Variability in flood disturbance and the impact on riparian tree recruitment in two contrasting river systems

The vegetation within the riparian zone performs an important ecological function for in-stream processes. In Australia, riparian zones are regarded as the most degraded natural resource zone due to disturbances such as river regulation and livestock grazing. This study looks at factors influencing vegetation dynamics of riparian tree species on two contrasting river systems in Western Australia. The Blackwood River in south-western Australia is influenced by a Mediterranean type climate with regular seasonal winter flows. The Ord River in north-western Australia is characterized by low winter base flows and episodic, extreme flows influenced by monsoon rains in the summer. For both rivers, reproductive phenology of studied overstory species is timed to coincide with seasonal river hydrology and rainfall. An even distribution of size classes of trees on the Blackwood River indicated recruitment into the population is continual and related to the regular predictable seasonal river flows and rainfall. In contrast, on the Ord River tree size class distribution was clustered, indicating episodic recruitment. On both rivers tree establishment is also influenced by elevation above the river, microtopography, moisture status and soil type. In terms of vegetation dynamics riparian vegetation on the Ord River consists of long periods of transition with short lived stable states in contrast to the Blackwood river where tree population structure is characterized by long periods of stable states with short transitions.