Flood-deposited wood creates regeneration niches for riparian vegetation on a semi-arid South African river

Question: The formation of large woody debris (LWD) piles during floods has significant impacts on riparian succession through pioneering plants often establishing in association with wood. We assess the importance of LWD for seed regeneration of riparian plants after a century-scale flood disturbance in a semi-arid environment. Location: The Sabie River within Kruger National Park in the semi-arid northeast of South Africa. Methods: Our approach was to quantify the riparian soil seed bank, to record the frequency of establishment of riparian plants in woody debris piles, and to conduct experimental outplantings of common riparian trees in plots with and without LWD. Results: We found the abundance and diversity of seedlings were higher in soils taken from wood piles than from open reference areas, and most seedlings were herbaceous species. Surveys indicated that numbers of seedlings recorded within woody debris were significantly greater than in open reference areas or within established vegetation. Seedling establishment in various cover-types also varied for different riparian tree species. Experimental out-planting of seedlings of two riparian tree species (Philenoptera violacea and Combretum erythrophyllum) revealed that, after 433 days, planted seedlings survived only in woody debris piles. Conclusion: LWD formed after a large flood creates heterogeneous patches that may influence post-disturbance regeneration of riparian vegetation by providing a variety of environmental niches for seedlings establishment. We suspect that higher seedling survival in LWD is due to increased moisture (particularly in the dry season) and nutrients, and protection from seasonal flooding and herbivory.