De toename van Pilularia globulifera L. (Pilvaren) in Overijssel

Since 1990 the number of Pillwort populations in the Province of Overijssel, the Netherlands, has increased significantly. The observed increase of locations is correlated with nature engineering and restoration. In two areas created this way (Strootman and Hemelrijk), Pillwort populations have been monitored during 5 years. At Hemelrijk, large patches settled at the border of small lakes, in open Typha latifolia marshes, and at ditch sides. Two patches were formed during the first year. This number had increased to 21 patches four years later. The first few patches may have acted as a source for spores, thus creating new patches after spore dispersal by means of birds or transport by water after inundation. At Strootman, Pillwort colonised simultaneously 40 patches during the first year. This indicates germination of megaspores from a sporebank some tens of years old. The first lot died of drought. A second lot appeared at the border of a pond and formed a Pilularietum globulifera within five years.