Fern Species of Corkonian Limestone Walls

One of the most aesthetically pleasing aspects of Cork city and its environs are its numerous limestone walls. Many buildings of note up to the early 20th century were constructed or façaded with locally quarried limestone and may have limestone walls surrounding them. Even the river Lee as it passes through the city is bordered, for the most part, by limestone quay walls. These walls are also mortared with limstone. All of this mimics perfect habitats for plants that thrive on a calcareous substrate. Due to little if any organic matter, most flowering plants decline these walls in favour of the dirt-rich cracks where they meet the footpath. Often not even this is an option as the built up nature of the environment limits the light available. These conditions do lend themselves to an often perfect environment for certain fern species.

Limestone wall with Polypodium vulgare and Asplenium ceterach

The most frequent to be seen is the Common Polypody (Polypodium vulgare). This tends towards the upper third of walls, especially the top which it may often cover uninterrupted for some distance. Their bright green colour is complemented by vibrant yellow sori which ripen from July onwards. Common Polypody, along with the other species mentioned here,

Polypodium vulgare

Walls in the city centre contain three other fern species. Hart’s Tongue Fern (Asplenium scolopendrium) is often seen, but with a much reduced frond size from samples growing in areas of higher organic matter content, such as hedgebanks and ditches. Whereas fronds in these conditions can grow as much as 60 cm, on limestone walls they tend to between 10 – 20 cm. This reduction in size does not affect dispersal however, and once established populations will persist.

Asplenium scolopendrium

The species that tends to have the largest individual size is the Maidenhair Fern (Asplenium trichomanes). The prostrate growth habit of the fronds combined with their large relative length (up to 35cm on limestone walls) and tough, black stalks means this can often be the most evident fern on a wall.

Asplenium trichomanes

Probably the least conspicuous fern is the delicate Wall Rue (Asplenium ruta-muraria). The young fronds are easily confused with Cardamine spp., however the mature plant has a delicate beauty all of its own.

Asplenium ruta-muraria

Outside the city centre, all of the above species persist, along with one other. Rustyback (Asplenium ceterach) is so named because of the rust covered scales that cover the underside of the fronds. Its absence from the centre of the city is unclear as it's response to pollution and light intensity (both of which vary detrimentally closer center) is similar to the species mentioned above.

Asplenium ceterach

These fern species lend a beauty and character to the otherwise plain limestone walls on which they grow. Unfortunately many are being removed or treated with herbicide to clean up the walls. Unlike larger, woody plants (such as Buddleja spp.), their rooting and growing does not affect the integrity of the walls and so should be left to decorate the city.

The threat of the Australian Flatworm

Ireland has been subject to invasion by a number of alien species down through the years. Some, such as the beech tree, date from prehistoric times. However other, more recent invaders are having a detrimental effect on the country's native species. Montbretia (Crocosmia x crocosmiiflora) is displacing species like Irish Spurge (Euphorbia hyberna) in hedgebanks. Zebra mussels (Dreissena polymorpha) not only clog waterways, but also decimate microalgae populations. Rhododendron ponticum threatens the biodiversity of native oak woodlands of Glenveagh, Glengarriff and Killarney by a combination of shading over young trees and shrubs and exudation of toxins into the soil. Indeed, according to Invasive Species Ireland, there are 250 hectares infested with R. ponticum in Killarney National Park alone.

Another unwelcome intruder is the Australian Flatworm, Australoplana sanguinea alba. This was first reported in Belfast and Dublin in 1983 and in the interim has spread to the entire island. A. s. alba feeds on earthworms, however not to the same extent as the ravenous New Zealand Flatworm, Arthurdendyus triangulatus. Its impact on native species has not yet been fully understood. Research carried out at the University of Manchester by Giulio Santoro and Hugh Jones suggests that A. s. alba, while not having a significant effect on overall earthworm population, selectively fed on anaeic earthworms. These are species that burrow deep in the soil and their decrease in an area leads to a reduction in soil drainage. This is especially serious in heavy soils that are prone to water-logging. Since much of the grazing land in Ireland is of this type, the Australian Flatworm may pose serious problems not only to the island's biodiversity, but also to its agriculture in the future.

To walk among mosses

Spring is somewhat of a stranger this year. The record cold of the winter has dragged on to a point where now, in mid March, the swathes of yellow daffodils so common in other years are much delayed. Snowdrops are making a gallant effort in the open, but only the brave crocus has unsheathed its full regalia, leaving fairy sized cups of yellow, white and purple to break through the grass. Elsewhere the catkins of the alder and willow and the remaining copper leaves of the beech strive to bring colour to the cold, short days. 

However it is the green of mosses that are most conspicuous at this time of year. They lie verdant among the blackened remains of other plants ravaged by harsh frosts. It is their ability to survive these frosts that allows them to carpet areas at this time of year. Frost causes irreversible frost damage to plant cells due to mechanical forces generated by growth of extracellular ice crystals as well as cellular dehydration and increased concentration of intracellular salts (Manabu Nagao, 2004). Mosses have the cell structures and mechanisms to overcome these stresses.

On closer inspection, the range of mosses present in relatively small area is equally impressive as their resilience. The delicate fronds of the Tamarisk (Thuidium tamariscinum) vie with the common haircap (Polytrichum commune) for attention, while the shining Hookeria (Hookeria lucens) shimmers ghost-like between the two. Take time to inspect them before their showier relatives turn up!

Hookeria lucens

Thuidium tamariscinum

Polytrichum commune

Mnium spp.

Brachythecium spp.

Mnium spp.