Too Much of a Good Thing – Using Water to Control the Aquatic Invasive Yellow Flag Iris (Iris Pseudacorus L.)

Invasions of Iris pseudacorus L. (yellow flag iris) into wetland environments can result in changes to the functioning of the ecosystem. Field-based and greenhouse studies were initiated to study the effect of water depth 29 on regrowth rates of cut stems of yellow flag iris. The field-based experiment occurred at 41 independent 30 populations around the perimeter of a single wetland. The greenhouse experiment was conducted to further study 31 the effect of water depth and duration of submersion on rhizome mortality. In both studies, treatments were 32 compared against controls. In the field-study, yellow flag iris regrowth was significantly affected by water, though there was no relationship between water depth and percent regrowth. In the greenhouse study, there was a 34 significant positive relationship between duration of submersion and percent mortality of rhizomes. And, there was 35 no relationship between water depth and percent mortality; indicating that as little as 5 cm of water is sufficient to 36 kill yellow flag iris rhizomes, if the stems are cut to the base of the plant. Our results indicate a simple technique 37 can control yellow flag iris within aquatic ecosystems without the need for chemicals or laborious hand removal. We pose the questions: what depth of water is required and what is the optimal duration of treatment?


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Invasions of Iris pseudacorus L. (yellow flag iris) into wetland environments can result in changes to the 28 functioning of the ecosystem. Field-based and greenhouse studies were initiated to study the effect of water depth 29 on regrowth rates of cut stems of yellow flag iris. The field-based experiment occurred at 41 independent 30 populations around the perimeter of a single wetland. The greenhouse experiment was conducted to further study 31 the effect of water depth and duration of submersion on rhizome mortality. In both studies, treatments were 32 compared against controls. In the field-study, yellow flag iris regrowth was significantly affected by water, though 33 there was no relationship between water depth and percent regrowth. In the greenhouse study, there was a  (Lakela 1939;Preece 1964;Rubtzoff 1959) and can grow in water depths ranging from 0-100 cm (Preece 1964).

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While yellow flag iris is typically associated with sites with continuous high soil-water content, it can grow in dry, 51 sandy soils (Dykes, 1974in Sutherland 1990). Rhizomes placed indoors, without water, can continue to grow for 3 52 months (Sutherland 1990).

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Yellow flag iris tolerates a wide range of soil pH ranging from 3.6 to 7.7 (Unit of Collaborative Plant 54 Ecology, unpublished, in Sutherland 1990), but prefers high nutrient sites (Ellenberg 1979in Sutherland 1990).

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Once established, yellow flag iris is known to change the hydrology, and ecosystem complexity and functioning of 56 an area, reducing habitat suitability for native plant and animal species (Clark et al. 1998;Pathikonda et al. 2008; 57 Raven and Thomas 1970;Thomas 1980). The species has a very high carbohydrate storage capacity in the rhizomes 58 (Taylor unpublished in Sutherland 1990) and is able to quickly colonize from rhizome fragments. During peak 59 storage capacity, yellow flag iris rhizomes soluble carbohydrate values may be as high as 80% of the dry matter 60 (Hanhijarvi and Fagerstedt 1994). The high carbohydrate content may allow a single population to expand rapidly.

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It is known that gases diffuse through water 1000 times more slowly than through the atmosphere (Sairam 89 et al., 2008). Therefore, we hypothesize that water may act as a barrier to gas (acetaldehyde) diffusion and thus 90 water alone may result in yellow flag iris mortality. We expect a relationship between water depth and duration of 91 submersion and rhizome mortality; therefore, we hypothesis that a constant layer of water overtop cut yellow flag The field-based portion of the study occurred at Cheam Lake Wetlands Regional Park (49.1981, -121.7503)

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(Cheam), near Vancouver, British Columbia, Canada. Cheam is a 107 hectare wetland with a history of marl 98 dredging. In 1990, the area was designated a Regional park and Ducks Unlimited Canada installed a water-flow box 99 to control lake levels. Because the lake has been dredged, it is unsafe to venture into deeper water. Therefore, we 100 chose to drop the lake levels by 40 cm to expose the bases of the plants prior to initiating the experiment.

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September 1, 2017, the controls of the water-flow box were lowered approximately 3-4 cm per day for 14 days.

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We suspect that uneven ground surface and distance from rising water may have resulted in an uneven recharge rate 116 at the 39 sites. This uneven recharge rate would allow the treated yellow flag iris at some sites to remain exposed for 117 up to 10 days (the recharge time for the system) longer than other sites. During this time, the plant could recover 118 and form a leaf. In order to control water depth treatment, we conducted a greenhouse study.

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Results from the greenhouse study provided some excellent insight into water treatment depth and duration. Again,

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there was no effect of water depth on mortality (P=0.23) nor was there an interaction between water depth and days 149 of treatment (P=0.49); therefore, all depth treatments were combined and analyzed by days of treatment (P<0.0001).

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We found that while significant mortality started to occur at 105 days of treatment (69% + 24%), 230 days of 151 treatment were required to attain 100% mortality (Figure 3). At 230 days of treatment the Control plants exhibited 152 0% mortality.

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The nature of anoxia/hypoxia tolerance in aquatic plants is complex and species specific (Loreti and Perata 2020).
Regardless of the mechanism(s) of survival; for a species to be considered anoxia/hypoxia-tolerant it is not 156 necessary that every organ or tissue survive the oxygen deprivation. All that is required is the survival of the 157 essential organs that support regrowth for survival once plants are returned to favourable conditions. Roots are 158 relatively sensitive to oxygen deprivation, and the survival capacity of most tolerant species resides in the shoots or

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The advantage in the latter adaptation is that plants seem to outlast anoxia by reducing metabolic activity. However, 170 reduced metabolic activity cannot be sustained indefinitely; and thus, carbohydrate starvation has long been 171 regarded as one of the main causes of cell death under anoxia (Schlüter and Crawford 1982). The results from the 172 current study indicate that I. pseudacorus utilizes the second, quasi-dormancy, mechanism to survive anoxic 173 conditions. Ironically, the rhizomes of yellow flag iris hold more than enough carbohydrate reserves to produce a 174 leaf and reconnect with the atmosphere. Only 5 cm of surface water is required to prevent shoot growth, however 175 upwards of 240 days is required to completely kill the rhizomes through carbohydrate starvation.

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For land managers using water as a control mechanism for yellow flag iris some key protocols must be 177 observed to ensure success. Often, yellow flag iris populations are a mixed condition where part of the population 178 is terrestrial and part is in deep water. In a mixed condition population, it is imperative that the terrestrial portion be   The relationship between days of water treatment (5,10, 15, 20 or 25 cm water depth) and percent rhizome mortality of cut yellow ag iris plants grown under greenhouse conditions (± 95% CI)