Sunday, 30 October 2016

A Headache of Hawthorns

The 3rd Edition of Stace back in 2010 introduced Large-sepalled Hawthorn (Crataegus rhipidophylla) to British botanists, and encouraged me to start looking at hawthorns more closely. Away from ancient woodlands it was perhaps too easy to get blasé and expect Hawthorn (Crataegus monogyna) or its hybrid Crataegus x media. Sure enough, I first met with this species in Lincolnshire and I have found it in scattered locations thereafter. It is not common and, currently at least, is primarily of planted origin. However, it is undoubtedly under-recorded and other botanists should be looking more critically.

Large-sepalled Hawthorn stands out in the spring with its early and large flowers. The leaves are distinctive being redolent of Wild Service-tree (Sorbus torminalis) in outline with a saw-edged base to the leaf. In the autumn the large and typically barrel-shaped fruit appear and typically have erect sepals (depending on variety) that are longer than wide. What is also distinctive in the autumn is the early colour change and leaf drop, which starts in October well before Hawthorn and its kin.

Much more common than Large-sepalled Hawthorn is its offspring with Hawthorn i.e. Crataegus x subsphaerica. Indeed, having targeted this species this October I can say it is very common, undoubtedly overlooked and likely to be a real headache for botanists in the future. What has become most apparent to me over the last few weeks is just how variable this species is. In some guises it can easily be overlooked as Hawthorn or Crataegus x media when not in fruit. By the time the BSBI starts to think about the next Atlas (after the current 2020 project) it will likely be so ingrained in our countryside that we will become dependent on examination of fruit for definitive identifications.

When I first found C. x subsphaerica it was as the optimal form for ID i.e. hybrid vigour, leaves with the outline and toothing of Large-sepalled Hawthorn but the dissection of Hawthorn (forms with the influence of the heavily dissected monogyna f. schizophylla are particularly distinctive), and large fruit with erect sepals. However, all intermediates occur and now is a good time to look while leaves and fruits are still present. The leaves on different bushes can show every range of variation between the two parents. Leaves may have the saw-edge of Large-sepalled, or this can be reduced or vary from leaf to leaf, or can be absent entirely. The fruit size can vary from small to large. However, if there are erect sepals on the fruit (nothovar. domicensis) you can be confident of the ID. Note this character develops as the fruit mature, so don't make assumptions based on immature fruit.

This is the current distribution of the hybrid based on data in the BSBI database, and reflects where people are looking rather than the true distribution. It will only be the tip of the iceberg. My experiences around my local patch will undoubtedly be replicated across much of England and Wales, if not the UK as a whole.

Finally, to round off the hawthorns, a return visit to the fabulous large fruited (>1cm) form of Hawthorn (var. splendens). It is a stunning sight when you meet a bush of this in full fruit, dripping like jewels in a particularly vibrant shade of red. Despite the fact that this variety only occurs at very low density, i.e. one or two bushes here or there, it does seem more than a chance mutation. While the fruits of var. monogyna (and subsp. nordica if you follow Sell & Murrell, but the distinction betwen subspecies monogyna and nordica does not seem worth maintaining) and var. splendens do seem to vary in size on a single bush there does seem to be a well marked cut off in the size ranges between the two varieties. In other words, the fruit vary in size around their respective means, but do not grade into each other. A reasonable working hypothesis to explain this might be that the latter has a higher ploidy level.

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