Floodplain Vegetation Productivity Response to Wetting and Drying: Testing the Adaptive Cycle Model

Abstract

<p>Dryland floodplains are characterized by highly variable flooding and drying regimes. The spatial and temporal variability in flooding plays a significant role in the productivity of these ecosystems and is a key influence on the composition and distribution of vegetation in these floodplains. Dryland floodplains have been perceived to be boom and bust systems, in which the boom is characterised by an inundated floodplain and the bust is characterised by a dry floodplain in moisture deficit. The boom stimulates great primary and secondary production where as the bust brings contraction of the ecosystem into refugia and a reduction in primary and secondary production. This relatively simple two state model may not account for the transitions that may occur between flooding (boom) and dry (bust) floodplain states. Understanding the patterns of response at different scales is critical to our ability to manage these complex dryland systems and to be able to make predictions about their future condition over time.</p> <p>This thesis applied an adaptive cycle model in order to understand change in floodplain vegetation productivity through multiple periods of flooding and drying. Adaptive cycles are a key component of resilience thinking. In this adaptive cycle model, vegetation productivity is the ecosystem responder and hydrology, or floodplain flooding and drying, the main driver of change. I derived a series of sequential hypotheses that explored the applicability of an adaptive cycle for the response of vegetation productivity in the Narran floodplain. The Normalized Difference Vegetation Index (NDVI) which measures vegetation greenness was used as a surrogate for vegetation productivity. In this adaptive cycle floodplain inundation was considered to drive vegetation productivity response through a cycle of exploitation, conservation, release and reorganization phases. The adaptive cycle starts as floodwater inundates the floodplain in the wetting phase. The wetting phase corresponds to the exploitation part of adaptive loop, where the area of vegetation productivity and quality will increase because of the availability of water as an exploitable resource. The wet phase is the phase of maximum inundation and corresponds to the conservation phase of the adaptive loop. The conservation phase is a period of increased vegetation productivity and a stability of vegetation productivity. The contraction of floodwater triggers the drying phase and corresponds to the release phase of an adaptive cycle. Further, desiccation of the floodplain occurs with the draining of floodwaters until the floodplain reaches a dry phase, a phase of no surface water availability. The dry phase corresponds to the reorganization phase of an adaptive cycle. The results of this thesis represents an advance on previous studies of dryland floodplains as an approach for characterising and understanding the response of vegetation communities in large floodplains.</p> <p>The findings of this thesis demonstrated there to be marked differences in NDVI class area, number of transitions, directions of transitions, probability of transitions and NDVI class diversity between the dry phase and the combined wetting, wet and drying phases of inundation. Overall an anti-clockwise hysteresis relationship occurred between flooding and vegetation productivity, indicating a cyclic nature of vegetation response to floodplain inundation through dry, wetting, wet and drying phases. These results support the hypothesised adaptive cycle model for the response of vegetation productivity and its appropriateness for understanding the complexity of dryland floodplain vegetation response to wetting and drying. These results were also repeated over four flood events of different size. Although the four events exhibited an adaptive cycle, the duration and the nature of vegetation within each phase of the adaptive cycle differed. Likewise, the four different vegetation communities also exhibited response patterns in relation to flooding and drying that fit the adaptive cycle model. However, differences were evident in the timing of transitions between adaptive cycle phases and the duration spent in those phases in each vegetation community. The woodland community types of the Narran floodplain showed a higher productivity response during the drying or release phase. By comparison the highest productivity response for the grassland and shrubland was observed during the wetting or exploitation phase. Overall, the results showed the four vegetation communities are sensitive at different points in the adaptive cycle.</p> <p>A unique finding of this study result was the location of the exit point from the adaptive cycle, which is the potential point for a state change. The exit point from an adaptive cycle is characterized by a period of enhanced high instability. In the Narran floodplain, the patterns of response in vegetation productivity to flooding and drying indicate this occurred between the conservation and release phases and not between the reorganization and exploitation phases as hypothesised by adaptive cycle theory. Thus, the potential for a change in state in dryland floodplains is highest between the wet (conservation) and drying (release) phases.</p>