(aleman grass, water hyacinth, olive hymenachne and

Assessing the potential for controlling four invasive aquatic plant species (aleman grass, water hyacinth, olive hymenachne and para grass) using seawater in northern Australian coastal wetlands Benjamin Reid1, Tony Grice2, Joseph Holtum1, Mike Nicholas2, Jim Wallace2,3, and Nathan Waltham3 1

James Cook University, School of Environment and Marine Sciences, Townsville, QLD, 4814 CSIRO Land and Water Flagship, Australian Tropical Sciences and Innovation Precinct, Townsville, QLD, 4814 3 TropWATER, Freshwater Ecology Research Group, James Cook University, Townsville, QLD, 4814 2

Report No. 18/31 August 2018

Assessing the potential for controlling four invasive species (aleman grass, water hyacinth, olive hymenachne and para grass) using seawater in northern Australian coastal wetlands

Report No. 18/31 October 2018

Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) James Cook University Townsville Phone : (07) 4781 4262 Email: [email protected] Web: www.jcu.edu.au/tropwater/

CSIRO Land and Water Flagship, Australian Tropical Sciences and Innovation Precinct, Townsville, QLD, 4814

Information should be cited as: Reid, B., Grice, T., Holtum, J., Nicholas, M., Wallace, J., Waltham, N. 2018, ‘Assessing the potential for controlling four invasive species (aleman grass, water hyacinth, olive hymenachne and para grass) using seawater in northern Australian coastal wetlands’ Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) Publication 18/31, James Cook University, Townsville and CSIRO Australia, 21 pp.

For further information contact: Dr Nathan Waltham Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) James Cook University [email protected]

This publication has been compiled by the Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER), James Cook University. © James Cook University, 2018. Except as permitted by the Copyright Act 1968, no part of the work may in any form or by any electronic, mechanical, photocopying, recording, or any other means be reproduced, stored in a retrieval system or be broadcast or transmitted without the prior written permission of TropWATER. The information contained herein is subject to change without notice. The copyright owner shall not be liable for technical or other errors or omissions contained herein. The reader/user accepts all risks and responsibility for losses, damages, costs and other consequences resulting directly or indirectly from using this information. Enquiries about reproduction, including downloading or printing the web version, should be directed to [email protected]

Acknowledgments:

We thank James Cook University students Jade Gould and Melanie Hammond for their assistance in the setting up of the experiment and data collection respectively. We would like acknowledge the Traditional Owners of Mungalla Wetlands, the Nywaigi people and pay respect to their elders past and present. This study has been part of CSIRO’s Restoring Nywaigi Country and Mungalla Wetlands project.

Controlling freshwater invasive wetland plants using seawater – TropWATER Report no. 18/31

EXECUTIVE SUMMARY Natural coastal wetlands of northern Australia are heavily influenced by tides, rainfall and seasonal changes in stream flow. Anthropogenic tidal barriers have significantly altered these influences in many wetlands and this has facilitated invasion by non-native freshwater species such as aleman grass (Echinochloa polystachya), water hyacinth (Eichhornia crassipes), olive hymenachne (Hymenachne amplexicaulis) and para grass (Urochloa mutica). Removing these tidal barriers can reinstate tidal flows, which may help manage the reduction in freshwater species if they are susceptible to saline water. To determine this for the above species each was exposed to a range of treatments of salt water (30%, 50%, 70% and 100% seawater and a freshwater control) for 22 days. For comparison, salinity treatments were also applied to a common native freshwater sedge (Cyperus platystylis). In each treatment the effect of salinity on each species was assessed using measurements photosynthesis, leaf number, relative growth rate and mortality. All of the species examined were negatively affected by exposure to salt water, but the effect varied between species; para grass could tolerate up to 70% seawater for as long as 22 days, while water hyacinth could not tolerate more than 3 days in 30% seawater. The three other species studied (aleman grass, hymenachne and the freshwater sedge) had intermediate salinity tolerences. Information on these species’ tolerances of salinity can help guide management decisions involving the removal of tidal barriers for freshwater weed control or other purposes.

Page i


TABLE OF CONTENTS EXECUTIVE SUMMARY .............................................................................................................................. i 1 INTRODUCTION ................................................................................................................................... 3 2 METHODOLOGY .................................................................................................................................. 4 2.1 2.2 2.3 2.4

Plant Specimens........................................................................................................................... 4 Salinity Treatments ...................................................................................................................... 4 Measurement of Photosynthesis.................................................................................................. 5 Growth Measurements ................................................................................................................ 6

3 RESULTS .............................................................................................................................................. 6 3.1 Photosynthetic response to light .................................................................................................. 6 3.2 Impact of salinity on photosynthesis ............................................................................................ 8 3.2 Plant Growth and survival ............................................................................................................ 9 4 DISCUSSION ...................................................................................................................................... 12 4.1 4.2 4.2 4.3

Salt tolerance ............................................................................................................................. 12 Limitations of the Study ............................................................................................................. 13 Practical Implications and Recommendations ............................................................................ 14 Conclusion ................................................................................................................................. 15

5 REFERENCES ...................................................................................................................................... 16

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1

INTRODUCTION

Coastal wetlands of northern Australia are ecologically, socially and economically important ecosystems. They are valued for their roles in hydrological function and effects on water quality, providing habitat for juvenile fish, recreational opportunities and the biodiversity that they support (Grice et al., 2008; Barbier et al., 2011, Arthington et al., 2015. Waltham and Fixler, 2017). Many coastal wetlands naturally experience fluctuations in salinity under the influence of tides, rainfall and seasonally varying stream flows. Anthropogenic influences have altered hydrological flows, water quality and salinity regimes (Boys et al., 2012; Sheaves et al., 2014). In particular, in many locations, construction of tidal barriers has created freshwater wetlands on their landward side and led to a shift from salt tolerant native species to freshwater aquatic invasive species (Scott and Sindel, 2000). Invasive species include aleman grass (Echinochloa polystachya (Kunth) Roberty); Olive hymenachne (Hymenachne amplexicaulis (Rudge) Nees); para grass (Urochloa mutica (Forssk.) TQ Nguyen, syn Brachiaria mutica); salvinia (Salvinia molesta D.S.Mitch); and water hyacinth (Eichhornia crassipes (Mart.) Solms). Three of these are Weeds of National Significance; H. amplexicaulis, S. molesta and E. crassipes (Thorp and Lynch 2001). Invasive species, in particular the exotic grasses such as aleman grass, hymenachne and para grass, can drastically alter the structure, composition and function of wetlands and riparian zones (Douglas and O'Connor, 2003; Humphries et al., 1991). Each was introduced to Australia as pastures for cattle but has naturalised and in some cases become dominant over native species (Grice et al., 2010). Para grass grows in a variety of habitats, both wetland and riparian, where it often dominates and reduces species richness (Douglas et al., 2001; Douglas et al., 2006). While it thrives in seasonally inundated wetlands, aleman grass and hymenachne can occupy deeper water bodies (Grice et al., 2010). Water hyacinth was introduced to Australia as an aquatic ornamental plant; it also has naturalised to dominate bodies of freshwater (DAFF, 2013b). The replacement of native aquatic plants by these freshwater weeds has serious implications for the conservation of wetland biodiversity (Houston and Duivenvoorden, 2003). Management of invasive plants can involve physical; chemical; and biological removal methods and integrated management using these and other techniques requires a sound understanding of the weed’s environmental requirements (Vitelli, 2000) and the complex and dynamic ecosystems that they invade (Grice et al., 2008). Most literature on salinity in Australian wetlands relates to the detrimental effects that increases in salinity can have (Bennett and Virtue, 2005; Hart et al., 2003; Nielsen et al., 2003a; Nielsen et al., 2003b), but few, if any, consider the potential benefits that salt water may have in freshwater weed control. This study provides useful information on the impact of different levels of salinity on four key weed species that have invaded many of the coastal wetlands in north Queensland. Tidal barriers, such as bund walls and levees, have been constructed on almost every coastal stream in the Bowling Green Bay Ramsar-listed coastal wetland (Carter et al., 2007; Sheaves et al., 2014). Bund walls are also present at the Cromarty wetlands at Wongaloo station, south of Townsville and the Mungalla wetlands, near Ingham. These wetlands have been highly impacted by freshwater weed invasion, but there are opportunities for cost-effective actions (e.g. bund breaching and/or removal) that reinstate natural hydrological processes and restore ecological function (Sheaves et al., 2014). This may help control saltsensitive invasive freshwater plants and there are also other positive consequences including improving fish passageways, restoring seasonality of wetlands and the inland encroachment of mangroves leading to protection from storm surge. Plants exposed to salinity are stressed in two ways. The high external salt concentrations (in soil or water) make it difficult for the roots to extract water, and high internal salt concentrations can be toxic (Munns and Tester, 2008). The most immediate effect of salinity on plants is via the plant’s stomatal response, which reduces transpiration and leaf photosynthesis (Munns and Tester, 2008). A plant’s photosynthetic response to salinity is similar to its response to drought (Chaves et al., 2009). As salt stress can inhibit photosynthesis, elevated salinity can reduce the growth of plants, or even result in death (Sudhir and Murthy, 2004). This is caused by the inability of the plant to extract water from the external environment and/or by the toxic effects Page 3

Controlling freshwater invasive wetland plants using seawater – TropWATER Report no. 18/31 of salt on the plant’s cells (Greenway and Munns, 1980; James et al., 2003). There are three mechanisms whereby plants cope with salinity; tolerance; acclimatisation; and avoidance (James et al., 2003). This study focused on assessing the tolerance of four invasive species of northern Australian coastal wetlands. Further studies on a longer time scale would be required to determine if the plants acclimatise to changes in salinity. It is well documented that many Australian native freshwater aquatic plants are salt-sensitive, with water with as little as 4% to 7% seawater (electrical conductivity (EC) of 2000-4000 µS cm–1) potentially lethal in some of the more sensitive species (Hart et al., 1990, 1991; Hart et al., 2003; James and Hart, 1993). The leaves, stems and roots of submerged and semi-submerged aquatic plants come into direct contact with saline water (James et al., 2003). There have been few studies conducted on the salt tolerance of invasive plants in Australia. Information is based mainly on visual field observations. Hymenachne shows signs of stress in brackish wetlands and its abundance declines sharply with increasing salinity (Wearne et al., 2010). The growth of aleman grass and para grass is also inhibited by saltwater intrusion. An early study on water hyacinth concluded that a salinity level of 6-8% of seawater can be lethal (Muramoto and Oki, 1988). This study aimed to identify the impact of salinity on each of these four invasive freshwater species. The native freshwater sedge (Cyperus platystylis (R.Br.)), one of many naturally occurring species at Mungalla Wetlands, Ingham, was also included in the study for comparison. This study provides insights relevant to the management of coastal wetlands of northern Australia where restoration of natural tidal flows is an option.

2

METHODOLOGY

2.1

Plant specimens

Aleman grass (E. polystachya) and olive hymenachne (H. amplexicaulis) were collected from freshwater at Mungalla wetlands (18o43’S, 146o15’E, 5 m above sea level) on 29 January 2014. Para grass (U. mutica) and freshwater sedge (C. platystylis) were collected from freshwater in the Ross River (19o18’S, 146o46’E, 14 m above sea level) on 6 February 2014. Water hyacinth (E. crassipes) was collected on 9 March 2014 from freshwater in the Ross River (19o19’S, 146o44’E, 17 m above sea level). Entire plants were collected facilitating quick establishment. The grasses and the sedge were potted within a day of collection and placed in a ventilated shade house. Pots (190 mm x 195 mm) were lined with shade cloth and filled with coarse sand. Each plant was fertilised with 5 ml of all-purpose slow release fertiliser (Scotts Osmocote, Bella Vista, NSW, AUS) and 400 ml of all-purpose soluble fertiliser (Yates Thrive, Padstow, NSW, AUS). Five plants of each species were randomly allocated to each of five metal trays (1800 mm x 90 mm) that contained freshwater to a depth of 120 mm. Hence, a total of 125 plants were used in this study. The study was completed in a greenhouse at James Cook University to control ambient air temperatures, with natural sunlight following a daily 12:12h day/night cycle.

2.2

Salinity treatments

Salinity treatments were based on a percentage of seawater (Table 1). Seawater has an electrical conductivity (EC) of ≈ 55,000 µS cm–1 (appslabs.com.au/salinity.htm, accessed 23 May 2014). Seawater was collected from the ocean at the Australian Institute of Marine Science Headquarters, Townsville (via James Cook University’s Marine & Aquaculture Research Facilities Unit) with EC levels varying from 51 000 to 58 000 µS cm–1. Freshwater used in establishing the range of EC levels was sourced from Townsville’s main domestic water supply.

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Controlling freshwater invasive wetland plants using seawater – TropWATER Report no. 18/31 Table 2.1. Salinity treatments applied to five replicates of E. polystachya, H. amplexicaulis, U. mutica, C. platystylis and E. crassipes in each tray. Water levels were maintained at 120 mm and EC levels were kept within ± 10 000 µS cm–1 of their allocated treatment for the duration of the study Salinity treatment (% of seawater) 0 30 50 70 100

Electrical conductivity (EC) (µS cm–1) < 800 ≈ 16 000 ≈ 27 500 ≈ 38 500 ≈ 55 000

Three days after collection of E. crassipes, the freshwater in each tray was drained, algal growth was manually removed and salinity treatments were applied. Salinity, conductivity and temperature of the water in each tray were recorded periodically throughout the study using a YSI handheld meter (Model 30/10, Yellow Springs, OH, USA). The EC levels of the water in each tray were kept relatively stable within ± 10 000

µS cm–1 of the levels set for each treatment (Figure 2.1).

100 90 80 70

40

60 50 40 20

30 20

Salinity (% seawater)

Electrical Conductivity (mS cm-1)

110 60

10 0

0 0

2

4

6

8

10

12

14

16

18

20

22

Days after treatment Figure 2.1.

2.3

Electrical conductivity (EC) levels in relation to percent of seawater for each treatment for the duration of the study. (○) Control; (■) 30% of seawater; (▲) 50% of seawater; (♦) 70% of seawater; and (●) 100% of seawater. EC levels for each tray were kept within ± 10 000 µS cm–1 of their allocated treatment.

Measurement of photosynthesis

Photosynthesis was measured using an infrared gas analyser (Li-Cor LI-6400, Lincoln, NE, USA) at a block temperature of 35oC, flow rate of 500 μmol s-1 and reference CO2 level of 400 µmol. Before the salinity treatments were applied the relationship between rate of photosynthesis (A) and the level of photosynthetically active radiation (PAR) was determined for each species. Measurements of PAR were made on the Li-cor meter for each species measured from zero to 1800 (μmol m-2 s-1). The different light levels were generated using natural ambient available sunlight in the greenhouse. Photosynthesis measurements were made on the day before treatments were imposed (day -1) and on nine further occasions (days 1, 3, 7, 9, 11, 13, 15, 20 and 22). On each day a total of 10 photosynthetic rate measurements were made on each species; using two plants (one leaf per plant) per species, per tray. Leaves chosen were the healthiest green leaf with a width of > 60 mm. Where a leaf was not wide enough to fill the 60 mm x 60 mm sampling area on the Li-Cor (often the case with para grass and the freshwater sedge), the Page 5

Controlling freshwater invasive wetland plants using seawater – TropWATER Report no. 18/31 leaf was measured and area was calculated using photography and ImageJ (NIH, Bethesda, MD, USA). Photosynthesis rates per unit leaf area were calculated using the computer program LI6400 Simulator (Lincoln, NE, USA).

2.4

Growth measurements

Measures of plant size and indices of biomass were used to derive growth measurements. For the grasses and the sedge, the number of green leaves on the central tiller of each plant was recorded on the day treatments were imposed (day 0) and subsequently every seven days for 21 days. At the same times, leaf expansion of a young leaf (marked on day 0) on the same tiller was quantified (length from point of attachment to the stem to tip of marked leaf). For water hyacinth, the total number of green leaves was recorded.

2.5

Data analysis

The relationship between photosynthesis (A) and photosynthetically active radiation (PAR) was examined using non-rectangular hyperbolae (Thornley and Johnson, 2000); 𝐴=

𝛼𝐴𝑚𝑎𝑥 𝑃𝐴𝑅 𝛼𝑃𝐴𝑅+𝐴𝑚𝑎𝑥

− 𝑅𝑑

(1)

where α is rate of change of A with PAR at low light levels (also known as the photosynthetic efficiency), Amax is the maximum rate of photosynthesis and Rd is the dark respiration rate. Separate hyperbolic functions were fitted to the mean A and PAR data for each species using an iterative least squares fitting procedure. A repeated measures ANOVA was performed to test for the effects of the different saltwater treatments (x) on rates of photosynthesis (y) using SPSS statistical software (IBM, St Leonards, NSW, AUS). The rate of photosynthesis averaged across the 22 days of the experiment was used as a fixed factor in the analysis. A post-hoc paired sample t-test was used to compare the average, and the variance, rates of photosynthesis over time for each treatment. This was repeated for each species examined here. Samples from days -1, 3 and 7 were used in separate repeated measures ANOVAs to compare the average rate of photosynthesis (y) across all treatments (x). A post-hoc paired sample t-test was used to compare the average, and variance, rates of photosynthesis per day between treatments. This was completed three times for each sample day (days -1, 3 and 7) and repeated for each species. For each species a paired sample t-test was used to compare leaf expansion (length) between days 0, 7, 14 and 21 for each treatment.

3

RESULTS

3.1

Photosynthetic response to light

The photosynthesis v light curves for the five species examined are shown in Figure 3.1. The nonrectangular hyperbolae used (equation 1) gave good fits to the data (r2 ranging from 0.927 to 0.999; Table 2.1). Three species, aleman grass, para grass and water hyacinth, had similar response curves with the highest maximum photosynthesis, Amax, and photosynthetic efficiency, α (Table 2.1). These species did not reach their light saturation values (Amax) even when PAR was ~ 1800 μmol m-2 s-1 and this is characteristic of plants with a C4 photosynthetic pathway. In contrast, olive hymenachne and the freshwater sedge had significantly lower rates of photosynthesis (A) for any given light level and there was little increase in A once PAR was greater than ~ 1000 μmol m-2 s-1. This kind of ‘light saturation’ is associated with plants with a C3 photosynthetic pathway. These measurements Page 6


demonstrate that the three C4 species (aleman grass, para grass and water hyacinth) have a higher potential for growth than the two C3 species (olive hymenachne and the freshwater sedge).

Figure 3.1.

Rate of photosynthesis (A) for each species (aleman grass, hymenachne, para grass, sedge and water hyacinth when exposed to varying levels of photosynthetically active radiation (PAR). The regression statistics for the fitted curves are given in Table 3.1

E.polystachya

H.amplexicaulis

U. mu ca

C. platystylis

E. crassipes

35

A (μmol CO2 m-2 s-1)

30 25 20 15 10 5 0 -5 0

400

800

1200

PAR (μmol

Table 3.1.

1600

2000

m-2 s-1)

Parameters derived from the non-rectangular hyperbolae (Equation 1) fitted to the photosynthesis (A) v PAR data for each species (see Figure 3.1). Also given are the correlation coefficient (r2) for each species and its photosynthetic pathway Maximum Dark respiration Photosynthetic photosynthetic correlation Photosynthetic Species rate, R d efficiency rate coefficient pathway r2 Rd α A max (μmol CO 2 m2 s-1)

(μmol CO 2 m2 s -1)

Aleman grass (Echinochloa polystachya)

2.80

0.075

43.7

0.999

C4

Olive hymenachne (Hymenachne amplexicaulis)

1.72

0.060

19.2

0.948

C3

Para grass (Urochloa mutica)

2.60

0.091

39.2

0.946

C4

Freshwater sedge (Cyperus platystylis)

0.80

0.084

25.3

0.927

C3

Water hyacinth (Eichhornia crassipes)

2.40

0.091

40.8

0.948

C4

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3.2

Impact of salinity on photosynthesis

The effect of exposure to saline water on each species is shown in Figure 3.2. Initially there were no significant differences between treatments one day before treatments were applied (p > 0.05) (C. platystylis p = 0.08). In freshwater (Figure 3.2 a), photosynthesis rates declined with time in all species, by between 25 and 40% over the 22 day study. This is simply due to the leaves ageing, a common feature of tropical pasture plants where after full expansion the photosynthetic rate steadily declines (e.g. see Ludlow and Wilson 1971). However, increasing salinity eventually reduced photosynthesis in all plants (compared to the freshwater control), but the size of the effect differed between species. As early as 3 days after treatments were applied, all species showed significant differences between salinity treatments (p ≤ 0.001 for E. polystachya, H. amplexicaulis, U. mutica and E. crassipes; p = 0.039 for C. platystylis). On day 3 there were also significant differences between some treatments and controls in rates of photosynthesis of both E. polystachya (p = 0.062, t = 10.3, n = 2) and E. crassipes (p = 0.064, t = 9.89, n = 2). For each salinity treatment, E. crassipes was the first species to show signs of intolerance to salinity and was the first species to have a zero photosynthetic rate after only three days in the 100% seawater treatment and after seven days in salinity treatments of 30% seawater or above. It took 7 days before H. amplexicaulis showed a significant difference between the 30% treatment and the freshwater control (p = 0.004, t = 147, n = 2), and in 50% (or above) seawater photosynthesis was zero after 7 days exposure. C. platystylis did not show any significant differences between treatments and controls up to 7 days post treatment (0.05 < p < 0.06, 10.5 < t < 12.2, n = 2), but in 50% seawater it had the greatest decline in photosynthetic between day -1 (27.2 μmol CO2 m-2 s-1) and day 22 (7.7 μmol CO2 m-2 s-1). Overall. C. platystylis had similar photosynthetic rates to E. crassipes, indicating its high sensitivity to salinity. After its initial decline in 30% seawater the photosynthetic rates E. polystachya were maintained in 30% and 50% seawater, and it was only in 70 – 10% seawater that photosynthesis eventually diminished to zero (~ 11 days after treatment). U. mutica also maintained a relatively steady rate of photosynthesis after day 11 in the 30%, 50% and 70% seawater treatments, and is the only species that tolerated long exposure in the 70% seawater treatment. In fact, U. mutica only showed a significant difference between the control and 100% seawater (p = 0.011, t = 68.2, n = 2).

Page 8


E. polystachya 50

50

40

40

H. amplexicaulis

U. mutica

C. Platystylis

E. crassipes

(b)

(a) 30

30 20

50

20 5010

-10

40 50


0

(e)

( )

0

(c) 40 30

-2

30 0

2

4

6

8

5020 20

40

(d)

30

10 20

(e)

10 4010



10

(e)

12

20

18

20

22

24

0 -1-1 11

40

16

10

0

30 050 -1

14

1

3

5

7

9

11 13 15 17 (e) 19 21

33

55

7

99

11 11 13 13 15 15 1717 1919 2121

Days af ter treatment


30

20

10 10

0 0 11 13 13 15 15 17 -1 -11 1 3 3 5 5 7 7 9 9 11 17 19 19 21 21


Figure 3.2.

3.2

Rates of photosynthesis (A) over time for five species (E. polystachya, H. amplexicaulis, U. mutica, C. platystylis and E. crassipes) in the five treatments; (a) freshwater control; (b) 30% seawater; (c) 50% seawater; (d) 70% seawater; and (e) 100% seawater. The value in parenthesis is considered an outlier

Plant growth and survival

The number of green leaves on control plants increased for all grass species and the sedge (Figure 3.3). The leaf count of E. crassipes peaked at day 7, but decreased thereafter. E. crassipes is the only species to show a decline in leaf number in the 30% seawater treatment under which all other species remained stable in relation to the control. E. polystachya and U. mutica maintained the highest leaf count in the 50% seawater treatment. U. mutica continued to display green leaves in the 70% treatment under which all other species had no green leaves from day 14. Interestingly, H. amplexicaulis and U. mutica displayed a few green leaves in the 100% seawater treatment until day 21.

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E. polystachya 50

H. amplexicaulis

U. mutica

C. Platystylis

E. crassipes

12

(b)

(a)

10

40

8

50 6

30

(e)

4

20

40


2 12 0

10

10

0

A (μmol CO2 m-2 s-1) Number of green leaves

12

(c)

10

8

8

50

-10

-2

40

30

20

10

66

0

2

4

6

8

30

(d) E. polystachya 20

10

(e)

4

H. amplexicaulis 14U. mutica 16 18 C. platystylis

12 10

42

22

24

E. crassipes

0

0

2 12 10

20

-10

1

3

5

77

9

21 11 13 1415 17 19 21


(e)

0

8 6

4 2

0

0 -1 01

3

5

77 9

11

13 14 15

17

19

2121

Days af ter treatment Figure 3.3.

Green leaf count for each species (E. polystachya, H. amplexicaulis, U. mutica, C. Platystylis and E. crassipes) over time post treatment, per treatment. (a) freshwater control; (b) 30% seawater; (c) 50% seawater; (d) 70% seawater; and (e) 100% seawater.

Measurements of relative leaf expansion showed a significant increase in leaf expansion for E. polystachya (p = 0.004, t = 5.15, n = 6), H. amplexicaulis (p < 0.001, t = 12.17, n = 6), U. mutica (p = 0.002, t = 5.607, n = 6) and C. platystylis (p < 0.001, t = 8.49, n = 6) after 14 days in the control treatment (Figure 3.4). After 7 days of applying treatments, leaf expansion slowed (compared to the control) in all of the species and was lowest in the highest salinity treatment. Both U. mutica and C. platystylis showed significant expansion in the 30% treatment after 7 days (p < 0.01, t < 4.1, n = 6) and C. platystylis showed further expansion after another 7 days (p = 0.031, t = 2.98 n = 6). Both U. mutica and C. platystylis showed significant expansion in the 30% treatment after 7 days (p < 0.01, t < 4.1, n = 6) and C. platystylis showed further expansion after another 7 days (p = 0.031, t = 2.98 n = 6). There was an anomalously low control leaf expansion rate for H. amplexicaulis after 7 days, but the control rate recovered on days 24 and 21. In this species leaf expansion increased steadily in the 30% seawater treatment, but was very low in the higher salinity treatments. Page 10


U. mutica had an even greater relative leaf expansion in the 50% seawater treatment than in the 30% seawater treatment. This is the only case where a species performed better in a higher level of salinity. All the other species showed very little expansion in the 50% seawater treatment after 7 days (p > 0.1). However, there was significant expansion in E. polystachya (p = 0.026, t = 3.14, n = 6) and U. mutica (p = 0.004, t = 5.12, n = 6) after 14 days. U. mutica was the only species to show significant leaf expansion after 14 days in 70% seawater treatment (p = 0.019, t = 3.41, n = 6). However, leaf expansion ceased at higher salinity levels. There was no significant leaf expansion for any species in the 100% seawater treatment (p > 0.05). Control

Relative Leaf Expansion (%)

Control 5045 40

50% of seawater

30% of seawater

(a)

45 40 Control 35 40 30 45 30 35 20 40 25 30 10 35 20 25 015 30 2010 20 25 15 5 (c) 2015 10 0 1510 0 5 10 50 5 0 0 0 0 0

Relative Leaf Expansion (%) Relative Leaf Expansion (%)

30% of seawater

50% of seawater 30

25 50% of seawater 20

30% of seawater

70% of seawater

100% of seawater

70% of seawater

100% of seawater

(b) 70% of seawater

100% of seawater

15 10 5 0 140 120

(d)

100 80

7

14

21

60treatment Days af ter 40 7

7

14 7

20 Days af0ter treatment 0 7 21 14

14

21

14

21

21

Days af ter treatment Figure 3.4.

Relative leaf expansion to length at Day 0 for each treatment (control, 30% of seawater, 50% of seawater, 70% of seawater and 100% of seawater) per species. (a) E. polystachya; (b) H. amplexicaulis; (c) U. mutica; and (d) C. Platystylis. y = T2 / T1 * 100 - 100

Mortality was defined by the absence of green tissue (Table 2). E. crassipes was the first species to show signs of senescence, with 17% mortality after 14 days in 30% seawater. C. platystylis was the next most sensitive species, but required > 14 days exposure in 50% seawater. C. platystylis was the first species to lose 100% of plants after 21 days. Mortality was high for E. polystachya only in the 100% seawater treatment from 14 days and there was no mortality in treatments with 70%) without being completely killed. However, their growth rates are reduced in salt water and this may hinder their competitiveness with other halophytic species. It should also be noted that para grass and aleman grass are C4 plants, who’s photosynthetic and growth rates are higher (at similar light levels) than those in C3 species, such as hymenachne and freshwater sedge. This will also influence inter-species competition. Salt water can be re-introduced into coastal wetlands either by the removal of anthropogenic tidal barriers or by the pumping of saline groundwater. This has a number of benefits, one being control of some invasive freshwater species. However, it is important to understand all environmental tolerances of these species prior to taking action. Acknowledging the wetlands as a dynamic and complex ecosystem with an abundance of invasive and native species is the first step towards holistic, adaptive and integrated management. This approach has the potential to guide successful ecological restoration of coastal wetlands of northern Australia. Page 15


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