Multiple consequences of flood events have differing effects on riparian vegetation
Riparian (or riverbank) plant community composition and diversity is often attributed to the hydrological characteristics of the riparian zone, including flood events. Floods have several simultaneous effects on riparian vegetation and thereby on community composition. Stress caused by underwater submergence may place a plant at a competitive disadvantage against individuals or species better able to tolerate the physical effects of inundation. Floods can also bury vegetation under sediments, as well as add nutrients to the riparian substrate. It is difficult to understand the relative independent contributions of these simultaneous consequences of a flood event through field observations. Understanding the effects of these abiotic conditions at a finer level may provide a greater understanding of the mechanisms of riparian community assembly.
We performed a greenhouse experiment examining the independent effects of submergence under water, burial under sediments, and nutrient addition on the growth of six herbaceous plant species. Of these, three (Bidens frondosa, Polygonum pennsylvanicum, and Prunella vulgaris) are commonly found in riparian plant communities along small streams in the Upper Hudson watershed (NY, USA), and three (Solidago flexicaulis, Polygonum virginianum, and Marrubium vulgare) are common in the surrounding woodlands of the region but rare in riparian zones.
All but one species (M. vulgare) experienced increased growth under nutrient addition. The three riparian species responded differently to submergence and burial treatments; while B. frondosa and P. vulgaris experienced the poorest growth when buried, P. pennsylvanicum had the poorest growth when submerged. All riparian species experienced high survivorship when submerged. Survivorship of B. frondosa was reduced when buried under sediment, while the other two riparian species showed similar survivorship between sediment burial and control treatments. M. vulgare showed 100% mortality in the submergence treatment, while the other non-riparian species had high survivorship when submerged. All non-riparian species showed poor survivorship when buried under sediment.
Our results suggest that burial under sediments may play a primary role in excluding our three non-riparian species from riparian zones, while the three riparian species differ in which consequences of floods facilitate or hinder growth or survivorship. These results begin to provide a better understanding of the specific abiotic conditions that might underlie the distribution of common riparian species either by facilitating species success or excluding some members of the regional species pool from the riparian zone. Future work will focus on the interactive effects of these flooding effects on species growth and survival.