The origin of the British Flora

Commentary: Richard J. Abbott* This article draws heavily from the work of Clement Reid on fossil deposits in East Anglia and most notably the Cromer Forest Bed Series (Reid 1899). This particular deposit was shown by Reid to contain plant fossils representing a temperate flora very similar to that existing in Britain and Ireland today. Above this deposit is a glacial till containing fossils of an arctic-alpine flora. Goodchild followed the belief of the time that the Cromer Forest Bed was laid down in the late Pliocene ' . . . not less than three millions of years back in the past'. He also suggests that immediately following that period an 'Age of Snow' occurred which prevailed until relatively recent times, i.e. until the current interglacial when the British and Irish Flora was re-established. It is now known that the Cromer Forest Bed is much more recent than thought by Goodchild and represents an interglacial period in the middle Pleistocene some 0.7 to 0.5 million years ago (Godwin, 1975). Although, Goodchild got his dates wrong, and was also incorrect in suggesting that an ice age extended from the time of the Cromer Forest Bed until recently, he was correct in emphasising that the current native British and Irish flora is very similar in composition to that which existed during previous interglacials, and that this temperate flora was re-established as a result of migrants moving across land-bridges from southern refugia. Subsequent palynological studies, and more recent molecular research, have since produced a much clearer picture of where in southern Europe most temperate plants native to Britain and Ireland survived the last ice age, and also the migration routes they followed back to these islands during the early Holocene. Of course, not all species that were previously part of this flora returned by natural means. For example, Rhododendron ponticum L. is one species that did not make it back of its own accord (Godwin, 1975), although subsequently became widely naturalised following its re-introduction by man. This brings up a point not touched on by Goodchild who focused only on the native British flora. In addition to native plants, the present-day British and Irish Flora contains a large percentage (nearly 50%) of naturalised aliens, most of which were introduced either by accident or design due to human activities over the last 500 years (Preston et al., 2002). Of further interest is that some naturalised species have hybridised with components of the native flora or with other introduced species, and on occasion this has led to the origin of new plant species (Abbott et al., 2003). Two notable examples of recently originated British