Drought-induced emergence of Physcomitrium eurystomum Sendtn. on the drawdown zone of Wilstone Reservoir, Hertfordshire, UK

Abstract

ABSTRACT Introduction Physcomitrium eurystomum Sendtn. is rare in Europe and threatened with extinction. In this study, its drought-induced emergence at Wilstone Reservoir, Hertfordshire, UK, was documented. Methods A preliminary search of the reservoir was carried out in late November 2022, and a detailed one 36 days later. Other local sites were also searched. Geographical coordinates of colonies were used to derive counts of occupied Ordnance Survey (OS) grid cells at resolutions of 1 and 100 m. Additionally, 1 m2 quadrats were used to determine sporophyte density and estimate the total number of sporophytes present. A phytosociological relevé was recorded. Key results Exposure of the diaspore bank through the cracking of mud and avoidance of xeric conditions allowed a population of Physcomitrium eurystomum to emerge and fruit on the rims of the cracks. Further emergence occurred as winter progressed, allowing widespread co-occurrence with P. patens (Hedw.) Bruch & Schimp. An occupancy of 7885 OS 1 m grid cells was recorded (72 times the area of the largest previously recorded UK population of P. eurystomum). The estimated population was at least 4,300,000 individuals with sporophytes. The hybrid Physcomitrium × stevensonii D.A.Callaghan was found. The non-native, invasive species Crassula helmsii (Kirk) Cockayne, a threat to P. eurystomum at sites in Britain and mainland Europe, had been suppressed by the xeric conditions. Conclusions Drought-facilitated emergence gave rise to a significant population of Physcomitrium eurystomum, making Wilstone Reservoir a principal site for its conservation. Additionally, vertical deposition of its spores into the diaspore bank was enhanced.