Saturday, 30 July 2016

Eureka Moment with Burdocks

I have made various post on burdocks over the last couple years. A tricky group where the discriminating characters are limited and the literature repeats the same unclear information over and over again. I also think it not unfair to say that there have been many decades of misunderstanding, uncritical recording (Arctium minus agg.) and likely widespread misidentification (NB. I don't claim to have all the answers here or to never have been part of the problem). What was needed was clear photographs, a good understanding of local populations and how they vary locally and nationally, and most of all I needed to find Wood Burdock (Arctium nemorosum). I was starting to believe the latter was mythical, until an opportune trip to Swaledale (VC65) this week, where it is frequent. However, something was still not sitting right in my head, but finally 9 pm last night it finally clicked into place and I provide my view on the situation below in the hope that it helps others. I apologise in advance for the dry text upfront, but lots of photos follow below.

In the following account I will use Arctium pubens for brevity and clarity, not A. minus subsp. pubens. I think really we should all be treating this plant as a species for recording purposes, lets call it 'Confused Burdock' for want of a common name. I'm not suggesting there are not challenges here (more later), and this species is a PhD study in itself BUT lumping with Lesser Burdock (Arctium minus) is not helpful and is a major part of the problem. My rationale is:
1) Arctium pubens is probably the most common species over much of England and perhaps the UK as a whole, but too much data is being lost under the agg.
2) Lets be honest it (with notable exceptions, see photos) looks reasonably distinct, and to treat it as a form of Lesser Burdock for recording purposes is unhelpful and potentially dangerous (in the same way, for example, that it is untenable not to segregate the native and non-native forms of Lamiastrum when recording - think of the implications for nature conservation and scientific understanding).  I suspect that Wood Burdock may be over-recorded because many botanists find but are unaware of Arctium pubens and how much it typically differs from Lesser Burdock. They can also quite clearly see it is not good Lesser Burdock, so 2 + 2 = 5 it must be Wood Burdock. Lumping with Lesser Burdock implies a level of similarity that usually is not there in practice.
3) All botanists are more than capable of, and should be, recording burdock taxa to species level when in flower - don't let the variability of Arctium pubens deter you. I hope this account helps with this, most species are readily picked out at distance (from a car even) once known (but in some cases an ID may need refining to Arctium pubens on close examination). If you don't record to species then I see little value in recording the aggregate, its just perpetuating the historic problem and taking us further from a solution.
4) I really don't understand why Arctium pubens is considered closer to Lesser Burdock than it is to any of the other species, nor how it has derived certain traits (particularly the often exaggerated pubescence) from its putative parents.

Before getting to the photos, it's worth giving quick consideration to the one character that can be quickly used to split Arctium pubens from Lesser and Wood Burdocks. The former has peduncles greater than 8mm, the latter two have peduncles up to 8mm with capitula more usually +/- sessile. This comes back to my point about the poor quality of published descriptions. This trait relates to the distal part (upper part away from the middle) of the branch, but what is meant by distal is never defined i.e. is it immediately just above the mid-point or if not how far up? At a certain point down the branch the peduncles start 'trying' to be branches (you can't have both sessile capitula AND a raceme in Lesser and Wood Burdock without allowing for non-sessile stalked capitula) and clearly if you measure these you will quickly go wrong. For the last few years I have been concentrating on the top 15 cm or so of the branch, and this works well for all species except Wood Burdock. My eureka moment was realising distal would be better phrased as the terminal part of the branch. It may be that distal in Lesser Burdock and Wood Burdock are not quite the same thing, this may sound like a fudge to suit me but I feel that this comes back to age old problem of writing keys to separate taxa. Quite often it is very difficult to explain how taxa reliably differ in a key, but the human eye can tell the difference at a glance. This is the case here and the photos below will show this. Wood Burdock in particularly is a completely different beast from Lesser Burdock and Arctium pubens.

So lets start with Wood Burdock, as its a plant yet to feature on the blog, and then work through the spectrum from sessile species to the corymbose pedunculate Greater Burdock (Arctium lappa).

Wood Burdock (Arctium nemorosum)

Sell & Murrell are probably right to rename this Northern Burdock, records from the south need revisiting. As I have noted previously, I have searched in vain for this species in my Yorkshire VC, and have only found Arctium pubens. Note the relatively large capitula and the terminal clusters of sessile capitula at the tips of the main and side branches. This is the key jizz of this species. The Swaledale (you will also find Arctium pubens in Swaledale) plants are all uniformly short and I wonder if this is typical of the species, if so this is also useful jizz. Unfortunately I did not photograph the plant as a whole, this would have been helpful as it would have shown how the morphology of each branch contributes to the look of the plant as a whole.

Note also how quickly long peduncles start to appear below the terminal cluster - so the ambiguity in use of the word 'distal' is not helpful. This species seems a bit of a scruff-bag - the ugly duckling of the group.

Final bit of advice - don't trust Google!

Lesser Burdock (Arctium minus)

This is the plant of the south of Great Britain, see Sell & Murrell for their detailed views on distribution. Capitula are +/- sessile but in a different way to Wood Burdock, with the terminal capitula usually solitary (but not necessarily remote).

Warren Hill (VC31) - capitula much smaller than Wood Burdock and a taller more refined plant as a whole, note the obviously exserted flowers

Warren Hill (VC31)

Below is the Yorkshire form which is tenable for inclusion in this species if we accept my terminal/very distal rather than distal hypothesis. To me it would seem to sit better here than as a Arctium pubens, something that has been giving myself and Mike Wilcox much to contemplate recently. I may be wrong, but having seen it again today it is not too divergent and therefore I don't feel it too controversial to include it here. My only concern is that it is less inclined to form colonies and only occurs as an odd plant here and there in a landscape otherwise populated by Arctium pubens. It seems fully fertile though.

Jinny Moor Lane, Leventhorpe (VC64) - tolerable 'northern' Lesser Burdock, this plant had the exserted flowers but they are not well captured in this photo

Little Preston (VC64) - full seed head from a 'northern' Lesser Burdock

'Confused Burdock' (Arctium pubens)

This is were the fun begins and this species needs urgent genetic study to determine its origins, relationships, and whether it hides a 'real' species within a hybrid swarm. Also research based around whether any of the various geographic variants merit description at the rank of variety or forma. There are distinct forms that are consistent over large areas, indicating that they are capable of breeding true and maintaining themselves. There is variation in capitula size, branching pattern (corymbose to racemose), degree of pubescence, and degree of browny-red pigmentation. Its often simpler to start with 'have I got Arctium pubens, if not why not, therefore it must be species X?'. This is best achieved by looking at the peduncles.

Thorp Arch (VC64) - large heads and very pubescent, subcorymbose to corymbose

Oulton (VC63) - very similar to above but many kilometres away, green stems will be a shade effect, subcorymbose

Ledsham Banks (VC64) - racemose form, approaching nemorosum/minus in character but plants when seen as a whole are nothing like those species

Fleet Lane, Woodlesford (VC63) - the common brown-flowered racemose form, note how it might be confused with Wood Burdock but the longer peduncles are there (slightly clearer in the next photo) and the capitula are less agglomerated

Fleet Lane, Woodlesford (VC63) 

Swillingon (VC64) - 'pseudo-lappa' form

And the problem forms .....

Occasionally atypical forms are found e.g. with very small capitula. Current concepts mean you either treat as a Lesser x Greater Burdock hybrid (Arctium x nothum) on equivocal grounds, or you lump in Arctium pubens on the basis that this is of the same hybrid origin and segregation and recombination may occur throwing surprises.

Neither is a satisfactory answer, the following plant is from ancient woodland (also with Arctium pubens but no other taxa). Lesser Burdock is very rare in VC64 and the location is outside the local range of Greater Burdock, also rare, so this plant has clearly not been derived in situ as a hybrid from these parents. It is possible that Lesser Burdock occurred historically, so if not Arctium pubens sens. str. can we contemplate a hybrid origin through a cross with the latter? A problem for another day.

Hayton Wood (VC64) - small flowered 'pseudo-minus' form, note long peduncles so not Lesser Burdock. Lesser Burdock is very rare in VC64 (and some records need revisiting)

Woolly Burdock (Arctium tomentosum)

The holy grail species! Inflorescence approaching corymbose. Completely unmistakable, based on my two encounters with this rare plant, with the combination of brilliant white woolly capitula and rich purple flowers. There are supposedly less hairy variants in Europe - hmmm, I can't comment but if Continental botanists face the same problems as UK botanists then perhaps this statement is best treated as equivocal. Don't believe the key in Sell & Murrell - a case of over-emphasising one trait that is not reliable - the inner involucral bracts can be hooked. Trust your eyes instead.

Castle Hills (VC64) - what a beauty!

Greater Burdock (Arctium lappa)

If this species is rare in your area and you are not familiar with it be careful, some forms of Arctium pubens have inflorescences and capitula redolent of this species. That said it is unlikely to cause problems. Large plants with a corymbose inflorescence and green glabrous or very sparsely pubescent capitula. Note the solid petiole is only of use for separating from Lesser and Wood Burdocks. It is not reliable for Arctium pubens.

Appleton Roebuck (VC64)

Appleton Roebuck (VC64) - note diffuse corymbose inflorescence

Appleton Roebuck (VC64)

1 comment:

  1. very useful account - thanks - was momentarily worried that an extra-woolly pubens might be tomentosum, but the photos here help confirm it (think scientific name for woolly burdock is a typo?)