Flow-divergence feedbacks control propagule retention by in-stream vegetation: the importance of spatial patterns for facilitation

Facilitation (enhancement of propagule retention in this case) is increasingly recognized as an important driver of biodiversity, but it is still unknown if facilitation during dispersal and colonization is affected by self-organized spatial pattern formation. We investigated the ability of in-stream submerged macrophyte patches to trap the vegetative propagules of three species (Berula erecta, Groenlandia densa, Elodea nuttallii in two size classes: 13–22 and 40–48 cm long), and to potentially benefit the colonization of these three species. We tested the effects of propagule traits, hydrodynamic forcing, and spatial patch configuration on propagule trapping. Propagule buoyancy was negatively correlated with trapping chance, while propagule size did not influence trapping. Species-specific differences in buoyancy were maintained for weeks after fragmentation. Propagule retention was interactive and conditional upon the interplay between incoming flow velocities and vegetation spatial patterning. In the flume experiment at low flows, a patchy configuration (one patch filling 66% of the flume width) retained more surface-drifting propagules (B. erecta, G. densa), than near-homogeneous cover (two patches close together, filling the entire flume width). In contrast, retention of sinking E. nuttallii propagules increased in the two-patch configurations. In flume and field releases where patches did not completely fill the channel width, water flowed around the patches rather than over or through them. This resulted in low-flow velocity areas within patches where canopies were upright and propagules were retained, and higher velocity flows around patches. In contrast, when vegetation filled the channel width, water could not be diverted laterally around the patches and preferentially flowed over them, causing the canopies to bend and reduce their trapping capacity. In flume experiments at high flows, retention of all species decreased, regardless of vegetation configuration, as propagules passed over the reconfigured vegetation canopies. These findings on the interplay of water movement and patch reconfiguration suggest that environmental heterogeneity generated by the self-organizing behavior of aquatic plants might enhance colonization of sessile organisms, calling for landscape-scale processes like dispersal to be better investigated.