Impacts of Common Reed & its management

Preparation:

Before beginning this activity, make copies on transparency sheets of the grid overlay chart—one per student group.

Engage:

Before beginning this activity, ask students to describe first cattails, then Phragmites, a.k.a. common reed.  Also ask where they’ve seen each of the plants before.  If they’ve done any driving along highways, they’ve very likely seen both in wetlands or drainage areas (though Phragmites tends to dominate these disturbed environments).  Confirm and solidify their thoughts of what cattail and common reed look like using the PowerPoint associated with this lesson.  Having students think of the image (or lack thereof) in their head first can help them engage more and look more closely for what they are unsure of when you show them the images.

Also show them the images of Iona Island and the some of the different birds that inhabit tidal marshes (and are referenced in the handout).

Explore:

Divide students into groups or pairs and distribute copies of the Iona Island maps. There are three maps: 1991, 1997, and 2005. Give each pair one map year, but give each student their own map and transparency. 

Ask students to classify the percentage of different vegetation.  Students should focus on only: Phragmites australis (common reed, pink) and Typha angustifolia (cattail, green).  There are other vegetation types but it is difficult to tell the difference between the shades of green, and the main changes take place with the above-mentioned plants.  

After students have calculated the % of cattail and Phragmites, have students share their results with each other.  Make a chart on the board and ask students to give you the answers they came up with to fill in the chart. 

Next, give the students the marsh bird data along with the information on the effects of Phragmites removal on marsh nutrient cycling.  Based on this information, students should be able to make some inferences about the benefits and drawbacks of removing the plant. 

Explain:

Common reed (Phragmites australis) as a species is native to both North America and Eurasia.  However, the plant evolved separately for a very long time, causing the North American type to have a very different physiology from the Eurasian type.  Together with genetic evidence, these differences have led scientists to consider the North American form the subspecies “americanus,” while the Eurasian type is considered to be the subspecies “australis.”

The native Phragmites australis var. americanus has quite nearly disappeared from much of its northeastern range, being confirmed in only one county in New York.  Phragmites australis var. australis (from here on, referred to simply as Phragmites or common reed) expands rapidly and as a result, plant and bird biodiversity decrease markedly in areas where it dominates.  The data tables show that at Iona marsh, most of the birds are red-winged blackbirds.  At the other marsh locations, a more diverse bird community flourishes. This disparity is likely in response to the severe decrease in plant diversity at Iona compared with cattail-dominated marshes, as well as the fact that large Phragmites stands tend to be poor habitat for most vertebrates.  Go over the photos of the plants and birds once again after they have completed the activity to engage students and help them visualize the community changes that have taken place at Iona.

            While removal of the plant has been shown to significantly improve native plant diversity, any management action can also have negative or unanticipated outcomes.  For example, the removal of Phragmites can decrease the denitrification capacity of the plot area substantially for the first couple of years before other plants re-vegetate the area.  Denitrification is the conversion of nitrate to nitrogen gas and occurs in poorly-oxygenated soils, such as those found in wetlands. This process is an important ecosystem service that wetlands provide us.  Ammonium concentrations in the sediment waters (porewater) are also found to increase after removal of the reed.  If too much ammonium builds up in the sediment, then the area could act as a ‘source’ rather than a ‘sink’ of N for the surrounding area.  However, two years after removal most of the restoration area had become re-vegetated with higher plant diversity, and both ammonium and denitrification potential had returned to pre-removal levels. 

            While some students may be able to reason through the implications of the nitrogen changes, without having experience with the nitrogen cycle, many will find this part of the lesson challenging.  However, even if students haven’t had a lot of experience with nutrient pollution, it is a good discussion point when talking about the range of possible impacts of removing an established invasive plant. 

Extend:

For homework, students should read the Hudsonia article by Erik Kiviat.  Once they have completed the reading and the class work, they should be able to come up with a good list of ideas of what else they would want to know before making a decision about whether common reed should be removed from Iona Marsh.  Students should draft a two-page (or more/less) essay answering the following question: Do you think we should spend money to remove common reed from Iona Marsh?  Why or why not?  (See worksheet for further prompts to this question.)

Ask advanced students to read the complete article by Findlay et al. (2003) or Erik Kiviat (2013, cited with links below).  Students can create discussion questions or debate the merits of this type of invasive plant removal. 

Evaluate:

Assess student understanding from their responses to the worksheet questions, paying particular attention to the synthesis essay (Part 4/ “Extend” portion of the activity).