More Exotic and Fewer Native Plant Species: Riverine Vegetation Patterns Associated With Altered Seasonal Flow Patterns

Abstract

Natural flow regimes are important for sustaining riverine vegetation. The regulation of river flows to provide water for agriculture often results in changes to flow timing. This study assesses the impact of altered seasonal flow patterns on riverine flora. Within temperate Australia, we surveyed the vegetation of five lowland rivers, three of which have large dams that alter their seasonal flow patterns; the other two are unregulated. From four to six sites were selected on each river, and these were classified into three levels of regulation based on the extent to which the timing of their seasonal flow patterns were altered. Sites were surveyed in winter and the following summer. Permanent quadrats were also established at a number of the surveyed sites and resurveyed every 3 months. Of the 267 plant taxa identified, 145 were exotic (non‐native). More exotic taxa and fewer native taxa were associated with increasing level of seasonal flow inversion (regulation). In particular, greater numbers of short‐lived exotic terrestrial taxa and fewer native woody taxa were associated with increasing level of regulation. Some exotic woody species (e.g. willows) were more common in the unregulated rivers and may have life‐history traits favoured by the natural seasonal flow patterns of study area. Multivariate analyses showed that level of regulation had a significant effect on the overall composition of the riverine vegetation. Our results provide support for the hypotheses that flow regulation adversely affects native species diversity and increases the vulnerability of riparian zones to invasion by exotic species; however, these effects are dependent on plant species' life‐history strategies. Our study highlights the importance of natural seasonal flow patterns for sustaining native riverine plant communities. Flow management aimed at maintaining or restoring ecological values should consider seasonal flow patterns. Winter/spring flow peaks may be particularly important for the recruitment of native riverine plants, especially trees and shrubs, and reducing the extent of exotic annuals and grasses. Copyright © 2012 John Wiley & Sons, Ltd.

Soil seed bank dynamics in response to an extreme flood event in a riparian habitat

“…Such an extreme event can be considered as a form of disturbance, and through its influence on sediment reworking, plays an important role in re-shaping of wetland communities and ecosystems (Naiman and Decamps 1997;Nilsson et al 2010;Greet et al 2012). One such an extreme event happened in the later months of 2010 and continued into mid January 2011 in eastern Australia (Fig.…”

“…2) The magnitude and direction of flooding effect upon the viable seed bank will vary, depending on the plant functional group and/or their life history traits. For example, theory predicts that the trajectory pathways of invasive exotics are significantly different from those of native or non-invasive species (e.g., Ho¨lzel and Otte 2004;Catford et al 2011;Greet et al 2012). Flooding, being a form of disturbance that can creates new niches, should favour exotic invasive species over native species because of their higher competitive ability in these modified environments (Greet et al 2012).…”

“…For example, theory predicts that the trajectory pathways of invasive exotics are significantly different from those of native or non-invasive species (e.g., Ho¨lzel and Otte 2004;Catford et al 2011;Greet et al 2012). Flooding, being a form of disturbance that can creates new niches, should favour exotic invasive species over native species because of their higher competitive ability in these modified environments (Greet et al 2012). Wetland and aquatic plant species are better adapted to moisture, and hence their seeds, following flooding can float and survive better than those of species that are less adapted to such condition (especially terrestrial specialists).…”

“…Similarly it can be expected that the response of opportunist species (annual/biennials, herbaceous and graminoids) that complete their life cycles in short periods of time will be different from the response of long-lived perennials, trees and shrubs. Thus it is predicted that in the postflood environment, annuals, short-lived perennials, hygrophilous, and invasives will make-up the majority of plant species found in the seed bank (Bagstad et al 2005;Catford et al 2011;Greet et al 2012). …”

“…Thus, in the process of searching for resilience of riparian ecosystem to rare, extreme disturbance event, we quantify (1) the impact of the major flood in Queensland, Australia of 2011 on soil seed bank composition and functional group assemblage, and (2) the effect of the flood disturbance relative to yearly, seasonal and micro-topographic factors. Seasonal observations are important because riparian vegetation often displays dramatic seasonal changes (Reid and Quinn 2004), and thus vegetation and seed-bank changes associated with flooding are hypothesised to be more apparent at different seasons of the year (Ho¨lzel and Otte 2004;Greet et al 2012). …”