The long-term development of urban vegetation in Helsinki, Finland: A pollen diagram from Toolonlahti

A core from Toolonlahti, a bay close to the central business district of Helsinki, the capital of Finland, was analysed for its pollen stratigraphy. The extrapolated age-depth curve based on Cs-137 dating, sphaeroidal carbonaceous particle analysis and historical data suggests that the core goes back to the 17th or early 18th century. Four local pollen assemblage zones (LPAZ) were identified. Detrended correspondence analysis was used to detect the main phases of change represented by these zones. The basal LPAZ indicates that the vegetation was characterized by forest dominated by Pinus sylvestris and Picea abies during the early history of the city of Helsinki and that human impact remained relatively minor until the end of the 19th century. A drastic change in vegetation took place at the turn of the 19/20th century when both Pinus and Picea declined rapidly while pollen values of several anthropogenic indicators rose. These changes were synchronous with the rapid expansion of population and urban area in Helsinki. Rarefaction analysis shows that palynological richness was highest during the early phase of increased human impact but then started to fall. During the 20th century the main vegetational trend has been the proliferation of introduced park trees. Palynological richness has decreased.