Changes in Vascular Plant Biodiversity in the Netherlands in the 20th Century Explained by their Climatic and other Environmental Characteristics

In the Netherlands nation-wide databases are available with about 10 million records of occurrences of vascular plant species in the 20th century on a scale of approximately 1 km². These data were analysed with a view to identifying relationships between changes in botanical biodiversity and climatic and other environmental factors. Prior to analysis the data were corrected for several major forms of survey bias. The records were broken down into three periods: 1902–1949, 1975–1984 and 1985–1999. Using multiple regression analysis, differences between successive periods were related to plant functional characteristics as explanatory variables. Between the periods 1902–1949 and 1975–1984 there were small but significant increases in the presence of both thermophilic (‘warm’) and psychrophilic (‘cold’) species. However, in the final decades of the 20th century there was a marked increase in thermophilic species only, coinciding with the marked increase in ambient temperature observed during this period, evidence at least of a rapid response of Dutch flora to climate change. Urbanisation was also examined as an alternative explanation for the increase in thermophilic plant species and was found to explain only 50% of the increased presence of such species in the final decades of the 20th century. Besides temperature-related effects, the most important change during the 20th century was a strong decline in oligotrophic and a marked increase in eutrophic plant species.