Variation in sexual reproductive output among exotic taxa of Spartina (Poaceae).
Sexual reproduction can enhance the colonization and spread of invasive plants. We aimed to evaluate ecological factors influencing sexual reproduction and dispersal capacity of exotic Spartina taxa. Density of inflorescences, spikelets per inflorescence, percentage of spikelets with caryopses, and density of caryopses were recorded for North American S. patens, South American S. densiflora, and reciprocal hybrids of invasive S. densiflora with native S. maritima in salt marshes at the Gulf of Cádiz. For S. patens and hybrids, we also assessed pollen viability, and the percentages of reabsorbed caryopses and spikelets with all stamens exserted. For fertile S. densiflora, we evaluated plant sexual traits and characteristics of the sedimentary environment, and conducted germination experiments. We found interannual variation for reproductive traits of S. densiflora which also differed among habitats. Plants with highest inflorescence and caryopsis densities occupied low marshes and saltpans, lowest caryopsis production was in low marsh where abiotic stress was high, while viability of caryopses tended to be the highest in middle and high marshes. Our results indicate neither hybrid produced any mature caryopses, due to lack of pollen viability. However, male fitness did not explain infecundity of S. patens with ca. 80% of spikelets with all stamens exserted and pollen viability was ca. 60%. To reduce invasive spread and impact, conservation efforts to protect biodiversity should prioritize rapid eradication of Spartina hybrids before they attain sexual reproductive capacity. The removal of S. densiflora within 2 years of establishment before production of viable caryopses is strongly recommended.