Phenology of the dioecious shrub Baccharis halimifolia along a salinity gradient: Consequences for the invasion of Atlantic subhalophilous communities

Atlantic subhalophilous communities are habitats of high conservation value that are seriously threatened by plant invasions. The dioecious alien shrub Baccharis halimifolia L. (Asteraceae) is among the most aggressive invaders in estuarine communities in Europe, especially in sea rush communities of Juncus maritimus. We investigated the growth and flowering phenology of B. halimifolia through fine-scale environmental gradients, and we assessed the intersexual differentiated responses. We monitored 120 individuals of B. halimifolia growing in rush communities differing in salinity and waterlogging through one growing season in Urdaibai Biosphere Reserve. Ground water salinity and water table level were also recorded. Overall, B. halimifolia shoot growth occurred from May to December and flowering in late August, but males flowered earlier than females. However, we found high intrapopulation and intersexual variation in the phenology of B. halimifolia, related to fine-scale variations in edaphic conditions. Growth rate and leaf production were lower in high-salinity rush communities. Females and males showed different phenological responses to environmental conditions. Females appeared to be more sensitive to salinity than males, but males experienced higher levels of attack by natural enemies. The implications for the invasiveness and management of sea rush communities are discussed.