggish plant uh it takes over Gardens I like to think I
Bought it at the instigation of my partner rather than by my own choice but I can’t honestly remember um but can be difficult to decide whether plants are spreading directly from Gardens or whether they’re self- sustaining on the Pavements and I happen to photograph it
But cimri Orient used to be a feature of one section of the transa just one area of path where it grew as a weed in adjacent Gardens in one of two moris derel houses and on the pavement well the garden plants disappeared when the derel houses were renovated and since
Then the pavement population has declined mind to Extinction nearly and I think as a species it flowers in May and June often at exactly the same time as the herbicide is applied so it’s very vulnerable to that and I think in retrospect the pavement plants obviously need a topping up from the adjacent
Garden plants which are unsprayed the other side of the garden rails right well to return to my summary table um I now like to go here 49 the highest total of all species recorded in just one Street in just one year and of these 49 at least 23 were present as
Just a single plant probably more I haven’t been very good at keeping a record of of Singletons and many of these are garden escapes and there’s an almost infinite number of species I think that might germinate from a garden onto the street um not all of them are
Likely to survive so flowering and thus be recorded by me but still it’s potentially a very large number and my more unusual plants include euphobia kapner uh which appeared here one plant here this was cut down to the very base within 10 days of me finding it and
Recording it um Salvia mofler is uh another one that appeared as a single individual and uh selenium pseudo capsicum and and I must thank here uh Alan lesie a friend in Cambridge who’s done a great deal to help me identify these uh erent Horticultural species and
Uh if you’re judging him uh against your apps Michael I think Alan would come uh very much higher than they do in his accuracy um this single corn caliz suspect came from Wildflower seed which was directly scattered on the street for some reason rather than from dispersed
From a cultivated plant uh but I have no idea how this tiny example of the arable weed desia came to be on its pavement and I have no idea how this single massive Hemlock came to get onto the tree base either these are all just one species one year um some of the
Singletons are plants which are native elsewh though they may well not have been uh introduced the streets from native stock uh there’s the lovely marshmallow Alia this was the first record from the Cambridge Square TL 45 since the late 18th century and then we’ve had this year a single uh for
Basam thsis I’m pretty sure this wouldn’t have escape the herbicide regime in earlier years these bals are particularly vulnerable and then um on the dry Street of Castle Street uh Veronica kator although this is a street that leads down to the river so perhaps it didn’t come up from the
River so so far I’ve dealt with three groups the common Staples the rare persists and the rare transients but there are of course many other cases uh more frequent casuals uh species that tend to sew themselves out of hanging baskets around pubs or window boxes around houses of which the most frequent
In the transit is liilia recorded in all years and from six streets in total and I found tomato in three streets and I don’t think it’s a coincidence that there are the three streets uh in which there are pubs and we didn’t have in John’s mechanisms for
Introducing uh seeds into urban areas uh plants introduced in human vomit but I think it is one Anna Leslie puts it in his Flor of cambri rather more delicately he talks about them being distinguished being dispersed by regurgitated fruit this one that I photographed is actually there you can see it’s only a
Short stagger from the exit to this I don’t know how uh occasional brassas these are these just crop up from now again again with some cereals particular along the basis of trees perhaps you can help me explain what’s happening there we do have Hala fights I think these are introduced from the
Major roads leading into the city um codan is normally just the odd casual plant but there is a small population that has um begun to get established it’s lasted for four years now along one of the Pavements but pinellia distand which we saw in the previous talk that’s
Nothing more than a casual in the transa never found uh more than two years in succession on the same street and one final category are plants that may be in the first stages of permanent colonization and I’m pretty sure this is one polycarpon tum it’s been established on Cambridge Street since
2011 but only arrived on the transa in 2021 he’s now on two streets and I’m pretty sure it’s here to stay and this is one I think John showers polypogon monell Enis started off as a casual in 2019 on the transic but it seems to be rather more slowly becoming
Established well this discussion of longer term change brings me to my second uh survey which is a resurvey of uh network of 14 streets about the same length as the 4 kilm streets of the transit which was originally undertaken in 1998 and 1999 by uh my friend Philip Oswald in a
Joint study with Arthur chater Arthur looked at a comparable group of streets in abist withth uh Philips streets are mainly in the new town area of Cambridge um this is the same date as edinburgh’s new town uh but very dissimilar in character I’m afraid it lacks the
Grandeur and it’s rather more subtle in its allegiance to the hanian succession um and like the transit streets I’ve talked to it has a AET a mixture of major streets and minor streets I haven’t quite re finished this um res survey this year so I’m only dealing with some interim results uh but
I put here each number represents a species and the number of streets Philip recorded them in here mine here so this two for instance means eight streets in Philips survey and five in mine now the first thing I think strikes me about this table is the huge number of
Species that were rare in both surveys so Philip found 55 species in the streets that I’ve been unable to refind I found 10 in the streets that he didn’t notice and I think this emphasizes the tremendous turnover of the street flora and I won’t enlarge on that because I
I’ve said enough about it with respect to the transit uh but it it is really very striking whereas down here at the bottom right we have the sort of core species that are found just 10 found in all streets in both surveys uh what I want to do is just mention the species
That have shown the greatest increase and the greatest decline pending a a more detailed analysis later and these are the ones that show the greatest increase ones that Philip didn’t find or scarcely found and ones which I found in over half the streets and here they are uh some of
Them will be familiar to you already from earli talks but I think the point I’d like to make is almost all of them follow National Trends if you look at the short-term Trend identified by the statistical analysis in the latest BBI Atlas you can see almost all of these are increasing nationally
And uh this is the irrigant we’re not quite sure Philip recorded an alien iRig and he in three streets he wasn’t quite sure of the name they were taxonomically uncertain then um and if you look at the BBI map for summer trenis um the pink area is the national range in 1999 you
See in Cambridge we were right at the edge of that and the red triangles are records that have been added later and clearly it’s now gone up and way past us and and I was rather pleased able to photograph this is iRig and I hope some a trenes actually growing outside the
House Philip lived in when he was doing the survey 33 Panton Street um also uh Missing You will have noticed cerian angustifolium and CIO squalidus both oh sorry there’s one more um increased both POA infirma and uh seems to be plant of the day polypogon viridis see again we we it’s it’s
Engulfed Us in its spread um as an increasing species but the I was going on to talk about the ones that had declined they’re the ones that Philip found in over half the spe streets but I found in either none or uh fewer than four or
Five and here they are and it’s puzzling here they they don’t follow National Trends anything like as clearly um there’s a indication of a strong decline in Roseberry Willow herb in the English records from the bsbi and as strong decline in Oxford ragwar but others include some increasing species so so
There isn’t this close tie up with the national Trend and Rose Bay has been it’s a very striking loss Philip had it in 12 streets and it’s clear from his notes which he passed on to me before his death that he’d um recorded it in several sites in some of the streets I
Just got four individuals and I photographed three two of them fling and two of them vegetative and that’s all and similarly CIO squalidus has gone uh and in general in Cambridge it seems to be retreating back to its Railways uh but I did find which Philip didn’t CIO
Viscosus in three streets and the recently arrived CIO in Aiden as one and the raid ground saw CIO Volaris has also gone he Philip found it in just one street but it was a feature of that street uh in his time and and and it’s missing now another complete
Disappearance found in 13 streets in Philip survey and not at all in mine is Elder and interestingly this was one of the commonest species on walls when John rbur surveyed Cambridge walls in a Pioneer survey in the 1940s but Jonathan Shanklin and I we surveyed 85 walls in
2020 and it was only present on one so there does seem to be a genuine uh loss and perhaps in the discussion people might have ideas as can help me to explain that other losses are equally puzzling um tomato isn’t any longer in Philip streets although as we’ve seen it
It is in my rather more disreputable part of Cambridge uh and leelo’s decline there too and I think only the only increase I think I can explain decrease is male Fern dryopteris Felix Mass which I think potentially an effect of uh the summer droughts uh which um it’s at the
Edge of its distribution in uh Cambridge uh it’s often found by leaky drain pipes and I think it may well have suffered from uh the the summer droughts I’ve shown you well that’s uh coming to the end of what I wanted to say uh and I I’d like
To step back from the sometimes sorted details of the urban Flora um this is a combination of uh weed killing and covid neglect and oh drought sorry it wasn’t we this a drought of uh 2022 and covid neglect and ask what conclusions can we draw well I hope I’ve
Given you an idea of the massive flux of the urban Flora at least in this area the relatively small core of frequent species and the transience of many of the species that you’re likely to find uh on the streets and superimposed on this is of course an element of
Long-term change as species expand and contract their broader ranges now of course other habitats have transient species too and I was interested to come across a paper by Matt Sutton who studied the plants of his pen Brookshire farm from year after year and he also notices these he refers
To them as the blink and you miss element in his Flora but the urban areas I’m talking about I think are particularly uh likely to show such high turnover the streets are lined by Gardens as I’ve said almost anything can s out of the gardens uh that is then
Very vulnerable to Accidental damage or deliberate persecution uh so as botanist surveying the urban Flora I would say that we should remember that just as the urban streets can be noisy places the data we collect is also likely to be F full of noise too thank you very [Applause]
Much Chris thank you very much that was a great great talk and a great piece of work uh questions I thoroughly enjoyed that talk absolutely fascinating and I what I wondered was whether the fact that you’re wandering these streets and people maybe get to see you month after
Month it’s the the guy with his hand lens and his beard again get the children inside that kind of thing but do you do you do you do you think your repeated surveying has had any effect on the way people treat the plants that are growing just outside their front
Gate no I don’t I sometimes wonder if um Wonder too uh people deliberately clear their front just to avoid the but I don’t I don’t think that’s that’s the case um what I am subjected to I don’t know if anyone of you find it is what I call
Nimes on the prow I’m always people coming are worried I’m from the council and I’m planning some sort of change to their area and and they don’t seem to realize that if I was from the council I’d be in a high Vis jacket and I wouldn’t be working on a bank holiday or
That sort of thing that they’re so terrified that I’m planning a wind farm at the end of the street or something like that uh but but you do get it a lot of conversations but I don’t think they protect and I’d be a bit worried because of course I wouldn’t want to influence
The the results of what’s Happening than you Michael we did a a resurvey of the wild plants of the Glasgow botanics a few years ago 20 years on from a definitive study by Peter mcferson and that threw up the notional statistic that in a given defined area such as your transect
The turnover or churn would be roughly between 1 and 2% per year um I I haven’t been able to do the maths quickly enough on your graphs but does that seem reasonable to you that seems a bit low to me but we we ought to
Do the maths and and a point I didn’t make I forgot to make was that in the Philip survey we’re looking at all plants including the non-flowering ones whereas I only look at flowering ones and clearly the the churn is going to be much greater for the vegetative ones but
I’d be very interested to to do the maths and to see try and work out with you how they compare but um it’s about I don’t what is it if it’s about 50 of my 300 in my transa are just in that over eight years have just
Appeared once that sounds a bit greater than 1 to 2% turnover doesn’t it which would be 3 to six a year yeah but we we ought to compare those two yeah you referred to herbicides and herbicide resistance and um it was my belief until fairly recently that most of the time
The councils use glyphosate uh but I did um see some Council workers using a gray powder and I wonder whether some councils instead of using glyphosate would which is um which degrades over over weeks and months whether they are beginning to use something more persistent that would last and would
Would keep the weeds away for um more than a year do do does anyone in the room even have um any any knowledge of of what exactly are the policies of local authorities before we ask ask other people I just say that they do there was a public information request and the
Counsiling Cambridge have said what they use I don’t remember the details but it’s characteristic in Milky dots appear on the plants they spray as a liquid milky but I don’t know if there are people who know more exactly I was just going to say the um pesticide Action
Network have got some really good Publications on pesticide use and I think they found so they didn’t information request to every Council in in in in Britain n not all of them replied but those that did I think 99% of the pesticides being used were glyphosate but people are turning now to
Other Alternatives as Chris has described in his talk with with gangs of people reluctantly going out and removing vegetation but also things like salt people are using salt um uh and and uh uh what sort flamethrower guns as well just to remove the actual surface vegetation so people are coming up with
Other Alternatives um but glyphosate is is is by far in a way the it’s interesting up in North Wales our local town has just been spray or sprayed a couple of months ago relentlessly with glyphosate and Petry of the wall shriveled along with everything else and I’m really interested in
This the the resistance has been noted in several countries around the world with several species but we’re just not not monitoring it it would be really useful If people could record if a Street’s been herbicid is there any evidence of species actually actually survive and and persist that you know
Persist after that after that application that that be released student project to grow grow progeny and and look at herbicide resistance yeah I was struck by the fact that some of the some of your photographs showed really beautifully grown plants in some places you know the The Mullins and I
Wondered if is there any evidence that people are actually looking after them or even planting them they some people do like them definitely and there is a counterbalance to the headlines I shown there have been a couple of streets where people have inv invited me to help them Count Their
Plants and determined to oppose the other people on the streets who want them cleared um I did one wonder how about the poppies whether they might be swn and I’m I’m pretty sure the conco was scattered because and I forget what it was it was something like uh Anem
Miss or striker but there was another signature plant of that sort of annual mix appeared in the the following month so there is a little bit of sewing going on yes but but certainly some people love the poppies although others think they’re absolute disgrace yes D you showed a showed a picture of
Brasas and cereals I just wondered whether that was a bird seed mix or somebody throwing down something yes I’m I I really would like to know how that gets there and it could be a bird seed mix but would people put it you know on the streets I don’t see people throw you
See people throwing seeds for pigeons oh it may yes it may if there are yes yeah I haven’t actually seen that but but that does seem the more likely yeah explanation yeah um yes so thank you for lovely talk so over the past few days we’ve heard about the amazing biodiversity in urban
Areas and uh the wealth of information that you have so what are the opportunities of feeding all that data to government policies on their biodiversity action plans I I spent my career trying to avoid policy and uh it’s I think I leave that to cleverer people than I am Sim I
I was always an embarrassed to me I never really understood what policy was and uh had to pretend that I did when uh discussions came around to it so that’s I’m glad to be free of policy and retirement and just looking at the weeds