Carex flava

Carex flava L. (Cyperaceae)

Large yellow-sedge

Status in Britain: VULNERABLE.

Status in Europe: Not threatened.

Typically in Europe, C. flava is a plant of base-rich fens, but in Britain, populations occupy a transition zone between the base of a shallow slope of limestone supporting ash woodland, and the edge of a lowland raised mire supporting Betula pubescens-Molinia caerulea woodland. It grows on peaty mineral soils kept moist by seepages and flushes from the neighbouring limestone outcrops, and does best where there are gaps in the canopy and in light shade. The canopy is of alder, ash and birch, with shrubs including Frangula alnus and Prunus spinosa. Associated species in the ground flora reflect the transitional nature of the habitat: Ajuga reptans, Brachypodium sylvaticum, Carex remota, Festuca gigantea, Filipendula ulmaria, Lysimachia nemorum, Mentha aquatica, Mercurialis perennis, Oxalis acetosella, Ranunculus repens, Valeriana officinalis, Calliergonella cuspidata and Plagiomnium undulatum. 

It is a tufted perennial with a compact inflorescence, usually carrying its ripe fruits in July when it is most readily distinguished from its near relatives. Seed usually germinates in early summer (mean day temperatures of about 15°C are required). Populations of C. flava often have 'persistent seedlings' which may grow very slowly for a number of years but, when released from near-neighbour competition, rapidly mature. Individual plants can be long-lived under field conditions: at least six years (Schmid 1986) and up to twenty years (Davies 1956).

In Britain, C. flava is known at a single locality: Roudsea Wood, Cumbria. Numbers at Roudsea have increased over the past twenty years, mainly due to selective felling and canopy-thinning, but there have been local declines over the same time span where shading has increased. Between 1967 and 1988, the numbers ranged between 1,300 and 2,300 plants, and there is no evidence that the population has changed significantly since 1988. If a light canopy is maintained, then populations are likely to remain relatively stable. Recent observations at Roudsea Wood reveal that the hybrid C. x alsatica (C. flava x C. viridula var. oedocarpa) is rare, comprising less than 1% of the C. flava population. It is also sterile, so that hybridisation does not pose a significant threat at the present time. C. x alsatica also occurs at Malham Tarn, in the absence of C. flava. There is an old record of C. flava from north-west Cumbria, and of presumed hybrids between C. flava and C. viridula ssp. brachyrrhyncha from Hampshire, Yorkshire and Ireland.

C. flava occurs throughout Europe though it is generally rare, and in the south confined to montane regions. In Finland, it occurs in a range of habitats including rich fens, spring feeds, marshy forests, and ditches in agricultural land (Pykälä & Toivenen 1994). It is found in eastern and western North America at roughly the same latitudes as in Europe and there are isolated relict populations in the Caucasus, Iran, Siberia and the western Himalayas. In studies of C. flava populations throughout Europe, Schmid (1982) and Bruederle & Jensen (1991) found very little evidence of C. x alsatica, and hybrid plants which were found were highly sterile.

C. flava forms part of a species-complex which has long challenged the minds of taxonomists, but it is only in the last decade that a clearer picture of this group of sedges has emerged. As many as nine species were formerly recognised in the complex, but Schmid's work in Europe (e.g. Schmid 1983) and that in North America (Crins & Ball 1989) strongly support the acceptance of just two - the relatively invariable Carex flava L. and the highly differentiated C. viridula Michaux.

I. Taylor