Seedling growth of Acacia tortilis and Faidherbia albida in response to simulated groundwater tables

The growth and survival of woody seedlings in arid and semi-arid riverine forests is partly determined by the ability of their roots to maintain contact with the alluvial water table. After a flood event, the water table declines in conjunction with the river stage and the rate of decline may have species-specific effects on the regeneration of riverine trees. In this study, a growth experiment involving seedlings of Acacia tortilis and Faidherbia albida, two keystone tree species along the regulated Turkwel River in northern Kenya, was conducted to assess the effects of simulated pre- and post-dam water table dynamics. Seedlings were grown in rhizopods with constantly shallow water tables (0 cm day−1 treatment), slow rates of water table decline (5 cm day−1 treatment), rapid rates of water table decline (10 cm day−1 treatment) and no subsurface water (rainfall treatment). The last treatment corresponded to the dam-induced paucity of floodplain inundation but allowed early seedling growth in response to rainfall. Results showed that F. albida attained larger shoot growth but shorter root lengths than A. tortilis, while water table decline promoted root elongation in both species. However, F. albida seedlings were adversely affected by moisture deficits under the rapid rate of water table decline and rainfall treatments. In contrast, the growth of A. tortilis seedlings was sustained under all treatments, suggesting that A. tortilis is a drought tolerant facultative phreatophyte, while F. albida behaves more like a near obligate phreatophyte. For F. albida, the ideal stage recession rate following a flood event is 5 cm day−1 or less, which is also favourable for A. tortilis. Although more research is still needed on the hydrological preferences of woody seedlings in the Turkwel River floodplain, it is evident that the regeneration of strictly riverine trees such as F. albida depends on slow rates of water table decline in the post-flood period.