Floral morphs of a heterostylous grassland herb differ in responses to grassland fragmentation and environmental...

Semi-natural grasslands represent biodiversity hotspots but face increasing threats from habitat fragmentation, land-use intensification, and climate change, which disrupt ecological interactions and reduce population viability. Species with heterostylous mating systems are particularly sensitive to these disturbances, as they depend on cross-pollination between floral morphs. Primula veris, a typical grassland heterostylous species with declining populations, has two floral morphs (S- and L-morph) and often exhibits skewed morph ratios in small and fragmented populations, suggesting potential differences between morphs in reproductive success, germination, or plant performance. Yet, such morph-specific differences remain poorly documented.

We conducted three complementary experiments assessing morph-specific variation in P. veris. We quantified seed production across 30 semi-natural grassland populations differing in size and habitat fragmentation, evaluated germination under controlled conditions using seeds from both S- and L-morph mothers, and tested plant performance of the resulting offspring under factorial combinations of competition, drought, and nutrient enrichment.

Seed production did not differ significantly between maternal morphs but was higher in fragmented habitats, potentially reflecting higher pollination efficiency associated with lower plant species richness and reduced reproductive interference. In contrast, germination rates were higher for seeds from S-morph mothers than for those from L-morphs, while population size and fragmentation had no effect. Under experimental stress, plant performance depended on interactions between morph, competition, and fertilization: S-morphs responded positively to fertilization and negatively to competition, with these effects acting independently of each other, whereas L-morphs benefited disproportionately from fertilization in the absence of competition but suffered more under competitive conditions when fertilized.

Our results demonstrate that germination success and stress responses differ among morphs, even when seed production appears similar, emphasizing the need to consider morph-specific ecology when predicting population persistence and morph frequency shifts in heterostylous species in fragmented grasslands.