BSBI Species Accounts ArchiveOrobanche rapum-genistae
Mon, 09/06/2010 - 09:33 | 
markd
Greater Broomrape, Orobanche rapum-genistae Thuill. (O. major auct.) is one of the most straightforward of the broomrapes. For a review of the taxonomy and identification of Orobanche in Britain, refer to Rumsey & Jury (1991).
Chromosome No.: 2n = 38 (Stace 2010).
Photography: A.J.Lockton
There have been thought- provoking accounts by Mike Foley in Scarce Plants (Stewart, Pearman & Preston 1994) and the New Atlas (Preston, Pearman & Dines 2002) in which he makes predictions about changes in the distribution of this species. As time goes by these theories can be tested as more data comes in. The maps in Scarce Plants were ground-breaking for the time, giving different distributions based on the recording periods used. Unfortunately, the technique that was employed is open to some doubt because the date periods are not comparable. One map shows all records, which accounts for 300+ years of recording, while another shows less than 14 years of recording for the 1980+ map. It is no wonder that a decline was detected. Bowen’s detailed account of it in Fl. Dorset (2000) illustrates the problem rather nicely. Roughly speaking, there has been one record of it in Dorset every decade for the last 200 years, meaning that 22 sites have been lost while only one remains. But he presents no evidence that there was ever a time when it was significantly more common than it is now.
For the Maps Scheme, we have tried to make the date classes more comparable, but with a species as noticeable and reliably recorded as this one, there is still undoubtedly some bias. What is needed is a detailed compilation of all records, giving the most accurate dates possible, to demonstrate conclusively that it is not simply an intermittent or mobile species. Many county recorders are currently making efforts to improve the quality of the early records, and questions such as this demonstrate the importance of such work. It is clear that there has been almost no change in the range of O. rapum-genistae, although it may have become more sparsely scattered within its area. Foley (op. cit.) has speculated that it may have retreated to coastal areas and that its decline may be climate-related; but these observations do not appear to be supported by recent records. An alternative theory could be that habitat loss is the main cause of any change or decline.
Click on the map to view full-size on the BSBI Maps Scheme website.
It is clear that there has been little change in the range of O. rapum- genistae, although it may have become more sparsely scattered within its area. In Scarce Plants Foley speculated that it may have retreated to coastal areas and that its decline may be climate-related; but these observations do not appear to be supported by recent records. An alternative theory could be that habitat loss is the main cause of any change or decline. O. rapum-genistae is a plant of scrubby grassland. It is difficult to find much ecological information on it - possibly because it is rare in most counties. I have seee it in U1 Festuca ovina grassland that is going over to W23 Ulex europaeus-Rubus fruticosus scrub and, ultimately, W10 Quercus robur woodland; but I have also seen it in other vegetation such as MG1 Arrhenatherum elatius grassland, perhaps. If the grassland is agriculturally improved, the scrub is cleared, or the woodland allowed to develop too far, the O. rapum-genistae disappears. Whether this sequence is typical or not, I do not know; Rodwell (1991-2000) lists no NVC communities for it.
How long O. rapum-genistae can persist at a site is a question that is in need of further investigation. Foley reports, in Scarce Plants, that it reappeared after the great storm of 1987 made clearings in some woods in southern England. In one site in Shropshire it was refound 87 years after the previous record (Shropshire Botanical Society Newsletter No. 16 (2007)) and Rumsey & Jury (1991) showed that it had persisted in one site in Lincolnshire for nearly 100 years. This suggests a ‘strategy’ of persisting in a vegetative state or in the seed bank until suitable conditions return; but, alternatively, it may simply be a good coloniser.
How big are the populations? Several authors state that populations are often small. This may be true, but large populations occur as well. At Old Oswestry there were some 200 flowering spikes in 2007, and Martin Rand reported 710 spikes at Summerlug Hill in 2004. Details of population size would be worth collecting, because this is clearly a plant with a risky lifestyle. Being parasitic upon a host that occurs in a transitional habitat, and killing that very host after a few years, means that there isn’t much opportunity to make long-term plans. Each plant produces thousands of seeds, and a large population must produce millions. Are these widely dispersed or do they build up a persistent seed bank?
- Bowen, H. 2000. The Flora of Dorset. Pisces Publications, Berkshire. Rumsey, F.J. & Jury, S.L. 1991. An account of Orobanche L. in Britain and Ireland. Watsonia 18, 257-295.
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