Review of Water Requirements for Key Floodplain Vegetation for the Northern Basin: Literature review and expert knowledge

Knowledge Gaps

There are obvious gaps in the current knowledge illustrated by the annotations in Tables 2–8. Information is particularly lacking for E. camaldulensis subsp. acuta that occurs in the Northern Basin, and all subspecies of E. coolabah. The knowledge base is improving for E. largiflorens, Acacia stenophylla and Duma florulenta, although demographic processes, thresholds and controls are not well known. More basic research on the reproductive processes and timing in relation to water regime is required.

Information is accumulating for E. camaldulensis subsp. camaldulensis, so that we are starting to be able to generalise the responses and tolerances of that species in the Southern Basin. The estimates of flood frequency are likely to be most reliable in the Southern Basin, but their applicability to populations of the same subspecies, and other subspecies in the Northern Basin is not yet known. Although we know that timing of floods for E. camaldulensis in the Southern Basin is best in the Winter-Spring, the best timing of flooding in the Northern Basin has not been assessed and could easily be later (i.e. Summer).

The knowledge gaps in Tables 2–6 relate mainly to water regime. In particular there is uncertainty regarding duration of flooding for most species. The data gathered for flow duration is often at the landscape level, and while this will provide response models that are applicable at the landscape level, determination of variability in responses within a community is a consequence of individual tree condition and position on the floodplain. Essentially it is a multidimensional response (tree age, stage, condition, position, distance to groundwater, response to antecedent rainfall, distance to channel, elevation etc.), when the approaches to measurement are necessarily made in fewer dimensions.

The monitoring and remote sensing data that has been collected for these species as part of Commonwealth and State research projects, since the Millennium Drought and subsequent floods, will provide the basis for further investigation, including demographic tracking of individuals over time (for all species listed in this report). We still don’t know how long individuals of any of these species can live, or have lived. We can hypothesise that they are K-selected (Macarthur and Wilson 1967), stress-tolerators (Grime 1977), that have high juvenile mortality, in common with many tree and shrub species, and the creation of significant, multiple, long-term monitoring sites (far longer-term than a funding-cycle, even longer than a human research career) would assist with understanding the ecology of these species for better management.

It is likely that the ecology of Acacia stenophylla and its role in nitrogen cycling in riparian systems (Brockwell et al. 2005) can be understood in a shorter time than the other key species in this review, since many Acacia species are typically short-lived (c. 10–20 years), although there is an estimate of 50 years for A. stenophylla.

The Northern Basin Review (Hale et al. 2014) identified the following knowledge gaps, and although the situation has not changed significantly, it is likely that projects that are currently in progress will fill some of these gaps.

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  Knowledge of the spatial distribution of key plant species, vegetation communities and vegscapes (i.e. vegetation maps). Although a range of vegetation mapping is available across the Northern Basin Scientific Review indicator sites and the Northern Basin generally, consistent vegetation maps (especially across the States’ border) are lacking.
  
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  Understanding of spatial variation in the character and condition of key plant species and vegetation communities across the Northern Basin. Knowledge of these is very patchy and inconsistent overall and especially lacking for the Barwon-Darling.
  
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  Understanding of historic variation in the character and condition of key plant species and vegetation communities (i.e. temporal variability). Very little information about historic variability of riparian, floodplain and wetland vegetation is available with the exception of some limited analyses of vegetation productivity (NDVI) and regeneration responses in the Lower Balonne (e.g. Good 2012).
  
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  Species responses to flow. A robust knowledge is lacking for key species in the Northern Basin, particularly in relation to if and how they differ from the same species occurring in the Southern Basin. Information concerning E. coolabah is particularly lacking.
  
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  Vegetation community level responses to flow. Knowledge of the water requirements of the understory of asset vegetation is particularly poor.
  
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  Vegscape responses to flow. Responses and variability in responses is poorly known, with the exception of limited analyses of vegetation productivity and regeneration responses in the Lower Balonne (Good 2012)
  

There are also significant ‘unknowns’ concerning the roles of competition and herbivory in structuring these plant communities. Since un-grazed sites are rare in the Northern Basin (and relatively recently created in the Southern Basin), the impact of grazing and its multitudinous, interacting consequences (see Casanova 2006 for a review) is unknown. The role of grazing in relation to life history stage and tree condition requires further research (Reardon-Smith 2011). The role of fire in structuring these plant communities is not well-known, and the role of weeds and litter accumulation or removal in demographic processes and nutrient cycling is a significant knowledge gap (Good 2012). The risk here is that if water is provided, because of some of these other stressors, the regeneration of species or improvement in condition that is expected, might not occur.

Given the significant diversity of species throughout the Murray-Darling Basin, the climatic variation within the regions, and elevation differences within each sub-catchment, it is not likely that the ecological processes and requirements of all species, or even all species of interest, will be able to be discovered in the near- or medium-term. There is a body of research supporting the use of species groups, rather than diversity indices or ‘key’ species in riparian vegetation management (see section 9), but this is hindered at the moment by a lack of consistency and application of the methodology throughout the basin. The use of WPFGs in the Murray-Darling Basin is likely to enhance prediction, management and communication of outcomes of watering to the general public if these limitations can be overcome.

This review recommends that:

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  Long-term monitoring be continued and enhanced, so that demographic processes and ecological responses and thresholds can be determined for the species of interest in the Northern Basin
  
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  The data that have been collected to-date, following the Millennium Drought and flooding events, on flood extent, duration and depth, tree response and condition, and ecological responses of the vegetation, be used as far as possible to understand the stress and recovery pathways for the key species. Initiatives such as that presented by Overton et al. (2014) are likely to make a significant contribution to our understanding, but these are (once again) limited by a lack of information about the ecology of key species in the Northern Basin.
  
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  The utility of WPFGs be investigated for prediction of responses and management of vegetation in the Murray-Darling Basin.