Forest restoration on floodplains mantled with legacy sediments: removing sediments appears unnecessary for successful restoration

Abstract

Recent studies suggest very low survival of seedlings planted in streamside areas containing thick (>1 m) deposits of legacy alluvial sediments. We planted 2,450 seedlings representing eight species in a streamside area with thick legacy sediments and monitored them for 5 years. The overall survival of approximately 60% (range across species: approximately 38–74%) was surprisingly high and mean overall growth per seedling (approximately 3.27 m) was very good, ranging between 2.5 and 4.7 m depending on tree species. Although both seedling survival and growth exhibited significant spatial variation, none of the results supports the idea that legacy sediment thickness up to 1.5 m is an important factor with regard to success of streamside reforestation. For survival, soil depth was significant for the sediment accretion zone but not the legacy sediment zone. For growth, the response was significant and positive, with the eight species on average growing significantly better as legacy sediment increased in thickness. The results suggest that the presence of legacy sediment up to 1.5 m thick should not preclude the successful restoration of natural forest along stream channels in the eastern Piedmont of North America. Finally, the study suggests that the U.S. federal criteria for reforestation success (i.e. 222 stems per hectare after 5 years) can still be met on legacy sediment sites by increasing the planting density approximately 25% from the required minimum of 296 stems per hectare to 370.