Calicium diploellum New to England!

After several years concentrating on the New Forest heathlands, I have begun to look at the woods again. This has produced an interesting crop of new records. These include Arthopyrenia nitescens, an oceanic species last collected from the New Forest from holly in the 19th century, refound on Beech; Opegrapha saxigena, a normally saxicolous species found on Beech and Ochrolechia arborea, a little known species, here found on well lit and weathered Oak dead wood.

Recent Finds

The most exciting find, however, was a very unexpected species. On the 24 March 2016 I had a casual lunch time dog walk in Great Stubby Hat (SU3011), in the Busketts Wood complex, looking at a few trees of interest on the way. Stopping at one attractive looking old Holly, I noticed a green specked thallus with some possible tiny stalked apothecia on lignum in a Holly lenticel. This looked very reminiscent of Calicium diploellum, a very rare species in Britain and Ireland, but one which I had seen a lot of recently during surveying in Co. Kerry, Ireland. As this had only been found in a few places in Ireland and one in western Scotland, however, this seemed very unlikely (distribution map, NB lacks some recent Irish records), but I collected a little just incase.

The Holly

I did not think much more about it and only just remembered to look at the specimen before going to bed. Under the binocular microscope it looked even more familiar, definitely stalked apothecia with a green speckled thallus. I extracted a couple of smaller apothecia, so as to leave a voucher specimen, just in case, and placed them under the microscope; it looked even more like Calicium diploellum. A quick squash and wow the spores! Brown one septate spores with spiral thickening and 7 – 9 x 4μm, **@!!! it was Calicium diploellum.

Under the microscope

At Little Stubby Hat the Calicium diploellum was confined to exposed lignum in lenticels where the adjacent bark was dominated by Mycoporum lacteum along with some Stenocybe septata. Nearby on the Holly were Schismatomma niveum, Mycoporum antecellens, Mycoblastus caesius and Thelotrema lepadinum. This is the typical habitat of Calicium diploellum in Ireland; the species is invariably found in lenticels, or damaged bark, where the non-lichenised fungus Mycoporum lacteum dominates. The reason seems to be quite simple; Mycoporum lacteum appears to be a bark chloroplast parasite; a fungus that lives like a lichen, but rather than developing a mutualistic relationship with an algae, it simply parasitises the chloroplasts in the bark of the Holly. For this reason Mycoporum lacteum avoids overgrowing the chloroplast free exposed lignum in the leticells. This leaves the lignum habitat free for Calicium diploellum. The main competitor for dry Holly bark is the fully lichenised Lecanactis abietina. This happily overgrows exposed Holy lignum, leaving no habitat for Calicium diploellum. The Mycoporum lacteum, however, does appear to be strongly competitive with Lecanactis abietina to the advantage of Calicium diploellum.

Finding Calicium diploellum in the New Forest was a surprise; the species is otherwise confined to old Holly in hyper-oceanic climates (see updated map below). However, it is very difficult to spot, even when well developed, so can be expected to be under recorded. The species is also restricted by the survival of old Hollies in the landscape, and the occurrence of strong populations of old Holly may be a more significant limitation to its distribution than climate. In Britain, these are only frequent in the New Forest, but are more frequent in Ireland. In Scotland, and even more so Wales, old Hollies are now very rare and restricted to relic trees, mainly on difficult to access cliffs. There is evidence to suggest this recent feature caused by some combination of increased coppicing, sheep grazing and the little ice age, and that old Hollies were a much wider feature of upland western medieval pasture woodland. The distribution of the old Holly habitat suitable for oceanic lichens is best represented by the distribution of Mycoporum lacteum. This is nearly always found on Holly, although it can rarely occur on Birch, Oak and Beech in areas with large populations of Mycoporum lacteum on Holly.

Calicium diploellum is an important discovery for England, it is assessed as Critically Endangered in Britain and is listed as priority Biodiversity Action Plan (BAP) species. All BAP species known to exist in England at the time were listed in Section 41 of the Natural Environment & Rural Communities (NERC) Act 2006. Species on this list are considered to be of "principal importance for conservation of biological diversity in England”. Like Strigula tagananae, also a BAP species added to the English list after 2006, Calicium diploellum is not on the Section 41 list but is a BAP species.

Finding the species is very much a learned nack. One needs to look out for the green speckling in Holly lenticels on the dry sides of old Hollies, especially where Mycoporum lacteum is extensive. This suggests the presence of the lichen. Then one has to squint sideways into the lenticels to try and spot the tiny (0.1 – 0.2mm) high pinheads. If well developed these can have yellow pruina on their heads, but this is usually absent; looking for this pruina may have lead to the species being missed in the past. Mostly the pinheads are very difficult to separate from general frass and rubbish, but with luck they can be spotted.

Neil A Sanderson 26/3/2016


On the 30th March 2016, I had another lunch time dog walk. This time I decided to look at Matley Wood (SU3307), a wood with a remarkable collection of ancient Holly pollards, just to see what I could find. I did find one Holly with some green speckling in the lenticels and some tiny round black specks that did not appear stalked. Looking at this under the microscope, the black specks turned out to be immature Calicium diploellum apothecia. They were very short stalked, but the stalk and the exciple looked identical to the older apothecia examined earlier and some had small distinctive looking fusiform asci identical to those seen in the mature specimen from Busketts Wood, and within one asci, a few one septate spores were just developing. So it looks like Calicium diploellum may be quite widespread, if rarely well developed in the New Forest.

Neil A Sanderson 31/3/2016

Final Update

On another dog walk on April the first, I took a picture of the first Holly I found Calicium diploellum on in Matley wood and had a look around. The adjacent old Hollies also had potential poorly developed Calicium diploellum thalli in their lenticels, but I did not collect these for conservation reasons. Further east I notice some more convincing material, and a small sample proved this was more Calicium diploellum, so there is a bit about. However, a quick look at a couple of woods with old Hollies in the morning of the next day, failed to find any convincing material so the species is not probably not frequent. I will report back latter in the year on the findings of a wider survey.

Neil A Sanderson 3/4/2016