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 (continued)

e) Conclusions and summary of research (assessed against objectives)

The project has answered the aims in section a). We found that:

  • the types of plant communities that established and reproduced from the same seed bank differed with water regime (Objectives 1, 4, 5);
  • sustainable seed banks developed in all water regimes of the new wetlands after germination, establishment and reproduction from a donor seed bank source (Objectives 1,3,5);
  • sustainable communities, as indicated by development of a viable seed bank, developed under each water regime within two years (Objective 5);
  • Grazing in interaction with duration, depth and frequency of flooding influenced plant growth and reproduction (Objective 2).

Objective 1. Determine the effects of timing, duration, and depth of flooding on the growth of a range of wetland plants with differing reproductive and growth strategies

In the five imposed water regimes in the UNE Experimental wetlands water regime influenced plant community composition but not water quality (Figures 3-7). In autumn 2000 after 30 months of establishment between 62-92 species from a range of functional groups grew in each water regime. Water regime influences included:

  • Permanently flooded ponds had well developed submerged plant communities with a narrow band of amphibious species at shallow depths.
  • Spring fill ponds had both submerged species and good representation of amphibious-tolerator and amphibious-responder species.
  • Autumn fill and Twice annual fill ponds had no submerged species but had amphibious-tolerators, amphibious-responders and terrestrial species represented.
  • Llangothlin mimic ponds had a narrow band of submerged species and broad areas of amphibious species with some terrestrials.
  • Although water regime did not select functional groups differentially for establishment it may select which plants groups are reproductive.
  • The timing of planting donor seed banks is important for reproduction in the first year. Planting in spring allows most species to seed before winter.
  • Preliminary studies showed that frog species were more likely to be found calling in water regimes where ponds were filling than drying down and Hylid frog spawn was only found in Spring fill ponds as they were filling in spring. In autumn macroinvertebrate were richer in taxa where ponds had been flooded for longer.

Under experimental tank conditions the timing, depth and duration of flooding influenced plant growth and reproduction and community composition:

  • Whether plants are flooded or exposed (damp) is important for growth and reproduction but depth of flooding is not particularly important for most species.
  • Duration of individual flood events is important; shorter floods give higher species richness and biomass.
  • Many amphibious species tolerate or respond to flooding whereas terrestrial species cannot tolerate flooding and submerged species cannot tolerate drying.
  • For submerged aquatics, the duration of flooding needs to long enough for flowering and seed set whereas amphibious species can reproduce when damp or flooded.
  • the timing of flooding can be important for flowering and seed set time: spring germination gives time to set seed before winter whereas autumn germination does not.

Objective 2. Investigate the effect of grazing and the interaction of grazing with water regime on production of reproductive units by aquatic plants

Water regime and grazing influence affect plant germination, establishment and reproduction both separately and in combination:

  • Water regime influences species presence and absence and mode of reproduction: the number of species reproducing sexually is greater in damp not flooded conditions whereas species were just as likely to reproduce vegetatively when flooded as when damp.
  • When species are both grazed (simulated by clipping) and flooded, sexual reproduction does not occur whereas species clipped in damp conditions reproduce both sexually and vegetatively . Unclipped plants reproduce in both flooded and damp conditions.

Objective 3. Determine the effects of water regime on the allocation of plant biomass to different reproductive modes

Water regime and clipping can affect the allocation of biomass to reproductive parts:

  • Damp conditions without flooding or drying stimulated the highest biomass and species richness but invasive terrestrials dominated. Although flooded conditions produced the lowest biomass and species richness species were aquatic and native. Clipping in flooded conditions reduces species number and reproduction further .
  • Short frequent floods promoted high species richness and biomass of amphibious species under fluctuating water levels but did not favour terrestrial species.
  • Patterns vary between species. Clipping increased reproduction and biomass for some amphibious species under conditions that fluctuate from flooded to damp (Lythrum and Limosella), whereas clipping had no influence on other amphibious species which have highest reproduction in constantly damp conditions (Centipeda and Cyperus). In contrast, the submerged species Vallisneria required flooding for growth and reproduction and clipping reduced both biomass and reproduction.

Objective 4. Validation of selected response patterns by studying wetland plants in temporary wetlands under a range of natural flooding and drying regimes.

Submerged, amphibious and terrestrial species in natural flooding and drying regimes have been assessed to perform similarly to plants under similar simulated experimental conditions. Therefore we feel we can extrapolate from our experimental results to field conditions in suggested in Attachment m. Some generalizations for most wetlands include (see attachment m):

  • Shallow habitats with amphibious plants change as water levels rise and fall.
  • Many plant species tolerate or respond quickly to water level changes.
  • Permanently flooded habitats have submerged plants that don't tolerate drying.
  • Terrestrial edge habitats tend to have weedy plants which don't tolerate flooding.
  • A mosaic of habitats provides a diverse set of conditions for wetland biota.
  • Fast lowering or raising of water levels leaves edge habitats bare of aquatic plants
  • Steep sided wetlands won't have wide zones of aquatic plants stimulated by water level fluctuations.

Objective 5. Model the responses of the different reproductive types under different water and grazing regimes from an 18-30 month data set

Substantial species-rich seed banks with species from a full range of aquatic plant groups developed under a all five water regimes in the UNE Experimental wetlands within 18 months. This seed bank will develop in species richness with further time. We believe that the development of a species rich seed bank is a significant indicator of the development of a sustainable wetland.

Responses of plant types under different water regimes over time can be extrapolated to other systems. Collaborative work in other wetland systems in Australia and selected systems the USA indicates that the zone of water level fluctuation, the amphibious zone, is the major place in which selection takes place for species adapted to aquatic habitats. Hydrological regime is likely to be the major selective force.

Objective 6. Present findings in a way that can be used by water managers to aid in the planning of flooding and drying regimes in temporary wetlands

The major target audiences are wetland users and managers. Our communication activities have been both written and face to face. Our major outputs are the delivery of two booklets on wetland plant management:

  • Are there plants in your wetland? Revegetating wetlands" (Brock and Casanova 2000, Attachment l).
  • "Does your wetland flood and dry? Water regime and wetland plants". (Brock, Casanova and Berridge, in press due for publication September 2000, Draft as Attachment m).