A Northumberland Alpine Gardener's Diary
This entry: 20 February 2016 by John Richards
Some Canarian ferns
Ferns from Tenerife
As intimated in the last entry, we have been to Tenerife again, our third visit in six years. Once again we have been to the north coast, staying for two weeks in a lovely apartment near a village called El Sauzal which lies east of Puerto Cruz above the precipitous coastline. Here are many so-called 'Urbanizations', extensive communities of luxurious and often gated residences which are built around one-way systems which scatter down the slopes. Our apartment was attached to one of the less ambitious houses, with its own extensive terrace with a spellbinding view and freezing pool. But it was lovely with a large and well-equipped kitchen and we had a lovely relaxing time. 'Relaxing' for the Richards family means getting out as much as possible, and several days we headed for the wettest parts of the island, Teno in the north-west or Anaga in the north-east. You might think it counter-intuitive to aim for wet areas when escaping the British winter, but these humid peninsulas contain the richest floras on this botanically bountiful island, particularly in the unspoilt laurel forest of Anaga.
You may also be surprised that these areas of Tenerife have a rich fern flora, until you visit these shaded, damp locations. During our visit it did not rain at all, and indeed it had not rained since before Christmas, two months previously. Nevertheless, in these often cloudy and misty regions the soil remained moist and the vegetaion lush.
Growing hardy ferns is a a notable subset of the alpine gardener's art, which is perhaps celebrated rather less than it deserves, except on the Show bench where superb specimens will invariably be found. Some of the Tenerife ferns also occur in the British Isles and are undoubtedly hardy, while others are found in Mediterranean mountains, and others again are Macronesian specialities. The latter are of particular interest to those who study fern taxonomy and evolution as some represent ancient parents of complex polyploid lineages which now occur on the European mainland.
Lets start with one of these. The only polypody on Tenerife is I believe Polypodium macaronesicum, also called P. cambricum ssp. macaronesicum. P. cambricum is of course a British native, popular in gardens, and a diploid progenitor of the commoner tetraploid and hexaploid polypodies in our islands. P. macaronesicum is to my eyes, very similar indeed.
One of the features of P. cambricum is that the sori tend to be impressed onto the top side of the frond and this characteristic is well shown in the next photo.
Another familiar fern at Anaga (Taganana road) was the soft shield-fern, Polystichum setiferum, which grows as a wild plant (although very likely a garden escape here) only a few hundred metres from my home at Hexham. I think this is the only Polystichum on the island.
There is a something of a pattern of scarce British and Irish ferns, ancestral to their groups, being restricted to the south-west of our islands and also occurring in Macronesia. Such a plant is Asplenium onopteris, one of the black spleenworts, rare in Britain (in fact mostly in Ireland) but the commonest spleenwort on Tenerife.
Superficially rather like Asplenium onopteris, but with very different sori which appears as blebs' or grain-like swellings on both surfaces of the leaf is Davallia canariense. This also differs in that the fronds arise from thick hairy-scaly rhizomes which creep over the surface of rock surfaces to which they cling with an ivy-like embrace. Also, the fronds are long-lived and evergreen. There are no davallias in the British Isles, but D. canariensis is also native to the wetter parts of south-west Iberia.
Another genus unrepresented in the British and Irish floras is Woodwardia. These large and impressive members of the Blechnaceae are most tropical, but one species does occur sparingly in the Mediterraean. I believe W. radicans only has a single station in Crete where I paid it homage some years ago, but it is also known in Sicily and in SW Iberia again. On Tenerife it are mostly confined to the wettest parts of Anaga near Chimorga, but here it is modestly plentiful.
I can scarcely leave the Blechnaceae without mentioning our own familiar hard fern, Blechnum spicant, which occurs quite commonly in Anaga. I must confess that this looks so different from the plant which grows just up the road from here in Hexham that I had expected it to be some endemic species, but apparently only B. spicant is recorded as a native (but see below) although it is a good deal larger and more impressive than our plant. Perhaps it deserves recognition as a subspecies?
However, I did see the next plant on several occasions, usually growing on walls near villages. It did not behave as a native, and I believe it is an exotic blechnum, possibly B. brasilense.
I think there are only two dryopteris (male and buckler ferns) in Tenerife, neither of which are known in Europe. However, D. guanchica, named for the original denizens of the island, is interesting as it has been shown to have arisen after hybridization between our scarce British fern D. aemula and another Macronesian fern, D. maderensis. This is interest as it infers that D. aemula may once have occurred in the Canary Islands, and indeed when I first found D. guanchica, this is what I thought it might be. Like D. aemula it is three-pinnate, but the deflexed bottom pinnae are distinctive.
Dryopteris oligodonta also occurs in the Anaga region, and this also is a non-European species, considered to be a rather primitive Macronesian relict. Unlike D. guanchica it is only two-pinnate, with rather ladder-like pinnae.
One of the most appealing Tenerife ferns to Alpine Gardeners would be Asplenium hemiotis. This is like a tiny version of our harts-tongue fern, but with divided fronds, much more to our concept of suitable scale.
Nearly all the species mentioned occur in shaded humid laurel forests. An exception is Cosentinia vellaea, (formerly Notholaena vellaea) which is commoner in the dry hot south of the islands where it occurs in shady rock faces, although out in the open.
Finally, not a fern but a fern-ally, Selaginella denticulata is quite common clinging to vertical shady damp rock-faces in Anaga. When stressed by drought it often turns a beautiful red.