Population status and light regime at the occupied niche of the globally threatened liverwort Lejeunea mandonii (Steph.) Müll. Frib. on the Lizard Peninsula, UK

ABSTRACT Introduction This study was carried out to investigate the population status and light regime at the occupied niche of the globally threatened liverwort Lejeunea mandonii on the Lizard Peninsula, West Cornwall, UK. Methods Surveys were undertaken at all locations from where the species has been known and counts made of individual-equivalents (occupied 1 m grid cells). Light loggers were used to measure the light regime across a full annual cycle at a control site and at colonies of the liverwort within deciduous woodland and heathland habitat, from which percentage shade was calculated. Key results and conclusions The species was refound at all eight locations at which it has been known previously. A total of 157 individual-equivalents were found and it is estimated that the real total is 189–300 individual-equivalents, with the largest populations at Bonython (64 individual-equivalents counted; 41%) and Poltesco (44 individual-equivalents counted; 28%). High illuminance (> 10,000 lux) was recorded commonly at the control site (29.5% of measurements; n = 52,560), rarely in heathland (2.6–6.6%) and never in deciduous woodland. Compared with heathland, shade in deciduous woodland is higher (annual mean = 78.4–91.1% vs 98.4–98.5%) and more consistent throughout the year. The status of the species across the Lizard Peninsula seems to be favourable, but the small size of the population and its localised occurrence mean that it will remain vulnerable.