Seedling germination and survival has been observed to be highly variable across the Murray-Darling Basin, with seedlings scarce in some areas and abundant in others. Providing woody seedlings with optimal conditions for growth and survival helps to ensure their establishment. Understanding these optimal conditions helps to manage growth in less desirable locations. This study investigated what flow regimes best support seedling establishment within populations of long-lived floodplain vegetation species. We were interested in the effect of flood timing (defined as early or later during seedling life stages) and multiple floods on seedling establishment. We were particularly interested in the impact on root development, hypothesising that flood sequences that result in greater root growth are likely to provide the plant with greater access to water and nutrients, increasing their capacity to survive dry periods.
A mesocosm experiment was conducted over seven months investigating the survival and condition of seedlings of River Red Gum, Coolabah and Black Box against five flooding regimes: (i) permanent flooding, (ii), permanent dry, (iii) inundation during either early or (iv) later seedling life stages, and (v) multiple wetting and drying periods during seedling establishment. Measurements of mortality, seedling height, leaf number and area, root length, total above and below ground biomass were recorded to assess survival and condition.
Responses were largely species specific, however some general patterns were observed. Species with a constant flooding regime displayed slower growth rates than all other flooding regimes. While seedlings that received late flooding had greater growth rate response compared to those that were flooded in earlier life stages. Root length was typically greatest with seedlings that received a late flood, though below ground biomass responses were species specific.
Outcomes from this study will be used to refine seedling water requirements and inform the design of flows to support seedling establishment.