ABSTRACT
Riparian forests are structured and maintained by their hydrology. Woody riparian plants typically adapt to the local flood regime to maximise their likelihood of survival and reproductive success. Understanding how extant trees form and reproduce in response to flood disturbance is crucial for predicting vegetation changes and informing restoration. Working in a temperate evergreen riparian forest, we aimed to determine whether disturbance-based responses of plants found in other ecosystems also typify woody plants in riparian forests where disturbances are often mild or chronic, non-lethal, annual events. Using plant surveys and 20-year modelled hydrological data, we examined whether (1) the morphology (main stem diameter, height, crown width, crown extent, stem leaning) and (2) reproduction type (sexual and asexual reproduction) and extent of three dominant woody species (Eucalyptus camphora, Leptospermum lanigerum and Melaleuca squarrosa) vary with flood regime (flood frequency and flood duration); and (3) whether different morphology is associated with different reproductive strategies. Increased flooding generally resulted in increased stem numbers and greater stem leaning-morphologies associated with asexual reproduction-of our study species. More frequent flooding also reduced plant size and sexual reproduction in E. camphora. Sexual reproduction in the studied species was more common in taller plants with single, more upright stems in good condition. Flexible morphology and plastic reproductive strategy may constitute an adaptation of trees to mild or chronic disturbance in floodplains. Our findings suggest that flood regime (i.e. variable frequency and duration of flooding events) is critical to the structural integrity and self-maintenance of species-diverse riparian forests.
ABSTRACT
Flow regulation impacts on riparian vegetation composition, often increasing the prevalence of exotic and terrestrial plant species. Environmental flows may benefit native riparian vegetation via the promotion of plant recruitment from riparian soil seedbanks, but this is dependent on an intact native seedbank. Thus, we assessed the composition of the soil seedbank of different riverine geomorphic features to determine its potential response to environmental flows. Soil seedbank samples were taken from channel bars, benches and floodplains at six sites along the Campaspe River, Australia, a heavily regulated river that receives environmental flows. These geomorphic features represent a gradient in elevation and thus flooding frequency from frequently flooded (bars) to infrequently flooded (floodplain). Seedbank samples were 'grown out' in a glasshouse, and seedlings identified and classified according to taxa, flood tolerance and origin (native or exotic). We identified 6515 seedlings across all geomorphic features and sites, with monocots most abundant. Soil seedbank composition varied between geomorphic features. Overall, seedling abundances were greater for in-channel features (bars and benches) than floodplains, but taxa richness did not vary likewise. Soil seedbanks of in-channel features were dominated by flood tolerant and native taxa, while flood intolerant and exotic taxa were generally associated with floodplains. The dominance of native flood tolerant taxa in the soil seedbanks of in-channel geomorphic features suggest these seedbanks can play an important role in the resilience of native riparian plant communities. Moreover, environmental flows are likely to play a positive role in maintaining native riparian plant communities given such conditions.
