Effects of Fertilisation on Grass and Forb Gamic Reproduction in Semi-natural Grasslands

Studying the effects of fertilisation on the seed production of grassland species can help understand the vegetation changes and biodiversity losses due to soil eutrophication. On a temperate hay-meadow, the seed production of fifteen grasses and seventeen forbs was studied under three fertilisation treatments: 0-0-0, 0-54-108 and 192-108-216 kg N, P2O5 and K2O respectively, per year. Fertile shoots collected at the seed maturation stage were analysed for all main traits of the gamic reproduction. On average, forbs produced more ovules and viable seeds per shoot (199 and 65, respectively) than grasses (112 and 35, respectively). Fertilisation increased the number of inflorescences per shoot in both grasses and forbs and had a limited but variable effect on germinability and viability in the two functional groups: viability increased in grasses but often decreased in forbs. This pattern resulted in 55% and 11% increases in viable seed production in grasses and forbs, respectively. At the higher level of fertilisation, shoot density was positively related to the number of viable seeds per shoot in grasses and to the seed size in forbs. Higher grass seed dormancy likely allowed for later grass seed germination in autumn when vegetation cover was low, contributing to the dominance of a few light-seeded nutrient-demanding grasses and poor forb number in fertilised meadows.