Salix herbacea L., Dwarf Willow
Account Summary
Native, very rare. European arctic-montane, also in N America.
1950; MCM & D; Cuilcagh summit ridge.
July to October.
Growth form and preferred habitats
The smallest willow species of B & I, S. herbacea is a dioecious, prostrate, mat-forming shrublet with an extensive branching underground rhizome. The dwarfism is very striking, aerial branches of the shrublet rising only 5-10 cm above the surface of the ground. Woody aerial shoots can be distinguished from the generally (but not always), subterranean rhizome, by their being segmented by bud-scale scars (Meikle 1984; Beerling 1998).
Being very low-growing, S. herbacea frequents disturbed, unstable or physically severe habitats that prevent or limit overgrowth by taller, more competitive species. Thus it is characteristic of summit plateau and other very wind-exposed, high-altitude situations, generally involving some degree of snow cover, frost solifluction, unstable scree slopes, rock crevices or regular sheep and goat grazing, allowing the development and maintenance of permanently open, sparsely vegetated, montane-heath communities.
In NW Ireland, protective snow cover is seldom present for long in winter due to the mild, wet oceanic climate, and consequently S. herbacea occurs only very sparingly and rarely in shallow, rocky or stony, strongly acid, nutrient-poor, leached podsol soils, in fully exposed, sheep-grazed vegetation. The openness of the vegetation and the growth-limiting severity of the physio-chemical environment are essential to the montane survival of S. herbacea which, on account of its small size, is biologically a very poor competitor (Beerling 1998). Plants confined to north-facing rock crevices are considerably more sheltered than those on the typical mountain summits and slopes and thanks to regular, high amounts of rainfall and their shaded situations, they are never going to suffer dehydration.
Fermanagh occurrence
S. herbacea is decidedly rare in Fermanagh, having been recorded in two tetrads only, both on Cuilcagh mountain. It occurs at several spots on the summit ridge and also slightly lower at Tiltinbane Td on the northern flank of the mountain, where it grows with lichens amid sparse cover of other subshrubs such as Erica cinerea (Bell Heather), Vaccinium myrtillus (Bilberry) and Empetrum nigrum (Crowberry) in open, prostrate, sometimes moss-dominated montane heath vegetation.
The Robert Northridge and the current author have together seen the plant at Cuilcagh Gap, where it grows on the north side of two deep clefts or gullies in the rocks of the summit ridge itself. In one of these, there are six flattened mats of this tiny, dwarf shrublet scattered in moist crevices across only about 7 m of cliff. The plant appears able to spread in these crevices by means of its unusually woody rhizomatous growth, but, unfortunately, we have no idea as to whether or not it manages to set seed here, or if so, with what measure of success it establishes new plants.
Practically every aerial shoot the shrublet produces, however short it is, ends in a small, 3-9 flowered, either male or female catkin. These are sheltered and warmed between two subtending, shining, leathery leaves. An exhaustive Biological Flora account of S. herbacea in B & I produced by Beerling (1998) suggests that seed reproduction is rare and existing clonal populations are purely maintained by vegetative spread.
As has been found in other clonal species, both herbaceous and woody, individuals may be thousands of years old, older indeed than any of the most often quoted patriarch trees. These plants may be functionally immortal (Korner 1999, p. 289). However, just how comprehensive reliance on vegetative reproduction is throughout the very wide range of this arctic-alpine species, and whether any populations in B & I are exceptions to this general picture, are unanswered questions.
Irish occurrence
The New Atlas map shows that these Cuilcagh stations are the most inland extant sites of Dwarf Willow anywhere in Ireland, all other sites for the species being much closer to the coast. It is an odd fact of geography that the highest mountains of Ireland all lie on or close to the coast and it is essentially on the highest ground and on N-facing cliffs, sometimes at rather lower altitudes, that this typical arctic-alpine species survives (The Botanist in Ireland). The New Atlas map for Ireland displays post-1987 date-class records in 33 hexads, while both it and the previous BSBI Atlas (Walters & Perring 1962) display a total of 35 Irish hexads which had pre-1930 (and therefore pre-1970) records. Clearly the only comforting thing about such statistics is that the losses that occurred prior to 1930 have not continued at the same pace, or at least not at the hectad scale. A long list of willow hybrids involve S. herbacea (Stace 1975; Beerling 1998), so that fertility is not in question.
Fossil history
Fossil remains of S. herbacea have been found locally in Fermanagh in a full-glacial freshwater deposit of Middle Midlandian age, radio-carbon dated to 30,500 BP, discovered at Derryvree, near Maguiresbridge (Colhoun et al. 1972). The flora and fauna of this deposit indicated that open tundra vegetation and a periglacial climate prevailed at the time when it was laid down. S. herbacea is thus a glacial survivor which probably spread to Fermanagh in the very early post-glacial period, perhaps as much as 12,000 years BP, and through changing climate and land use it has clung on ever since. Current rapid climatic warming poses the greatest threat to its continuing survival, since the degree of change may well release potential competitors from their present constraints.
Conservation
The duty of conservation stewardship requires that the remaining small Fermanagh populations should be studied to determine not only their extent, but to determine the ability of the species to reproduce and maintain itself. It would also be interesting to know even an approximate estimate for individual longevity and have some measure of rate of population turnover. Studies of this nature are already underway in Scotland and the Environment and Heritage Service in Northern Ireland should consider following suit.
Threats
Dwarf Willow is protected by its small size and the remoteness and physical position of its mountain stations. Occasional grazing by sheep keeps the high montane vegetation open and some populations are protected from erosion by shelter in rock clefts near the summit ridge. The real threat is from global climate warming stimulating increased competition from more vigorous perennials.