How do Water Regime and Grazing Alter the Reproductive Capacity of Aquatic Plants?

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Dr Margaret A. Brock
Botany, Rural Science and Natural Resources, University of New England
Environment Australia, 2000

1. Research Findings

a) Aims

To achieve the stated objectives experiments were designed to examine:

  • if the types of plant communities that establish and reproduce from the same seed bank differ under different water regimes (Objectives 1, 4, 5);
  • if a sustainable seed bank can develop in new wetland from an introduced seed bank source (Objectives 1,3,5);
  • how long does it take for sustainable communities, as indicated by development of a viable seed bank, to develop under each water regime (Objective 5)
  • if grazing in interaction with duration, depth and frequency of flooding influences plant growth and reproduction (Objective 2)

b) Background and methods

I From reliability to reality

This project has combined the strengths of various scales of experimentation to achieve answers applicable at the field scale. Reliability of results was highest in the finer scaled experimental testing under glasshouse conditions, and outdoor experimental tank testing of water regime and grazing interactions (see Attachment a). Field scale experiments in the University of New England Experimental Wetlands and field observations added greater reality to interpretations. We believe this set of experimental systems is unique in Australia. Benefits and limitations of these techniques are outlined in Attachment b.

II University of New England Experimental Wetlands

The University of New England Experimental Wetland Facility was developed as a cooperative project between UNE, LWRRDC, NSW Department of Land and Water Conservation and The Wetland R & D Program. It involved the construction of 16 wetlands on the UNE Resource Management property Newholme. Each wetland (10m x 4m x O.7m depth) has a 10m long slope along which plant growth from seed bank material is monitored. The water intake and outlet on each wetland is controlled separately allowing the manipulation of depth, duration and frequency of flooding within each wetland. (See Attachments c and d) During the term of this project:

  • The UNE Experimental Wetlands were completed in spring 1997 as a facility in which the effects of water regime on the biota are being examined in a field experimental situation.
  • The 16 experimental ponds filled in October 1997 and the following five water regimes have been imposed since then (30 months). Water regimes are altered every two weeks. Each water regime treatment is replicated either 2 or 4 times as follows (Figure 1, See Attachment, c and d). Natural wetland water levels have also been monitored (Figure 2 ) and levels at Llangothlin Lagoon used to calculate levels for the Mimic water regime.
  1. Permanently flooded: 2 reps (P);
  2. Mimic of a natural wetland (Llangothlin): 2 reps (M);
  3. Autumn fill (1 cycle / year): 4 reps (AF);
  4. Spring fill (1 cycle / year): 4 reps (SF);
  5. Twice annual fill (spring and autumn- 2 cycles per year): 4 reps (TF).
  • Physical and chemical parameters of each pond have been monitored regularly (Attachment e)
  • Seed bank material from four New England Wetlands of known seed bank composition was mixed to maximize the species richness and the range of functional groups of species present. Seed bank material was introduced in strips to each pond in spring 1997 and autumn 1998.
  • The experiments are being monitored regularly to follow changes in the plant communities under different water regimes. Germination, establishment and reproduction of each species is recorded.
  • Some pilot studies on frog and invertebrate colonization of the wetlands have been undertaken (Attachment g and h)

III Development of methods

Methods to monitor and score plant germination, establishment and reproduction and seed bank development have been designed. The methods include:

  • Monthly scoring of germination, establishment, reproduction and plant abundance in quadrats on transects along planted seed bank strips (December 1997- March 2000). Scoring from March 1999 has been three-monthly.
  • Photo scores to assess changes in plant community and species dominance and functional groups across the whole of each wetland.
  • Seasonal sampling and germination of sediments to assess seed bank development in each wetland
  • Functional groups of aquatic plant determined by response to water presence and absence (Figure 8, Brock and Casanova 1997), plant presence and absence and abundance are used to interpret plant patterns.

IV Data collection and analysis from studies in the University of New England Experimental Wetlands, outdoor tank, glasshouse trials and field observations.

Data from the UNE Experimental Wetlands come from:

  • 30 months data from seed bank strips planted in spring 1997.
  • 24 months data from seed bank strips planted in autumn 1998.
  • 5 imposed water regimes

Data from experimental trials in outdoor tanks to assess the affect of water regime and grazing on plant growth and establishment from seed banks were collected in Spring 1997 and Autumn 1998 and 1999.

Glasshouse experiments were conducted to assess the development of seed banks in the UNE Experimental Wetlands by germination.

Field records of water levels (Figure 1) and plant species presence and abundance have been recorded for local New England lagoons on a seasonal basis. These have been used for comparison with the experimental wetlands. Observations from other wetland types and in other systems have been made where possible.

Data were analysed using univariate (ANOVA) and multivariate analysis (PATN) to explore trends within and between water regimes using abundance and species presence data. Functional group patterns (see Brock and Casanova 1997and Figure 8) have been explored to assess groups of plants with different modes of growth and reproduction.

c) Changes, developments, constraints, and limitations

  1. Initially this project was designed as a 5 year project. As the Wetland R & D program funded projects for two years our objectives were trimmed to meet this constraint. The major effects of this have been:
    1. Testing of grazing in interaction with water regime has been limited to a tank trial in which clipping has been used to simulate grazing. It must be recognized that artificial clipping simulates only one aspect of grazing, so extrapolations and interpretations from this experiment are limited.
    2. Attempts to link the outcomes of this grazing trial with other grazing field projects have been explored but did not turn out to be feasible within the time frame and human resources of this project.
  2. The project was designed and initially implemented with a physiological as well as ecological approach to testing plant response to water regime and grazing to incorporate the specialist skills of the Post Doctoral Fellow, Dr Nigel Warwick, who is a plant physiologist. Dr Warwick stayed with the project for the first year only. The project was then redesigned to emphasize ecological outcomes using the ecological skills developed within the team. This has meant that:
    1. ecological rather than physiological answers to questions are provided, e.g. more emphasis on interpretation of trends through plant ecological types and functional groups and less emphasis on interpretations of biomass allocation.
    2. we have designed the opportunity to follow the development of viable seed banks as an indicator of wetland sustainability in different water regimes. This was not proposed in the initial project. We use this as a basis for modelling and predicting wetland plant community development (see Objective 5)
  3. It is unrealistic to consider aquatic plants response to water regime in isolation of other biotic components of the ecosystem . Hence we have initiated pilot studies to examine the influence of water regime on frogs and invertebrates.