Light quality and not quantity segregates germination of grazing increasers from decreasers in Mediterranean grasslands

Grassland plant species have traditionally been classified into two groups, increasers and decreasers, based on changes in their relative abundance in the presence or absence of grazing. However, the ecophysiological mechanisms underlying these opposing strategies remain unclear. Seeds of ten Mediterranean species of known grazing status were germinated in different light environments to test the hypothesis that grazing selects light-dependent germination amongst grazing increasers. Two light quantity levels (Photosynthetical Active Radiation, PAR) and five light quality levels (red:far-red ratio, R:FR) were used to quantify percentages of germination of viable seeds and number of days elapsed to reach 50% of germination (T50), measured using five replicates of each light treatment and species combination. There were no differences in response to light quantity between the two grazing status groups; germination was inhibited in both groups at higher light intensities. However, increasers had lower germination percentages at low R:FR ratios, while decreasers were not affected by light quality. Germination speed (T50) was affected differently by light quality in each grazing status. There was an increase in germination speed at the lowest R:FR ratio only in increaser seeds, whereas decreaser germination speed was not affected by the treatment. PAR affected T50, regardless of grazing status; germination was faster at lower light intensities. Increaser seeds germinated faster than decreasers. These results reveal that the differential control of germination by light quality is a key factor in determining the species composition of grassland plant communities affected by herbivores in semi-arid Mediterranean grasslands.