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Helleborus in Slovenia

Sometimes, when you start to look more closely at a plant, you realise that you understand it less and less well … 

Christmas rose

This seems the case with the genus Helleborus, the hellebores and the Christmas Rose. In fact, as long as you leave the local varieties and subspecies out of it, the Christmas rose is still pretty straightforward. There is just one species, which is widespread, and not hybridising with any other species in nature. Sure, you will find the odd-looking plant with some leaflets too many or a pinkish colour instead of pure white. This is in line with what Charles Darwin already found: common and widespread species tend to show most individual variation. So you can admire Christmas roses without worrying about which species you actually see. And they certainly give a reason for admiration: they are among the largest and most showy early spring flowers! They remind me of paper or silk flowers sometimes, only a lot better made 🙂

Helleborus niger

 

Three, four of five Species

In contrast, the hellebores, with flowers which vary in colour from yellowish green to bright green and even deep violet-black, are the ones which seem to cause an eternal confusion. Of those helebores, there are four species in Slovenia. Or three, or maybe five. Well, the consensus seems to be really four and since I am a zoologist, perhaps it’s best when I follow that. Apart from that, I have been looking quite a lot at these plants in the last two years so this is what I learned. The problem is two-fold:

A large hybrid zone

First, there is an unclarity between two types of plants: Helleborus odorus and Helleborus multifidus ssp. istriacus. These two appear to hybridise widely, so much even that you could argue that they are forms of a single species. That would mean that there are just 3 hellebore species in Slovenia, one of which has two subspecies. To solve this, it would be necessary to look at the contact zone between ssp. istriacus and H. multifidus ssp. hercegovinus in eastern Croatia. I did not do this, so I can also not have an opinion about it. 

High intra-population variation

Second, these hellebores are very variable from one plant to the next, even within populations. Because of this, it is often not possible to identify a single plant on basis of external characters. This becomes easier with the help of the general appearance of many plants which grow close together. To be on the safe side, it also helps to have the biogeography of Slovenia in mind. The latter may be problematic. As soon as you start to identify a plant on basis of the supposed distribution, your outcome will always fit with the generally accepted biogeographical zones. This way, you might overlook the interesting exceptions! In the case of hellebores, I think that the biogeographical zones should be used in the opposite way. As soon as you think that you find a species well outside it’s “typical biogeographical zone”, you have to look twice as hard at its characters!

The darkest hellebore

Helleborus atrorubens seems the easiest to identify. This is the only Slovenian hellebore with dark flowers. However, the colour of its flowers varies greatly, both between localities and between individual plants. It is customary to call the greener plants intermediates or hybrids with the green flowered Helleborus odorus. At the same time, they could also represent variation within populations. Some of the darkest flowering H. atrorubens can be found in the most exposed locations, where they receive most direct sunlight. Could the dark colour be a genetic adaptation to block the sun?

A too simplistic map

When you follow the current opinion, my map is too simplistic and the distribution of H. odorus should be patchy within the range of H. atrorubens … Likewise, I doubt that it is possible to find a clear border between H. odorus and H. m. ssp. istriacus.  And my map nicely ignores the many extralimital data, some of which are from well-respected authorities who I take very serious indeed … 

Biogeographical zones in Slovenia
Biogeographical zones in Slovenia
 
Distribution of Helleborus species in Slovenia
Distribution of Helleborus species in Slovenia

 

The delicate dumetorum

The least known hellebore is Helleborus dumetorum, which can be found in the northeastern corner of Slovenia. When I looked at the herbarium material of this species, it appeared to be yet another difficult to identify green hellebore. However, in the field, this species really surprised me. With it’s somewhat more delicate foliage and with it’s wetter lowland habitat, it isn’t so difficult after all! There are, however, a number of extralimital data in the Slovenian database which warrant a thorough second look … 

In conclusion: these plants are just like Darwin’s Barnacles, or like any other species complex in nature; endlessly interesting and never 100% clear. So let’s go out in a couple of weeks and try to identify those Slovenian hellebores! 

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