Can you Predict How Fast Phragmites australis Will Spread in Salt Marshes?

Introduction
Thank you to student scientists
Background
Goals, Objectives and Hypotheses
Study Sites and Methods
Results and Discussion
An explanation of the term null hypothesis
Answers

Introduction

At a presentation at the 2nd Annual Coastal Science Conference, Dave Burdick explained his analysis of the salinity data that he and other professional scientists have been collecting so far. Many students conducting the salt marsh science study are collecting data on sites similar to the ones that are analyzed by Dave Burdick. By collecting data at various sites, we have opportunity to learn a great deal about the impact of salinity on the growth of Phragmites. Already the work that students are doing is helping the Massachusetts Audubon Society gain a better understanding of the connection between salinity, flooding, and the spread of Phragmites.

The ideas here refer to a presentation By Dr. Dave Burdick: "Soil Salinity Can Predict Invasion Rates of Phragmites australis in Salt Marshes."


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Thank you to Student Scientists

Perhaps you are one of the students participating in this study. If you are, please let me thank you on the behalf of the Massachusetts Audubon Society. We appreciate the work you are doing. Your ideas and work are helping us as we work to protect and restore local wetlands. As you collect data, please think about what the data mean, and let us know what patterns you see. You may make discoveries, or have ideas that nobody has had before.

An important part of science is analyzing data. By looking at data, and noticing patterns, we can begin to make accurate predictions for the future. Dr. Dave Burdick has begun analyzing the salinity data he and Massachusetts Audubon Scientists have been collecting. This page helps describe his current conclusions. Dr. Burdick's conclusions may be supported by the data you are collecting at your sites. On the other hand, your data may contrdict his hypotheses, and challenge us to create new, different hypotheses. As you read through this page, refer to the data you have collected. Notice whether your findings are similar or different. Think about his hypotheses and predictions, and decide whether you agree or disagree. Scientists do not always agree. Often challenges to hypotheses help researchers decide that to investigate in additional ways to find new information. Please send me your thoughts and analysis regarding your own data.

There are questions in green are throughout this page, whichy ask you to think about and to analyze the data you are collecting.


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Background

As many of you know, the conditions of some salt marshes have been degrading, or getting worse, over time, as people have filled them in to build homes, businesses, and recreational grounds. Roads and railroads built through salt marshes have had additional consequences, which harm the marshes. Roads block the flow of the tide, and funnel in additional freshwater as it runs off impervious surfaces such as pavement. Both the addition of soil to marshes, and the changes in the flow and input of salt and fresh water to the marsh has lead to rapid increases in the growth of Phragmites australis, the common reed.

This tall plant, Phragmites australis, blocks the sun as it overshadows other marsh species. Without the sunlight, other plants cannot compete with Phragmites, and it begins to dominate a marsh. Phragmites also outgrow other plants with long, deep roots. Phragmites roots can grow many feet deep, which may mean that it can reach fresh water sources. We think that Phragmites has a hard time growing where salinity levels are high. With deep roots, it may be able to avoid the salt, drawing on fresher water at another depth.

Phragmites has grown in New England for centuries. However, its recent rapid spread in this region is raising concern. Scientists believe that a more aggressive genotype is rapidly advancing. It is thought to be poor wildlife habitat, a fire hazard, and a visual block to beautiful scenery. It's spread is often a symptom of negative human impact.


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Goal, Objectives and Hypotheses

Several years ago, Dr. Robert Buchsbaum from the Massachusetts Audubon Society and Dave Burdick from the University of New Hampshire set out to see if they could use a few simple measures to help us understand why Phragmites was spreading rapidly in the Plum Island Sound region. Dr. Burdick's goals and objectives are quoted or paraphrased below.

Goal:

"To understand how salinity affects the growth of Phragmites australis." They decided to use small wells placed at three depths in patches of Phragmites.

Objectives:

1) Examine soil water salinity in shallow, moderate and deep soil depths within Phragmites patches and other vegetation types over the growing season.

2) To measure how fast Phragmites patches are spreading- (Spread rates).

3) Compare patterns of soil salinity to the spread rates of Phragmites patches.

Dr. Burdick and Dr. Buchsbaum's Hypotheses:

Hypotheses:

1) Salinity varies seasonally with fresher periods in the spring and early summer (due to runoff and spring rains) and saltier periods in the late summer and early fall.

2) Phragmites australis expansion can be predicted from average well salinity and amount of tidal flow.

3) Salinity data will allow us to predict whether Phragmites will spread rapidly to an adjacent area (station 5) where Phragmites is not currently growing.


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Study Sites and Methods

These scientists designed and used the same methods that students are now using in sites in their towns. This web-page will discuss three of the sites studied by the professional scientists. Students may compare these to the sites they are studying. One site, "Oak Knoll #1" is a "unrestricted site". This means there is no road or railroad blocking the flow of the tide at this site. The sites that students from the Pine Grove School in Rowley, Nock Middle School in Newburyport, and the Rockport Middle School in Rockport are studying are also unrestricted sites. "Mud Creek" is a restricted site, where the tidal marsh is degrading because Route 1A is blocking the flow of the tide. Many schools are also studying sites that are restricted. This includes sites in Salisbury, Ipswich, Essex, Gloucester, and Danvers. "Railroad" is a Phragmites stand found in a recently "restored" marsh. As the MBTA rail line expanded to Newburyport, they installed a new larger culvert, allowing the flow of the tide to be restored.

Unrestricted Sites: Oak Knoll #1 (MAS Rowley) Railroad Ave. (Pine Grove, Rowley) Joppa Flats (Newburyport) Long Beach (Rockport)*

Restricted Sites: Mud Creek (MAS Newbury/Rowley) Conomo Point (Essex**) Mill Creek, Gloucester Town Creek, Salisbury Town Farm Road,** Ipswich Jeffreys Neck Rd.** Ipswich Forest River, Salem

Restored Sites
Dredging, Fill removed, ditches and/or salt pannes built: Joppa Flats (Newburyport)   *May soon be restored.	Tidal restriction removed: "Railroad" (MAS Rowley) Argilla Road, (Ipswich) Long Wharf (Gloucester) *Sites above are going through the permitting process and may soon be restored.

Like the sites that students are studying, the five salinity stations within each site, were established in the following arrangement: Three of the stations were placed along a transect that was established from the center of a Phragmites stand to a salt marsh creek.

Station #1: The first station was placed in the middle of the stand where only Phragmites is growing .

Station #2: The second was placed in the transition zone at the edge of the stand where Phragmites grew with other salt marsh grasses.

Station #3: The third was placed at the center of the salt marsh grasses between the transition zone and the creek.

Station #4 is meant to be a duplicate of station 2, and was also placed in the transition zone.

Station # 5 was placed the same distance from the upland as station 1, but where no Phragmites was growing.

At each station, a set of three wells, drawing soil water from three depth intervals: 5-20 cm, 35-50 cm, and 65-80 cm, were installed.

Some questions that we want to answer are:

Why isn't Phragmites growing at well #5?

Will it spread there in the future?

Fig. 2 (click to enlarge)

Questions that we hope to answer as a result of our testing:

• Is salinity higher in the shallow, medium, or deep well?
• Does this change over time?
• Does this change seasonally?
• What is the range of salinities that Phragmites encounters during the growing season?

Phragmites, like other plants in this region, begins growing in the spring, sprouting from it's roots underground. It grows through the summer, and then becomes dormant in the late fall through winter, after killing frosts.

Well #1 shows salinity levels where Phragmites is densest, and has overtaken the marsh, out competing all other plants.

Well #2 and #4 show salinity levels where Phragmites is mixed with other plants.

Well #3 and #5 measures salinity where Phragmites is not yet growing.

We wonder: Will Phragmites spread to wells #3 and #5?

Will Phragmites continue to spread, and out compete all the other plants at wells #2 and #4?

Based on the measurements you make at wells #1,# 2, and #4, notice the conditions Phragmites is growing in already and make predictions.

Well #5 is nearer the upland, where Phragmites typically grows. Well #3 is closer to the estuary (tidal river), and is probably flooded more often by the tide. People are beginning to notice that it may be a combination of daily flooding as well as salinity that affects the growth of Phragmites. (Nock Middle School students measuring salinity at Joppa Flats discovered very low salinity. Phragmites, however does not grow where the Spartina alterniflora is growing, where it gets flooded by the tide on a daily basis for a long length of time. O'Maley Middle School students in Gloucester discovered high salinity levels at the Mill Pond (Over 20 ppt). However, Phragmites is growing there. A tide gate, which forms the Mill Pond prevents tidal flooding onto the marsh. Despite high salinity, there is low flooding, and Phragmites is growing tall.)

Notice the height of the Phragmites you are studying. Where it is short (as tall as you, or shorter), it typically means it is stressed. It seems to have a harder time growing where it is flooded often by high salinities.


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Methods

In Dr. Dave Burdick's study:

The scientists measured salinity 15 times during the spring, summer and fall in 1996 and 1997. They completed a vegetation transect in the fall of 1996, and the fall of 1998. Dave measured the size of the transition zone (where Phragmites and other vegetation are mixed.) He measured percent cover of each plant species by putting down a square that was 1 meter by one meter, and then estimating how much of the area was filled by each plant species. He also measured the height of five Phragmites in each of these square meters.

An explanation of Percent Cover:

Imagine we are looking straight down at a one meter area above. The dark green represents one species of salt marsh grass, the light green another. We tally the area to get our percent coverage.

1. What percent of the the square above does the dark green fill? What percent does the light green fill? Answer

Percent cover allows scientists to notice how well a particular plant species is growing in a particular area. When we record a plant is present along a transect line, it does not indicate whether there is one plant or a hundred plants. Percent cover begins to document those differences. Students have not been recording "percent cover" as part of this study but have been noting the presence or absence of plants at different intervals along the transect line. We are measuring the height of Phragmites, along the transect line, and can tell how big a transition zone is by looking at your vegetation transect data. The area where Phragmites grows along with other plants is the transition zone.

2. How big is your transition zone? Has that changed over time?


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Results and Discussion

In 1997, at the Coastal Science Conference, Dave Burdick reported that salinity at the sites differed and was fresher in the spring, saltier and more stressful for Phragmites in the summer and fall (see Figure 3_2).

This led them to accept the following hypothesis:

Salinity varies seasonally, with fresher periods in the spring and early summer and higher salinity levels during the summer and fall.

3. Why do you think this happens? Answer

Comparing three different sites, One restricted, Not restricted, and Restored, Dave set about to explore whether he could accurately predict where Phragmites will expand by looking at his data.

4. Where would you expect the greatest salinity levels? In an unrestricted (Oak Knoll), restricted (Mud Creek), or restored site (Railroad)? Answer

The scientists noticed that salinity levels affected the spread rate of Phragmites, as well as the height of the plants. (Figure 4).

At the unrestricted site with highest salinity, the average plant height decreased. This shows that the plants were stressed by high salinity levels. Also, the patches of Phragmites did not spread where there was high salinity. In the restricted site (Mud Creek) where there was low salinity, Phragmites spread rapidly - about 1 meter per year over two years. The average height and the density of the plants increased as well.

Explanation: in some areas (where it is unrestricted) Phragmites may not be spreading out into the marsh at all. Phragmites may be limited by the flood of the salty tide, and will not grow out further into the marsh toward well #3. (It may, however spread sideways toward well #5!) In areas where the tide is restricted, Phragmites may be spreading at a rapid rate of a meter (or more?) per year.

5. Is Phragmites spreading at your site? How fast is it spreading? If it isn't spreading along the transect line, out into the marsh, perhaps you might set up a transect line to notice if it is spreading sideways, along the upland.

At the restored site, (Railroad), Phragmites is still spreading, but at a slow rate (.1 meters per year) and the average height of the plants is declining. (Figure 4) The restoration of the site occurred at the beginning of the 1998 growing size. In the future, we expect the spread of Phragmites on this site may be reversed because of the increased culvert size that is partially restoring the tidal flow at this site.

Take a look at the average salinities at each site:

6. Do you think that Phragmites will spread to well #5 at these sites? Will it spread out toward well #3? Will it spread quickly (1 meter a year) or slowly (.1 meter a year)?

Our predictions:

Dr. Dave Burdick expects that at the Oak Knoll site, Phragmites will not be able to spread toWell #5, because of its high salinity levels. At the other sites, Mud Creek, and Railroad, Dave expects that Phragmites will spread to well #5, and continue to spread rapidly. He suggests that Mud Creek should receive strong consideration for tidal restoration because the site is degrading. He looks forward to collecting further data to see if the restored site will reverse the spread of Phragmites.

Liz Duff, Education coordinator:
I think that Dr. Burdick may be right that Phragmites will not spread quickly to well #5. It does have a high average salinity. However, I don't think that Phragmites is limited by salinity alone. I think that the growth of Phragmites is limited by both flooding and salinity levels. (I have seen Phragmites growing, in Danvers, in a stunted form at salinity levels of 40 ppt! ) If well #5 is closer to the upland, and therefore not getting flooded daily, I believe that Phragmites will be able to grow. My prediction is that Phragmites will spread to Well #5 at Oak Knoll even though it is high salinity. Like Wells #1, #2, and #4, it is closer to the upland, and not flooded as much as well 3. I think that Phragmites will not spread to Well #3, but will continue to spread gradually sideways along the upland toward Well #5.

7. What do you think will happen? Explain your answer. What do you predict will happen on your site?

Is Phragmites spreading on your site? Do you think it will spread further? Will it spread quickly? Is your site going to be restored? What do you think will happen? Why do you think that?

Over time, we will be able to see which of our predictions is right.

Dr. David Burdick's Results:

H1o*: Salinity is consistent seasonally - REJECT

H1a: Salinity varies seasonally, with fresher periods in the spring and early summer (due to runoff and spring rains) and saltier periods in the late summer and early fall .- ACCEPT

H2o: Phragmites australis expansion cannot be predicted from average well salinity.

H2a: Phragmites australis expansion can be predicted from average well salinity.

Preliminary data suggest H2a is better than H2o, but this hypothesis needs to be tested using more sites.

H2o: If salinity at station #5 is similar to salinity at stations #1 or #2, we cannot predict whether Phragmites can expand rapidly if it colonizes the site.

H2a: If salinity at station #5 is similar to salinity at stations #1 or #2, we can predict that Phragmites will expand rapidly if it colonizes the site.

An experiment using potted Phragmites collected from the original patch and planted near station #5 could be used to help answer this question.

*What does H1o mean? That means: Hypothesis number 1, o means "null" or zero empty, void.


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Null Hypothesis (Scientific Method Explained.)

Dr. Burdick, like many scientists, uses what is called a "null hypothesis". Instead of trying to prove what you think will happen, you try to disproved that there is no relationship or connection between the two things. For example, instead of trying to prove that salinity affects the growth of Phragmites, the null hypothesis says Phragmites is not affected by salinity. It is often easier to disprove no connection than it is to prove a connection. That's why scientists often start with a null hypothesis. If they can reject the null hypothesis, then it is easier to accept their hypothesis.

Here is a hypothesis:

Salinity impacts the height of a Phragmites.

8. Write a null hypothesis to go with that: Answer

Ha is notation that means: your alternate hypothesis.

Design a method of studying to test your hypothesis.

Investigate!


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Answers

Question 1 Answer:

The dark green is one plant species represents 35% cover of this square. The light green covers 65% of the square. This totals 100%.

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Question 3 Answer:

Salinity is less in the spring due to runoff and spring rains. Salinity is higher during and late summer and early fall because of greater evaporation, and less rain during the hot, dry summer months.

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Question 4 Answer:

In all sites, salinity was greater later in the growing season. Salinity was highest at Oak Knoll, the unrestricted site. Mud Creek, the restricted site has the lowest salinity levels, especially at stations 1, 2, and 4. (Figure 3)

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Question 8 Answer:

Salinity level has no impact on the height of Phragmites.

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Write an analysis of your data, and post it on your web site, or send them to me to add to this site:

Elizabeth B. Duff, Coastal Educator/Education Coordinator
Massachusetts Audubon Society
346 Grapevine Rd.
Wenham, MA 01984
lduff@massaudubon.org

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