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Wetland Vegetation Responses to Environmental Water and Flooding: The Lower Lachlan River System, Australia

Author(s): Fiona Dyer; Alica Tschierschke; Will Higgisson; Sharon Bowen; Patrick Ver; Jane Roberts

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Abstract: Riparian and wetland plants have evolved within the context of the hydrological regimes they experience. They have developed specific reproductive characteristics, growth forms and dispersal mechanisms that enable them to flourish and persist within certain hydrological environments. As a result, the diversity, type and condition of riparian and wetland vegetation communities are strongly influenced by the frequency, duration and timing of inundation[1,2]. The alteration of flow regimes, as a consequence of the combined effects of flow regulation and abstraction, have had widespread and significant effects on the health of riparian and wetland vegetation[3,4], particularly in arid and semi-arid environments[5]. To address these effects, environmental flows are often used to try to improve vegetation condition and diversity[6]. The rivers of the Murray Darling Basin, Australia have been extensively regulated to provide water forhuman and economic needs. This has led to major changes in flow regimes with subsequent degradation of the ecological condition and character of rivers, floodplains and wetlands[7], including many significant floodplain and wetland vegetation communities[7,8]. In response, Australian governments have invested in providingwater to support environmental outcomes, including maintaining the extent and improving the condition of riparian and wetland vegetation[9]. The Lachlan river system, in semi-arid NSW, Australia, forms part of the Murray Darling basin. Theriver travels nearly 1400 km and, in most years, the river terminates in the Great Cumbung Swamp. The flow in the river is regulated using major dams, off stream storages and weirs which have significantly modified thehydrological regime[10]. The catchment of the Lachlan river contains many sites of high-value wetland plant communities and substantial areas of riparian fringing river red gum forest (Eucalyptus camaldulensis) and woodland, particularly in the lower reaches[10]. The Australian Government, through the CommonwealthEnvironmental Water Office (CEWO), use environmental flows to maintain and improve the riparian and wetland vegetation of the Lower Lachlan river system. To evaluate the response to the provision of environmental water, as part of the CEWO’s Long TermIntervention Monitoring Project, we monitored the diversity and condition of riparian and wetland vegetation in spring and autumn from 2014 to 2017 at 13 sites in the Lower Lachlan Catchment. The vegetation monitoring sites were selected to provide a sample from the different vegetation communities distributed across wetlands and riparian zones with different environmental watering probabilities[11]. At each site, species abundance and cover were recorded as well as measures of stand and tree condition (basal area, canopy openness, canopy extent, live/dead limbs). The climate conditions combined with water availability dictated the amount of environmental waterused for vegetation outcomes in each year. The three years of monitoring encompass vastly different climate and hydrological conditions. In the first year (2014-15) conditions were dry and a small watering action wasdelivered to provide connection to support reed beds in the Great Cumbung Swamp. In 2015-16, a greater availability of water enabled the delivery of watering actions designed to support wetland vegetation. These were accompanied by other flows which raised water levels in the main channel and inundated a number of wetlands. In 2016-17, the river system experienced the fourth largest flood on record which meant thatCommonwealth environmental water use was limited and all sites were inundated for extended periods of time. This paper reports observations of the vegetation responses to the range of different inundation conditionsexperienced. The floodplain and wetland vegetation communities of the Lower Lachlan river system displaysequences of dry and wet phases depending on regional climatic conditions. In 2014-15, conditions were dry and the sites were dominated by short, terrestrial vegetation species, mainly chenopods such as creepingsaltbush (Atriplex semibaccata), brassicas such as smooth mustard (Sisymbrium erysimoides) and grasses. In2015-16, large numbers of annual species such as burr medic (Medicago polymorpha), fumitories (Fumaria sp) and common twinleaf (Zygophyllum apiculatum) responding to the wetter conditions. The short durationinundation of some sites resulted in the appearance of species which are more adapted to damp conditions such as native mint (Mentha australis), nardoo (Marsilea drumondii), lippia (Phyla nodiflora) and swamp daisy (Brachyscome basaltica). Sites that were inundated also displayed an improvement in the condition of the fringing tree community. The major flooding in 2016-17 produced a distinct response in the vegetation and a strong interplaybetween the geomorphic template and the response of the vegetation to water was observed. The watering of sites that are periodically inundated to more than 0.4 m resulted in the appearance of new growth forms such as floating plant species such as azolla (Azolla filiculoides) and duck weed (Lemna minor), the elimination of all short and terrestrial species, and an improvement in the condition of the fringing river red gum woodland. At these sites, following recession of water a low diversity of amphibious species colonized the soil, dominated by common sneeze weed (centipeda cunninghamii) and damascisa (Glinus lotoides). These sites are generally low lying open water sites that are relatively easy to provide with environmental water but have a limited vegetation diversity response. In contrast, sites which are higher in the landscape showed a marked response to shallow (< 0.4 m) flooding with the appearance of a large number of aquatic and amphibious plants such as common spike rush (Eleocharis acuta), small flower umbrella sedge (Cyperus difformis), marsh club-rush (Bolboschoenus fluviatalis), water ribbons (Triglochin spp. ) and star fruit (Damasonium minus), as well as an improvement in tree condition. Many of these sites are unable to be watered with environmental water and require large natural floods for inundation. These findings have important implications for the establishment of site based vegetation diversityobjectives for environmental watering in the Lower Lachlan and similar catchments. The greatest opportunity to observe a vegetation diversity outcome is at sites that are able to receive a shallow watering with environmental water. These include some ecologically significant wetland sites for which we have very little historicalhydrological or vegetation data that could be used to identify environmental watering needs. The challenge at these sites is to identify the duration of flooding and frequency of flooding required to support the vegetation community.


Year: 2018

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