Monotropa hypopitys L., Yellow Bird's-nest

Account Summary

Growth form and preferred habitats

This small, pale yellow (sometimes tinged with red or brown), waxy-looking perennial has a few-flowered raceme, or rarely a solitary flower that droops at first but which later is held erect in fruit (Olson 1990). It appears amongst leaf litter, generally but not always in deep woodland shade between July and October. The plant contains no chlorophyll and until recently was considered saprophytic (deriving nutrients from decaying leaf litter by means of mycorrhizal roots and fungal partners). Recent research has shown it is actually epiparasitic, using Tricholoma fungi to extract nutrients from the roots of trees living in its vicinity (Leake et al. 2004).

In Fermanagh, where the species is at the extreme edge of its European range, it is typically associated with hazel or beech in woods or scrub growing on shallow limestone soils. It also occurs in the mixed oak-birch woodland of the Correl Glen NR where the geology is sandstone (although some of it is less acidic than normal it having been dolomitized). In other parts of its B & I distribution, M. hypopitys more usually grows on more acidic soils under oak or pine. In damp coastal dune-slacks, it is occasionally found growing under Salix repens (Creeping Willow) (F.J. Rumsey, in: Preston et al. 2002).

Variation

M. hypopitys occurs as two subspecies in B & I and the 1957 Flora authors stated that all plants examined by them belonged to subsp. hypophegea, rather than subsp. hypopitys (Meikle et al. 1957), the occurrence of which is very scattered in England and unknown elsewhere (Stace 1997; Sell & Murrell 2014). Subsp. hypophegea produces fewer flowers (ie eight or less), they are less hairy inside and the style is shorter than in subsp. hypopitys (eleven or fewer flowers) (Stace 1997).

Fermanagh occurrence

Overall, M. hypopitys is currently known from nine tetrads in Fermanagh and, as the distribution map shows, seven of them are scattered along the Lough Erne basin, three of them on the Castle Caldwell peninsula in the NW corner of Lower Lough Erne. M. hypopitys has been known from the Ely Lodge south shore of Lower Lough Erne since 1904, but two stations nearby have been destroyed in recent years; Praeger and Carrother's 1945 site, "opposite the gate of Ely Lodge" (BEL voucher sheet) was planted with conifers, and the 1980 site of c 120 plants beside Castlehume Lough was destroyed by the development of a golf course around 1991. Two other moderately stable stands of up to a dozen spikes survive in the Ely Lodge lakeshore lowlands, in mainly coniferous plantation. Happily, two additional Fermanagh sites have been more recently discovered: the first on the limestone Knockninny hill overlooking Upper Lough Erne in 1995 and the second in the Correl Glen NR in 2003. Both are quite distant from the Lower Lough Erne stations and the latter is much more upland.

The number of aerial flowering shoots in stations of the plant is typically very variable from season to season (Söyrinki 1985; Olson 1990) and the population extent is thought by some to correlate with maximal and minimal annual rainfall. However, while observed numbers are distinctly variable it should be remembered that the aerial shoots are often small and retiring, variable in the timing of their appearance and quite difficult to find and, therefore, easily overlooked. The edge-of-path locations of many of our discovered plants may simply reflect the rather inconspicuous, late-season appearance of the plant and the fact that even when we are exploring a woodland area for plants, we tend to follow pre-existing tracks or paths.

Despite the welcome addition of the two new sites to the Fermanagh listing for the species, Castle Caldwell on the shores of Lower Lough Erne still seems to be the M. hypopitys stronghold in the VC. This always inconspicuous plant has been independently found there by three people in the last decade, a fact which encourages RHN and the current author to believe it is probably quite widespread in that particular estate.

Flowering reproduction

The inflorescence is nodding at first but becomes erect later in development. It is a raceme of usually 3–8 flowers, the terminal one with parts in fives and the remainder with parts in fours. Above the ovary is a single, short, stout style expanding into a bright yellow, disk-shaped stigma. The flowers are fragrant and produce nectar from swellings at the base of the five-lobed ovary. The petals are expanded and saccate at their bases to hold the nectar. An insect visitor must push its proboscis (often erroneously referred to as its 'tongue') down between the edge of the stigma and the 8-10 anthers to reach the food reward, and thus pollen can be transferred between flowers.

Self-pollination is frequent, however, and close examination by median microtome sectioning showed very clearly that the flower has contact autogamy (Hagerup 1954). The cavity of the flower is narrow and the petals press the long stamens tightly against the thick ovary. The anthers are pressed inwards against the broad stigma, whose large mucilage plug retains a quantity of the pollen, with the result that the canal of the style is soon densely filled with pollen tubes (Hagerup 1954).

A recent study of genetic variation in the Fermanagh populations at Castle Caldwell, Ely Lodge and Knockninny found they were fertile and reproducing sexually, although the plants are self-compatible and inbreeding is rife. In contrast with the closely related boreal species Orthilia secunda (Serrated Wintergreen), surprisingly low levels of clonal patch development were found in Fermanagh populations of M. hypopitys, confirming that the predominant mode of reproduction is seed production. It is likely that the high levels of inbreeding observed are the result of self-pollination, particularly given the small numbers of genetic individuals in most of patches of the plant. Being self-compatible means, however, that M. hypopitys does not face the same problems of complete loss of sexual reproduction and/or rapid population extinction that can threaten populations of obligately outcrossing clonal plants (Beatty & Povan 2011).

Seed dispersal

In view of the restricted, and, in Fermanagh and in Ireland in general, sporadic, isolated nature of the populations of this rare species, it is puzzling to contemplate just how M. hypopitys achieves dispersal and establishment. The globular fruit capsule opens by means of five vertical, slit-like valves to release the numerous slender, white, extremely lightweight seeds it contains (Butcher 1961, 2, p. 117). The contents of two capsules counted by Salisbury (1942, p. 94) contained 1,953 and 1,980 seeds and since the number of flowers per raceme ranges from one to twelve, he reckoned that the mean seed output would be around 16,000 per shoot. The mean seed weight is just 0.000003 g, very similar to those of native terrestrial orchids and we may assume, therefore, that wind provides the chief method of seed dispersal.

However, seeds or roots might also be transferred inadvertently by man in soil, a circumstance most readily envisaged when imported tree material is being planted. Oddly enough, since the two most recent finds in NI were both in already established nature reserves where tree planting is not a feature, it may be necessary when searching for a vector to consider transfer on the muddy boots or clothing of habitual nature reserve visitors, a suggestion supported by the edge of path locations of many (but not all) M. hypopitys plants!

Germination and seedling establishment

The embryo is tiny, consisting of only three cells, and the nutritive endosperm is also reduced to just nine cells, so that even with the assistance of its mycorrhizal fungus, the odds against successful germination and establishment must be absolutely enormous (H.L. Francke (1935) quoted in Salisbury (1942)). The huge number of very small, biologically reduced seeds is undoubtedly responsible for the low reproductive capacity in terms of seed germination and seedling viability found in M. hypopitys. However, since these biological properties are also the case with saprophytic orchid species, this reproductive strategy obviously gives greater assurance of efficient dispersa and thus facilitates colonisation of very scattered, specialised sites suitable for growth and reproduction of these species.

Irish occurrence

In the rest of NI, M. hypopitys is known from only two other sites. One was discovered in 1997 at Straidkilly NR near Glenarm, Co Antrim (H39) and the other was in Co Londonderry (H40), although it has not been seen since 1944 and presumably has become extinct. In the RoI, Yellow Bird's-nest has been seen since 1970 at just two sites in Co Galway (H15, H16), two in Co Donegal (H34, H35) and one each in Cos Clare (H9), Wexford (H12) and Leitrim (H29) (New Atlas). If this is the true situation, it represents a long continuing decline of this always rare species in the RoI since before the turn of the 19th century (Praeger 1901, p. 209). However, the authors of the Irish Red Data Book believed M. hypopitys is, "clearly under-recorded" due to its inconspicuous nature (Curtis & McGough 1988). In its Wexford site, Yellow Bird's-Nest was very abundant in a pine plantation and Curtis & McGough (1988) suggest that it was very probably introduced along with the tree material.

Conservation status in NI

On account of its rarity and vulnerability, M. hypopitys is listed for special conservation protection on Schedule 8 Part 1 of the Wildlife (NI) Order, 1985. For wider based scientifically-drawn biodiversity aims, it is also included on the NIEA list of Priority Species of special concern requiring local conservation action.

British occurrence

In Britain, Yellow Bird's-nest is prevalent in south central England and Anglesey, being rather thinly scattered south of a line between Preston and Hull and very rare northwards to the Scottish Highlands. Even within the southern England area of most records, many sites have disappeared both before 1930 and during the last 50 years, presumably due in the main to woodland disturbance and habitat destruction (F.J. Rumsey, in: Preston et al. 2002).

European and world occurrence

M. hypopitys s.l. (consisting of a total of four subspecies) is widely distributed in temperate regions of the Northern Hemisphere across Europe, Asia and North America, although throughout its range it is generally found scattered at low abundance (Hultén & Fries 1986, Map 1444; Beatty & Provan 2011).

Threats

Woodland clearance through ignorance of the species' rarity and interest.

PRIMULACEAE – Primrose family

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