White Beak-Sedge

Appearance

''Rhynchospora alba'' is a perennial herb between 10 and 50 cm in height, though plants up to 75 cm tall can be found in North America. The plant grows in tight clumps, meaning it is often difficult to distinguish individual stems.

The plant consists of a single erect stem, which is three-angled and thin, usually 0.5-1mm thick. The leaves attached to the stem are three-ranked and parallel veined, extending up to 15 cm in length, though none overtop the stem. Each leaf is differentiated into a green or straw coloured sheath, which hugs the stem, and a grey/green blade, which is flat and slender and tapers to a blunt tip. In some specimens, the margins of the blade are sparsely covered in hairs. Unlike many sedge species, there is no ligule . At the base of the plant, the leaves have no blade, and only the sheaths are present. These are often subtending a 10-20mm bud, which will overwinter and grow a new plant in spring.

While most ''Rhynchospora'' have large rhizomes , ''R. alba'' has very small rhizomes, or none at all, and very shallow root systems. This reflects its different life history to many other sedges – ''R. alba'' loses all but the basal overwintering bud during the winter, while most other species retain and store nutrients in well developed rhizome and root structures.

Naming

The species was first described by Linnaeus in 1753 under a different genus and name, ''Schoenus albus'', but was subsequently reclassified into the novel genus ''Rhynchospora'' by Vahl in 1805. It has a wide range across the Northern Hemisphere, extending from the inland wetlands of North America, across Europe to the Korean Peninsula. Due to this large range, there is considerable variation between populations, and numerous varieties have been identified. The plant has few uses, though it is used as an ornamental in the UK.

Distribution

Although most species of ''Rhynchospora'' are found in tropics, ''R. alba'' is more restricted to the higher latitudes of the Northern Hemisphere, where climatic conditions favour the establishment of bogs and fens. It has a wide boreal distribution and is commonly found in the US , Canada, Europe, the Caucasus, China, Japan and the Korean Peninsula.

Status

''Rhynchospora alba'' is currently assessed by IUCN as least concern, as it is widespread and "habitat loss and degradation is not occurring at a scale to qualify the species for a threatened or near threatened category". On a regional scale, however, it is threatened by land conversion and over grazing in Central and Eastern Europe. It is currently threatened in Switzerland, critically endangered in Croatia and has been reported as extinct in Hungary.

Habitat

''Rhynchospora alba'' favours acidic, nutrient poor conditions and is found across a range of wetland environments. It is most commonly found in ombrotrophic bogs and ''Sphagnum'' moss-dominated communities, where it is one of the few vascular plant species present. Studies into nutrient and mass allocation by ''R. alba'' found the plant exhibits much higher rates of nutrient accumulation and loss across the growing season than in other sedges, which rely more heavily on storage and remobilising of nutrients in rhizomes. This is likely related to the much lower levels of interspecific competition experienced by ''R. alba'' in these bogs than other sedges in more grass-dominated environments.

It is also found in peatlands alongside other sedge species such as ''Carex'' species. It has a persistent seed bank, with seeds living up to 5 years. It is therefore often an early re-coloniser in disturbed environments, where it can become the dominant species in so-called ''R. alba'' sedgeland. It is much less dominant of more established communities, however, as it is less capable of outcompeting sedge species with more developed root and rhizome systems.

''Rhynchospora alba'' is wind-pollinated and wind-dispersed, so has few close interactions with insect pollinators, but is a major food source for a number of bog-dwelling species, such as ''Paraphlepsius'' leafhoppers in the US.

Reproduction

''Rhynchospora alba'' flowers in August, and these flowers are arranged into white inflorescences in a hierarchy of units. Each individual flower is surrounded a white , leaf-like structure called a 'glume'. These glumes are grouped into egg-shaped 'spikelets' between 3-6mm in length, and two to seven of these spikelets are clustered together into a hemispheric cluster, making the inflorescence. As with many other ''Rhynchospora'', there are 4-5 glumes in each spikelet, with the bottom one or two glumes being sterile and the top three glumes alternating fertile, sterile, fertile. As such there are usually one or two flowers per spikelet. Most plants have between one to three inflorescences growing off the stem, each subtended by a short leaf-like bract, about 2-3x the length of the inflorescence. Each flower is bisexual, comprising a superior ovary with a bifurcating style and 2-3 anthers . The petals and sepals are homogenous and highly modified, forming ring of 9-13 bristles with downward-facing barbs.

After pollination the flower develops into an achene – a dry fruit that is indehiscent and contains a single seed. This is composed of a 2mm x 1mm, egg-shaped achene body and the remnants of the style base of the flower, which forms a 1mm beak-like structure called the tubercle. The perianth bristles are also retained, and these are shorter than, or the same length as, the achene and tubercle combined. The lengths of both the tubercle and perianth bristles are key characters for distinguishing ''R. alba'' from other ''Rhynchospora'' species.