Weed seed dispersal via runoff water and eroded soil

High rainfall intensities on slopes produce runoff and erosion, but seeds are also carried down slope into streams by surface wash, causing weed infestation in lower parts of a river system. The study area was a cultivated valley (Houay Pano, Laos) farmed by smallholders and equipped with eight gauging stations: one draining the main catchment (60 ha) seven draining subcatchments (20-0.6 ha). All stations were used for sediment sampling, then sediments were scrutinized for seeds. The sampling took place simultaneously during, and immediately after, a single high intensity rainstorm. The event produced a total discharge of 1374 m(3) measured at the outlet of the main catchment, in this water 4600 Mg suspended soil left the catchment together with approximately 1.34 million floating and suspended seeds. The rainstorm produced 2227 Mg of stream sediments (bedload), containing about 103,300 viable seeds. In total 131 species were identified, most seeds were weeds. The catchments were mostly covered by vegetation (fallow and forest) just 13% was cultivated, unprotected soil. Only unprotected soil produced runoff and erosion, whereas well-protected soil was subject to high seed loss. Seed dispersal without soil erosion occurred because the discharge needed to transport light-weight seeds is less than the force necessary to move soil particles. An enrichment factor of 85 was calculated, comparing number of seeds in running water leaving a field (20 seeds m(-3)) with seeds in stream water (1706 seeds m(-3)). Upper slope sites, when cultivated, yielded not only locally eroded seeds, but seed enrichment took place in running water on the way down to the stream, even under dense cover where no soil erosion occurred. As long as the drainage channels on the lower slopes are not fully recovered, any cultivation of uphill sites will re-open the old gullies and the mass of seeds accumulated in the channels will be re-mobilized and delivered directly into the stream. Our experiment demonstrates that the effect of preferential erosion of seeds, i.e. dispersal without erosion, is far greater than dispersal in sediments, making hill slope processes the major contributor to seed dispersal, while the trapping of seeds in wetlands and by the riparian area was only locally important.