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Questions The order of plant species arrival can affect recruitment and subsequent plant community development via priority effects, but is often overlooked. Priority effects occur when early-colonizing plant species affect the establishment of later-arriving species, and are hypothesized to depend on species identity and habitat conditions. In riparian ecosystems on the banks of rivers, a strong moisture gradient induces a zonation of plant species with different degrees of adaptation to soil moisture. Further, riparian zones receive seeds during floods and later in the season via wind dispersal. As such, we questioned if recruitment in riparian zones is primarily affected by (1) environmental conditions (i.e. soil moisture), (2) arrival order, and (3) species identity, or an interaction between these factors.

Location Riparian zones of tributaries in the Vindel River catchment, northern Sweden.

Method We designed a controlled greenhouse experiment and a large-scale field experiment where we sowed five plant species representing different dispersal events and habitat moisture preferences. We sowed seeds in three arrival order treatments (all species simultaneously, species group A phased 3wk before group B, and vice versa) and under different soil moisture treatments in the greenhouse (dry, dry-after-wet and wet) and under a range of moisture conditions in the field.

Results We found strong priority effects as early-arriving species grew bigger and often produced higher seedling densities compared to later-arriving species, both in the greenhouse and after two growing seasons in the field. Priority effects in the greenhouse were strongest in the dry and dry-after-wet treatments and weaker under wet conditions. Consistent but weaker patterns were observed in the field after the first growing season. The relative abundance of species in plant communities assembled without phased arrival interacted with soil moisture and species identity. Priority effects were strongest for species with a low relative abundance (i.e. less competitive species).

Conclusions Our findings that priority effects influenced recruitment and interacted with soil moisture suggest that priority effects should be considered when addressing riparian vegetation changes after shifts in flooding regimes. This is especially important because floods will not only affect habitat conditions, but also the phasing of seed arrival.