Seed retention by pioneer trees enhances plant diversity resilience on gravel bars: Observations from the river Allier, France

International audiencePioneer riparian trees which establish in river active tracts on gravel bars enhance fine sediment reten- tion during high flows within their stands and in their lee side, forming obstacle marks. Fine sediment retention can be accompanied by deposition of seeds transported by water dispersal, i.e. by hydrochory. We tested the hypothesis that pioneer riparian trees significantly control seed deposition on gravel bars by forming sediment obstacle marks. We described the seed bank structure and compared samples col- lected from obstacle marks and bare coarse-grained bar surfaces. At the surface (at 2 cm depth), seed abundance ( N ) and richness ( S ) (expressed as mean ±sd) were significantly higher in areas directly af- fected by riparian trees, i.e. obstacle marks, ( N : 693 ±391; S : 17 ±3) than in bare surfaces ( N : 334 ±371; S : 13 ±5). Surface and sub-surface (at 20 cm depth) samples were also significantly different, with the sub-surface samples almost devoid of seeds (respectively N : 514 ±413; S : 15 ±5 and N : 3 ±6; S : 1 ±2). These results suggest a biogeomorphic feedback between sediment and associated seed retention medi- ated by hydrochory, vegetation growth and local seed dispersal mediated by barochory. Such feedback may improve plant diversity resilience on gravel alluvial bars of high-energy rivers