Liparis loeselii (L.) Rich. in eastern England: constraints on distribution and population development

Abstract

The fenland form of Liparis loeselii is currently known from only three sites in eastern England, having been lost from about 30 former localities since the start of the nineteenth century. The reason for its loss is sometimes uncertain, but in only a small number of instances can it be attributed confidently to drainage. The habitat of the current Liparis populations, all in the Norfolk Broadland, can be summarized as ‘base-rich, wet, low-fertility, herbaceous fen’. Vegetation management, usually summer mowing, helps maintain the relatively low-growing, herbaceous vegetation that the orchid requires. In addition, all localities are in reflooded turbaries, the plant growing on a relatively buoyant mat of vegetation. Many former Liparis localities were apparently of similar character. Demographic studies on tagged plants in ten permanent plots in one population (Sutton Fen) between 1983 and 1990 showed that individuals were mostly short-lived but developed quickly. Small (young) individuals had particularly high mortalities and the majority died without flowering; individuals frequently, but not invariably, increased their leaf dimensions in successive years; flowering was almost entirely restricted to 2-leaved plants, especially robust individuals, and most flowering individuals re-appeared in the next growing season. The main period of emergence was between mid-June and mid-July, though some individuals that emerged in mid-June disappeared by mid-July (apparently due to mollusc grazing). The number of plants in the population showed considerable flux (though not all plots showed the same trends). A peak of numbers in some plots in 1986 and 1987(due to both high rates of recruitment and low mortality) was followed by a crash in 1988 and 1990. This affected the entire population (1990 numbers were about one third of those in 1983) but was particularly evident in plots which had shown an earlier strong increase in numbers. The population peak occurred after a winter burn and may have occurred in response to this. The cause of the subsequent decline is not known with certainty but it was associated with lack of vegetation management, low fen water levels (lack of surface inundation) at the time of emergence of Liparis plants and mollusc grazing damage. The effects of these events are difficult to disentangle, but grazing damage, perhaps facilitated by lack of shallow flooding at the time of shoot emergence, may be the proximate cause of loss. The short-lived character of many Liparis individuals and the quite rapid population turnover suggests that effective conservation management must provide conditions suitable for recruitment of new individuals as well as the maintenance of established plants, but the character of suitable regeneration niches is not well understood. It is possible thatperpetuation of Liparis populations may require some disturbance (perhaps winter burning or light trampling) as well as mowing. Rejuvenation of the hydrosere, by further peat removal, may be another long-term requirement.