Floristic shifts of ancient woodland in recently urbanised landscapes seen through historic species records

Abstract

Ancient woodland in the United Kingdom is a habitat of high conservation concern where continuity of forest cover has allowed a distinctive ecological community to develop. In England, ancient woodland sites are classed as ‘irreplaceable habitat’ and are protected in the planning process, but urban expansion in the 20th century has meant that over 8% of ancient woodland is now adjacent to or surrounded by urban fabric. In this context, these habitats can be threatened by impacts such as high visitor pressure, pollution and the introduction of alien species. We used historic and recent botanical records to investigate differences between the flora and floristic change (pre- and post-1990) of urban and rural ancient woodlands in order to assess the impacts of urbanisation on ancient woodland flora. We collected data from 43 sites across three regions of England where urbanisation has occurred in different periods and at different rates: in north-east Buckinghamshire the ‘new town’ of Milton Keynes grew rapidly from the last quarter of the 20th century onwards; in Warwickshire urban development occurred mainly in the mid-20th century; in Greater London development occurred over a longer period of time, mostly before the mid-20th century. Urban ancient woodland showed species richness that was comparable with rural ancient woodland, including richness of ancient woodland indicators, illustrating their value as sites for nature conservation. Floristic change in regional urban species pools was driven by increases in alien species, probably largely from deliberate or accidental garden escapes and municipal planting. Earlier nearby urban development was associated with greater floristic change at regional and site scales, indicating that the impacts of urbanisation take place over several decades. Practical implications: We highlight the need for improved monitoring and awareness of the spread of alien species into urban ancient woodland. Visitor management and large buffer zones may also be needed to conserve these species-rich habitats.