8.17.22

Mowing for monarchs, Part II

In Part I you explored data that showed monarchs prefer to lay their eggs on young milkweeds that have been mowed, compared to older milkweed plants. But, is milkweed age the only factor that was changed when Britney and Gabe mowed patches of milkweeds? You will now examine whether mowing also affected the presence of monarch predators.

A scientist measuring a milkweed plant.
A scientist, Lizz D’Auria, counting the number of monarch predators on milkweed plants in the experiment.

The activities are as follows:

The bright orange color of monarch butterflies signals to their enemies that they are poisonous. This is a warning that they do not make a tasty meal. Predators, like birds and spiders, that try to eat monarch butterflies usually become sick. Many people think that monarch butterflies have no enemies because they are poisonous. But, in fact they do have a lot of predators, especially when they are young.

Monarchs become poisonous from the food they eat. Adult monarchs lay their eggs on milkweed plants, which have poisonous sap. When the eggs hatch, the caterpillars chomp on the leaves. Young caterpillars are less poisonous because they haven’t eaten much milkweed yet. And monarch eggs are not poisonous at all to predators.

Britney and Gabe met with their friends, Doug and Nate, who are scientists. Doug and Nate thought that Britney and Gabe’s experiment might have changed more than just the age of the milkweed plants in the patches they mowed. By mowing their field sites they were also cutting down the plants in the rest of the community. These plants provide habitat for predators, so mowing all of the plants would affect the predators as well. These ideas led to another potential explanation for the results Britney and Gabe saw in their data. Because all plants were cut in the mowed patches, there was nowhere for monarch predators to hang out. Britney and Gabe came up with an alternative hypothesis that perhaps monarch butterflies were choosing to lay their eggs on young milkweed plants because there were fewer predators nearby. To test this new idea, Britney and Gabe went back to their experimental site and started collecting data on the presence of predators in addition to egg number. Remember that in each location, they had a control patch, which was left alone, and a treatment patch that they mowed. The control patches had older milkweed plants and a full set of plants in the community. The mowed patches had young milkweed plants with short, chopped plants nearby. For the whole summer, they went out weekly to all of the patches. They counted the number of predators found on the milkweed plants so they could compare the mowed and unmowed patches.

Predators of monarch butterflies.
There are many different species that eat monarch butterfly eggs and young caterpillars. These are just a few of the species that Gabe and Britney observed during their experiment.

Featured scientists: Doug Landis and Nate Haan from Michigan State University and Britney Christensen and Gabe Knowles from Kellogg Biological Station LTER.

Flesch–Kincaid Reading Grade Level = 8.2

Additional resources related to this Data Nugget:

  • A news article discussing declining monarch populations and the causes that might be contributing to this trend.
1.11.22

Mowing for monarchs, Part I

A monarch caterpillar on a milkweed leaf.
A monarch caterpillar on a milkweed leaf.

The activities are as follows:

With their orange wings outlined with black lines and white dots, monarch butterflies are one of the most recognizable insects in North America. They are known for their seasonal migration when millions of monarch butterflies migrate from the United States and Canada south to Mexico in the fall. Then, in the spring the monarch butterflies migrate back north. Monarch butterflies are pollinators, which means they get their food from the pollen and nectar of flowering plants that they visit. The milkweed plant is one of the most important flowering plants that monarch butterflies depend on.

During the spring and summer months female butterflies will lay their eggs on milkweed plants. Milkweed plays an important role in the monarch butterfly’s life cycle. It is the only plant that monarchs will lay their eggs on. Caterpillars hatch from the butterfly eggs and eat the leaves of the milkweed plant. The milkweed is the only food that monarch caterpillars will eat until they become butterflies.

A problem facing many pollinators, including monarch butterflies, is that their numbers have been going down for several years. Scientists are concerned that we will lose pollinators to extinction if we don’t find solutions to this problem. Doug and Nate are scientists at Michigan State University trying to figure out ways to increase the number of monarch butterflies. They think that they found something that might work. Doug and Nate have learned that if you cut old milkweed plants at certain times of the year, then younger milkweed plants will quickly grow in their place. These new milkweed plants are softer and more tender than the old plants. It appears that monarch butterflies prefer to lay their eggs on the younger plants. It also seems that the monarch caterpillars prefer to eat the younger plants.

Britney and Gabe are two elementary teachers interested in monarch butterfly conservation. They learned about Doug and Nate’s research and wanted to participate in their experiment. The team of four met and designed an experiment that Britney and Gabe could do in open meadows throughout their community.

Britney and Gabe chose ten locations for their experiment. In each location they set aside a milkweed patch that was left alone, which they called the control.  At the same location they set aside another milkweed patch where they mowed the milkweed plants down. After a while, milkweed plants would grow back in the mowed patches. This means they had control patches with old milkweed plants, and treatment patches with young milkweed plants. Gabe and Britney made weekly observations of all the milkweed patches at each location. They recorded the number of monarch eggs in each of the patches. By the end of the summer, they had made 1,693 observations!

Featured scientists: Doug Landis and Nate Haan from Michigan State University and Britney Christensen and Gabe Knowles from Kellogg Biological Station LTER.

Flesch–Kincaid Reading Grade Level = 8.2

Additional resources related to this Data Nugget:

  • This research is part of the ReGrow Milkweed citizen science project. To learn more, visit their website or follow them on Twitter at @ReGrowMilkweed.
  • Britney, one of the scientists in this study, wrote a blog post about her experience in the NSF LTER RET Program (National Science Foundation’s Research Experience for Teachers) working with Doug Landis.
  • Learn about how this group of scientists responded to the COVID-19 pandemic to pivot to a virtual citizen science program in this blog post.
  • A news article discussing declining monarch populations and the causes that might be contributing to this trend.
7.19.21

Buried seeds, buried treasure

Marjorie (right) and David (left) digging up the seed bottle in 2021. This bottle was scheduled to be dug up in 2020, but the experiment was delayed one year due to COVID-19.

One of the world’s longest-running science experiments lies hidden in the soil beneath Michigan State University’s campus. Over 100 years ago, a scientist named William J. Beal had a question: how long do seeds survive underground? To find out, he started an experiment. In 1879 he filled 20 bottles with sand and seeds from local plants. William buried these bottles and created a map to document their location, hoping that future scientists would continue to dig them up to test whether the seeds would still grow long after his death.

These bottles and the map have been passed down from generation to generation, with each new scientist responsible for training their successor. To protect the seeds, only a select few scientists are let in on the secret. Today a team of four plant biologists hold the map, and they were the ones to dig up the most recent bottle in 2021. 

Early one Thursday morning, before the sun had risen, the team set out on their mission. Marjorie Weber, the first woman to be in charge of the study and currently the youngest team member, was the scientist who found the bottle and pulled it from the ground. This is a big deal, as back when William began the experiment women weren’t even allowed to be scientists!

Seeds of Verbascum blattaria germinating in 2021. This is the only species that germinated from the most recent collection.

Originally, the Beal Seed Experiment was designed to test seed viability, or how long seeds of different species stay alive in the soil and still germinate. Seeds don’t germinate as soon as they fall off their mother plant. They become part of a seed bank below the soil, waiting for the right conditions to tell them to sprout. William was working with local farmers in Michigan, and he was interested in helping them better understand how long weeds will continue to pop up in their fields after they start to plant crops. This is reflected in the fact that many of the species included in the experiment are weeds in agricultural fields. 

Despite all the changes that have taken place in the world since the seeds were buried 142 years ago, the main question remains the same: how long can seeds stay alive in the soil? In addition to helping farmers, Marjorie and the other scientists now have additional reasons for wanting to understand seed viability. Restoration of natural plant communities, conservation of endangered species, and removal of invasive plants from fragile ecosystems can all benefit from a knowledge of the seedbank. 

With this long-term study design, scientists can compare how many seeds sprout and which species are able to germinate through time. Originally, William dug up a new bottle every five years. Once scientists realized how long the seeds last, they made the interval between excavations longer; now they wait 20 years before digging up the next bottle. The experiment is set to go at least another 80 years. Imagine, future bottles will be dug up by scientists who are not even born yet!

Once a bottle is found and unearthed, it is taken back to the lab to see which species will germinate. Filled with sand and over a thousand seeds, each bottle contains the same mix of 50 seeds of 21 different species of plants. The contents are spread out on a tray filled with soil and are put into growth chambers. Scientists keep an eye on the trays to watch and see what germinates.

Featured scientist: Marjorie Weber from Michigan State University. 

Other scientists: Frank Telewski, David Lowry, Lars Brudvig, and Margaret Fleming.

Written by: Elizabeth Schultheis and Melissa Kjelvik.

Flesch–Kincaid Reading Grade Level = 9.7

Additional teacher resource related to this Data Nugget:

This experiment received a lot of press coverage. Have students check out these new stories and videos to learn more about the scientists and experiment:

YouTube video summarizing the search and the experiment:

4.20.21

How milkweed plants defend against monarch butterflies

Anurag looking at a monarch caterpillar on a milkweed plant.

The activities are as follows:

For millions of years, monarch butterflies have been antagonizing milkweed plants. Although adult monarchs drink nectar from flowers, their caterpillars only eat milkweed leaves, which harms the plants. This is an ecological interaction called herbivory. The only food for monarchs is milkweed leaves, meaning they have evolved to be highly specialized, picky eaters. But their food is not a passive victim. Like most other plants, milkweeds fight back with defenses against herbivory.

Monarch butterflies lay their eggs on the underside of milkweed leaves. After eggs hatch, caterpillars start to feed and quickly meet the plant’s first defense. Milkweed leaves are covered in thousands of tiny hairs, called trichomes, that the caterpillar needs to shave off before they can take a bite. The next challenge happens when the caterpillar takes a bite of the leaf. They get a mouthful of latex, which is sticky like Elmer’s glue. The caterpillars have to be very careful in how they feed. They cut the veins in the leaf to drain out the latex before continuing to feed on the leaf. Even after monarch caterpillars make it past the trichomes and latex, there’s another defense they need to overcome. Milkweed leaves have chemical toxins called cardiac glycosides, which are poisonous to most animals. As they feed, monarchs eat some of this poison.

Anurag is a scientist who has long been fascinated by plants and their defenses. He thinks this comes from the fact that his mother was such an avid gardener. She would grow food, such as peppers, squashes, and tomatoes. He looks back and has memories that are associated with garden plants and their defenses. For example, he remembers eating a bitter cucumber as a kid and spitting it out. He also can still recall the bitter aroma on his hand after brushing against the sticky tomato leaves. And plants that are tough and stringy, like kale, are not as tasty to eat. These traits are examples of plant defenses in action, making them harder or less enjoyable to eat, reducing herbivory.

Anurag collecting data on milkweed plants.

Anurag first started studying milkweeds 20 years ago, based on a recommendation from a friend. His friend told him of the bitter, sticky, and furry leaves that were treasured by the monarch butterfly caterpillars. This led him to study the paradox of coevolution. The milkweed and monarch have such a tight relationship that over time, milkweeds have evolved multiple ways to defend themselves against their herbivores. In response, monarchs have evolved to overcome those defenses because they need to eat the milkweed. This arms race may continue to shift back and forth over the course of evolutionary time.

This back-and-forth battle between caterpillar and plant intrigued Anurag. He wanted to know whether milkweed’s defensive traits are still effective against monarchs, or have monarchs evolved in ways that make them unaffected by the defenses? Because each defense trait might be at a different phase in the coevolution process, perhaps some would be effective defenses to herbivory, but others would not be effective. He predicted that monarchs would be harmed by all three milkweed defense traits (trichomes, latex, and cardiac glycosides), but that some would cause more harm than others.

To test his ideas, Anurag and his collaborators grew monarch caterpillars on 24 different North American milkweed species. They put a single newly hatched caterpillar on each plant and had five replicate plants per milkweed species. They recorded each caterpillar’s growth over the course of 5 days to see how healthy it was. They also measured the amount of trichomes, latex, and cardiac glycosides in each plant to determine their level of defense. Once they had their data, they looked for a relationship between caterpillar growth and plant defense traits to determine which made the best plant defenses. The better the defense, the less caterpillars would grow.

Featured scientist: Anurag Agrawal (He/Him/His) from Cornell University

Flesch–Kincaid Reading Grade Level = 8.5

Additional teacher resources related to this Data Nugget include:

  • Anurag has other examples, data, and related stories in his book: Monarchs and Milkweed, which is written for budding scientists and interested naturalists: www.amazon.com/dp/0691166358.
  • Students can learn more about Anurag, his research, and his lab at his website: www.herbivory.com which includes blog posts about monarch conservation, the community of insects on milkweed plants, videos of talks and presentations, and other things related to his research and teaching at Cornell University.
  • A scientific article based on this research: Agrawal, A. A., Fishbein, M., Jetter, R., Salminen, J. P., Goldstein, J. B., Freitag, A. E., & Sparks, J. P. (2009). Phylogenetic ecology of leaf surface traits in the milkweeds (Asclepias spp.): chemistry, ecophysiology, and insect behavior. New Phytologist, 183(3), 848-867.
  • Learn more about Anurag and his research in this YouTube video!

2.19.21

Getting to the roots of serpentine soils

Alexandria in the field observing the plants and soil.

When an organism grows in different environments, some traits change to fit the conditions. For example, if a houseplant is grown in the shade, it might grow to stretch out long and thin to reach as much light as possible. If that same plant were grown in the sun, it would grow thicker stems and more leaves that are not spread as far apart. This response to the environment helps plants grow in the different conditions they find themselves in.

Flexibility is especially important when a plant is living in a harsh environment. One such environment is serpentine soils. These soils are created from the weathering of the California state rock, Serpentinite. Serpentine soils have high amounts of toxic heavy metals, do not hold water well, and have low nutrient levels. Low levels of water and nutrients found in serpentine soils limit plant growth. In addition, a high level of heavy metals in serpentine soils can actually poison the plant with magnesium!

Combined, these qualities make it so that most types of plants are not able to grow on serpentine soils. However, some plant species have traits that help them tolerate these harsh conditions. Species that are able to live in serpentine soils, but can also grow in other environments, are called serpentine-indifferent.

Alexandria has been working with serpentine soils since 2011 when she was first introduced to them during an undergraduate research experience with her ecology professor. Alexandria was especially intrigued by this challenging environment and how organisms are able to thrive in it, even with the harsh characteristics.

Dot-seed plantain plants in the growth chamber.

To learn more, she started to read articles about previous research on plants that can only grow in serpentine soils. Alexandria learned that these plant species are generally smaller than closely related species. This was interesting, but she still had questions. She noticed the other experiments had compared plant size in different species, not within one species. She thought the next step would be to look at how plants that are the exact same species would respond to serpentine and non-serpentine soil environments. To explore this question, she would need to use serpentine-indifferent plant species because they can grow in serpentine soils and other soils.

Just as a houseplant grows differently in the sun or shade, plants grown in serpentine and non-serpentine soils might change to survive in their environment. Alexandria thought one of these changes could be happening in the roots. She decided to focus on plant roots because of their importance for plant survival and health. Roots are some of the first organs that many plants produce and anchor them to the ground. Throughout a plant’s life, the roots are essential because they bring nutrients to above-ground organs such as leaves. Because serpentine soils have fewer plant nutrients and are drier than non-serpentine soils, Alexandria thought that plants growing in serpentine soils may not invest as much into large root systems. She predicted plants growing in serpentine soils will have smaller roots than plants growing in non-serpentine soils.

To test her ideas, she studied the effects of soil type on a serpentine-indifferent plant species called Dot-seed plantain. She purchased seeds for her experiment from a local commercial seed company. About 5 seeds were planted in serpentine or non-serpentine soils in a growth chamber where growing conditions were kept the same. After the seedlings emerged, the plants were thinned so that there was one plant per pot. The only difference in the environment was the soil type. This allowed Alexandria to attribute any differences in root length to serpentine soils. At the end of her experiment, she pulled the plants out of the soil and measured the root lengths of plants in both treatments.

Featured scientist: Alexandria Igwe (she/her) from University of Miami

Flesch–Kincaid Reading Grade Level = 8.7

Additional resources related to this Data Nugget:

The topics described in this Data Nugget are similar to the published research in the following article:

  • Igwe, A.N. and Vannette, R.L. 2019. Bacterial communities differ between plant species and soil type, and differentially influence seedling establishment on serpentine soils. Plant Soil: 441: 423-437

There is a short video of Alexandria (Allie) sharing her research on serpentine soils.

There have been several news stories and blog posts about this research:

1.8.21

Fertilizer and fire change microbes in prairie soil

Christine collecting samples from the experimental plots to measure root growth.
Christine collecting samples from the experimental plots to measure root growth.

The activities are as follows:

Stepping out into a prairie feels like looking at a sea of grass, with the horizon evoking a sense of eternity. Grasses and other prairie plants provide important benefits, such as creating habitat for many unique plants, mammals, insects, and microbes. They also help keep our water clean by using nutrients from the soil to grow. When plants take up these nutrients, they prevent them from going into streams. High levels of plant growth also keeps carbon bound up in the bodies of plants instead of in the atmosphere.  

Prairies grow where three environmental conditions come together – a variable climate, frequent fires, and large herbivores roaming the landscape. However, prairies are experiencing many changes. For example, people now work to prevent fires, which allows forest species to establish and eventually take over the prairie. In addition, a lot of land previously covered in prairie is now being used for agriculture. When land is used for agriculture, farmers add nutrients through fertilizer. With all these changes, prairie ecosystems have been declining globally. Scientists are concerned that as they disappear so will the benefits they provide. 

Lydia and Christine are two scientists contributing to the effort to learn more about how to preserve prairies. They both became interested in studying soil because of their appreciation for prairies at a young age. For Lydia, she lived in an area that was covered by trees and farmland, but knew at one time it used to be prairie. This made her want to learn more about prairie environments and how places like where she grew up have changed through history. For Christine, she grew up surrounded by prairies where she developed a passion and curiosity for the natural world. Especially for the organisms living in the soil that you cannot see, called microbes. 

These are two different experimental plots within the large field experiment at Konza Prairie Biological Station. The one with lots of trees is an unburned plot, the one with lots of grass is a burned plot.
These are two different experimental plots within the large field experiment at Konza Prairie Biological Station. The one with lots of trees is an unburned plot, the one with lots of grass is a burned plot.

Lydia and Christine read about how grassland scientists have been doing research to learn more about what happens when fire is stopped and excess nutrients are added. These changes reduce biodiversity and affect which species of plants can grow in the prairie. However, Lydia and Christine noticed that the research had been mostly focused on what happens aboveground.  Lydia and Christine had a hunch that the aboveground communities were not the only things changing. They thought that belowground components would be changed by fire and fertilizer too. They turned their focus to microbes in the soil, because they also use nutrients. In addition, they thought these microorganism would be affected by the changes in aboveground plant biodiversity. 

To see if this was true, they used data that they and other scientists collected at Konza Prairie Biological Station from a large field experiment. The experiment was set up in 1986 and the treatments were applied at the field site every year until 2017! Lydia and Christine focused on the fertilizer (nitrogen) addition and prescribed burning treatments to answer their questions. The nitrogen treatment had eight plots where nitrogen had been added and eight with no nitrogen as a control. Similarly, the prescribed burn treatment was applied to eight plots, while eight plots had no burning as a control. These two treatments were also crossed with each other, meaning that some plots were burned and nitrogen was added.

Lydia and Christine expected the types of microbes in the soil to change in response to the nitrogen and burning treatments because of the different aboveground plant communities and difference in soil nutrients. Soil microbial communities can change in multiple ways. First, the number of unique species can increase or decrease, measured as richness. The other way is how many individuals of each species there are in the community, measured as evenness. Taken together, richness and evenness give a measure of diversity, which can be summarized using the Shannon-Wiener index. The value will get bigger if either richness or evenness increases because it incorporates both. For example, a community with five species that has equal abundance of each will have a larger Shannon-Wiener index than a community with five species where one species has a lot more individuals than the other four.  

Featured Scientists: Lydia Zeglin and Christine Carson from the Konza Prairie Biological Station. Written By: Jaide Allenbrand

Flesch–Kincaid Reading Grade Level = 10.4

12.10.20

Mangroves on the move

mangrove in marsh
A black mangrove growing in the saltmarshes of northern Florida.

The activities are as follows:

All plants need nutrients to grow. Sometimes one nutrient is less abundant than others in a particular environment. This is called a limiting nutrient. If the limiting nutrient is given to the plant, the plant will grow in response. For example, if there is plenty of phosphorus, but very little nitrogen, then adding more phosphorus won’t help plants grow, but adding more nitrogen will. 

Saltmarshes are a common habitat along marine coastlines in North America. Saltmarsh plants get nutrients from both the soil and the seawater that comes in with the tides. In these areas, fertilizers from farms and lawns often end up in the water, adding lots of nutrients that become available to coastal plants. These fertilizers may contain the limiting nutrients that plants need, helping them grow faster and more densely.

One day while Candy, a scientist, was out in a saltmarsh in northern Florida, she noticed something that shouldn’t be there. There was a plant out of place. Normally, saltmarshes in that area are full of grasses and other small plants—there are no trees or woody shrubs. But the plant that Candy noticed was a mangrove. Mangroves are woody plants that can live in saltwater, but are usually only found in tropical places that are very warm. Candy thought the closest mangrove was miles away in the warmer southern parts of Florida. What was this little shrub doing so far from home? The more that Candy and her colleague Emily looked, the more mangroves they found in places they had not been before.

Candy and Emily wondered why mangroves were starting to pop up in northern Florida. Previous research has shown nitrogen and phosphorus are often the limiting nutrients in saltmarshes. They thought that fertilizers being washed into the ocean have made nitrogen or phosphorus available for mangroves, allowing them to grow in that area for the first time. So, Candy and Emily designed an experiment to figure out which nutrient was limiting for saltmarsh plants. 

mangrove saltmarsh researchers
Candy (right) and Emily (left) measure the height of a black mangrove growing in the saltmarsh.

For their study, Candy and Emily chose to focus on black mangroves and saltwort plants. These two species are often found growing together, and mangroves have to compete with saltwort. Candy and Emily found a saltmarsh near St. Augustine, Florida, in which they could set up an experiment. They set up 12 plots that contained both black mangrove and saltwort. Each plot had one mangrove plant and multiple smaller saltwort plants. That way, when they added nutrients to the plots they could compare the responses of mangroves with the responses of saltwort. 

To each of the 12 plots they applied one of three conditions: control (no extra nutrients), nitrogen added, and phosphorus added. They dug two holes in each plot and added the nutrients using fertilizers, which slowly released into the nearby soil. In the case of control plots, they dug the holes but put the soil back without adding fertilizer.

Candy and Emily repeated this process every winter for four years. At the end of four years, they measured plant height and percent cover for the two species. Percent (%) cover is a way of measuring how densely a plant grows, and is the percentage of a given area that a plant takes up when viewed from above. Candy and Emily measured percent cover in 1×1 meter plots. The cover for each species could vary from 0 to 100%.

Featured scientists: Candy Feller from the Smithsonian Environmental Research Center and Emily Dangremond from Roosevelt University

Flesch–Kincaid Reading Grade Level = 8.3

3.26.20

Where to find the hungry, hungry herbivores

Carina and some pokeweed plants in Tennessee.

The activities are as follows:

Éste Data Nugget también está disponible en Español:

When travelling to warm, tropical places you are exposed to greater risk of diseases like malaria, yellow fever, or dengue fever. The same pattern of risk is true for other species besides humans. For example, scientists have noticed that crops seem to have more problems with pests if they grow at lower latitudes (closer to the equator). Locations that are at lower latitudes have warm climates. We don’t know exactly why there are more pests in warmer places, but it could be because pests have a hard time surviving very cold winters. 

Carina is interested in figuring out more about this pest-y problem. She first got excited about plants in school, when she learned that they use photosynthesis to make their own food out of light, air, and water. She thought it was fascinating that plants have evolved so many different strategies to survive. Even though they don’t have brains, plants do have adaptations that help them compete for light and mate in many different habitats. Carina continues to learn more every day, and especially enjoys researching how plants defend themselves against herbivores, or animals that eat plants. Herbivores pose a challenge because plants can’t run away or hide! 

Carina studies ways wild plants can defend themselves against herbivores. What she learns in wild plants could give us ideas of how to help crops defend against pests too. Scientists aren’t sure why crops have more pest problems in warmer places, but it would help to understand if wild plants also have the same pattern. 

Pokeweed (Phytolacca americana) is a common wild plant that grows all over the eastern US. Pokeweed has beautiful pink stems and dark purple berries. In fact, the Declaration of Independence was written with ink made from pokeweed berry juice!

So Carina decided to travel all across the eastern United States to measure herbivory on pokeweed, a common wild plant there. Carina drove a lot for this project! In one summer, she visited ten patches of pokeweed spread out between Michigan and Florida. Carina thought that the pokeweed found at lower latitudes (Florida, 27° N) would have higher herbivory than pokeweed at northern latitudes (Michigan, 42° N) because pests may not be able to survive as well in places with harsh winters. 

At each of the ten sites, she marked five very young leaves on 30 to 40 plants. That equals over 1,500 leaves! She then came back six weeks later to measure how much the leaves were eaten as they grew into large, mature leaves. When leaves are young, they are more tender and can be more easily eaten by herbivores (that’s why we eat “baby spinach” salad). To measure herbivory she compared the area that was eaten to the total area of the leaf, and calculated the percent of the leaf area eaten by caterpillars, the main herbivores on pokeweed. She then averaged the percent eaten on leaves for each plant. Some plants died in those 6 weeks, so the sample size at the end of the study ranged from 4 to 37 depending on the site.

Featured scientist: Carina Baskett from Michigan State University

Flesch–Kincaid Reading Grade Level = 9.4

There is one scientific paper associated with the research in this Data Nugget. The citation and link to the paper is below.

Baskett, C.A. and D.W. Schemske (2018) Latitudinal patterns of herbivore pressure in a temperate herb support the biotic interactions hypothesis. Ecology Letters 21(4):578-587.

12.11.19

A window into a tree’s world

Neil taking a tree core from a pine tree.

The activities are as follows:

According to National Aeronautics and Space Administration (NASA) and the National Oceanic Atmospheric Administration (NOAA), the years 2015-2018 were the warmest recorded on Earth in modern times! And it is only expected to get warmer. Temperatures in the Northeastern U.S. are projected to increase 3.6°F by 2035. Every year the weather is a bit different, and some years there are more extremes with very hot or cold temperatures. Climate gives us a long-term perspective and is the average weather, including temperature and precipitation, over at least 30 years. 

Over thousands of years, tree species living in each part of the world have adapted to their local climate. Trees play an important role in climate change by helping cool the planet – through photosynthesis, they absorb carbon dioxide from the atmosphere and evaporate water into the air. 

Scientists are very interested in learning how trees respond to rapidly warming temperatures. Luckily, trees offer us a window into their lives through their growth rings. Growth rings are found within the trunk, beneath the bark. Each year of growth has two parts that can be seen: a light ring of large cells with thin walls, which grows in the spring; and a dark layer of smaller cells with thick walls that forms later in the summer and fall. Ring thickness is used to study how much the tree has grown over the years. Dendrochronology is the use of these rings to study trees and their environments.

Different tree species have different ranges of temperatures and rainfall in which they grow best. When there are big changes in the environment, tree growth slows down or speeds up in response. Scientists can use these clues in tree’s rings to decipher what climate was like in the past. There is slight variation in how each individual tree responds to temperature and rainfall. Because of this, scientists need to measure growth rings of multiple individuals to observe year-to-year changes in past climate.

Jessie taking a tree core in the winter.

Jessie and Neil are two scientists who use tree rings for climate research. Jessie entered the field of science because she was passionate about climate change. As a research assistant, Neil saw that warming temperatures in Mongolia accelerated growth in very old Siberian pine trees. When he later studied to become a scientist, he wanted to know if trees in the eastern U.S. responded to changes in climate in the same way as the old pine trees in Mongolia. As a result, there were two purposes for Jessie’s and Neil’s work. They wanted to determine if there was a species that could be used to figure out what the climate looked like in the past, and understand how it has changed over time.

Jessie and Neil decided to focus on one particular species of tree – the Atlantic white cedar. Atlantic white cedar grow in swamps and wetlands along the Atlantic and Gulf coasts from southern Maine to northern Florida. Atlantic white cedar trees are useful in dendrochronology studies because they can live for up to 500 years and are naturally resistant to decay, so their well-preserved rings provide a long historical record. Past studies of this species led them to predict that in years when the temperature is warmer, Atlantic white cedar rings will be wider. If this pattern holds, the thickness of Atlantic white cedar rings can be used to look backwards into the past climate of the area. 

To test this prediction, Jessie and Neil needed to look at tree rings from many Atlantic white cedar trees. Jessie used an increment borer, a specialized tool that drills into the center of the tree. This drill removes a wood core with a diameter about equal to that of a straw. She sampled 112 different trees from 8 sites, and counted the rings to find the age of each tree. She then crossdated the wood core samples. Crossdating is the process of comparing the ring patterns from many trees in the same area to see if they tell the same story. Jessie used a microscope linked to a computer to measure the thickness of both the early and late growth to the nearest micrometer (1 micrometer = 0.001 millimeter) for all rings in all 112 trees. From those data she then calculated the average growth of Atlantic white cedar for each year to create an Atlantic white cedargrowth index for the Northeastern U.S. She combined her tree ring data with temperature data from the past 100 years.

Featured scientists: Jessie K Pearl, University of Arizona and Neil Pederson, Harvard University. Written by Elicia Andrews.

Flesch–Kincaid Reading Grade Level = 9.9

Suggestions for inquiry surrounding this Data Nugget:

These videos, demonstrating the science of dendrochronology, could be a great way to spark class discussions:

10.29.19

The carbon stored in mangrove soils

Tall mangroves growing close to the coast.

The activities are as follows:

In the tropics and subtropics, mangroves dominate the coast. There are many different species of mangroves, but they are all share a unique characteristic compared to other trees – they can tolerate having their roots submerged in salt water.

Mangroves are globally important for many reasons. They form dense forested wetlands that protect the coast from erosion and provide critical habitat for many animals. Mangrove forests also help in the fight against climate change. Carbon dioxide is a greenhouse gas that is a main driver of climate change. During photosynthesis, carbon dioxide is absorbed from the atmosphere by the plants in a mangrove forest. When plants die in mangrove forests, decomposition is very slow. The soils are saturated with saltwater and have very little oxygen, which decomposers need to break down plants. Because of this, carbon is stored in the soils for a long time, keeping it out of the atmosphere.

Sean is a scientist studying coastal mangroves in the Florida Everglades. Doing research in the Everglades was a dream opportunity for Sean. He had long been fascinated by the unique plant and animal life in the largest subtropical wetland ecosystem in North America. Mangroves are especially exciting to Sean because they combine marine biology and trees, two of his favorite things! Sean had previously studied freshwater forested wetlands in Virginia, but had always wanted to spend time studying the salty mangrove forests that exist in the Everglades. 

Sean Charles taking soil samples amongst inland short mangroves.

Sean arrived in the Everglades with the goal to learn more about the factors important for mangrove forests’ ability to hold carbon in their soils. Upon his arrival, he noticed a very interesting pattern – the trees were much taller along the coast compared to inland. This is because mangroves that grow close to the coast have access to important nutrients found in ocean waters, like phosphorus. These nutrients allow the trees to grow large and fast. However, living closer to the coast also puts trees at a higher risk of damage from storms, and can lead to soils and dead plants being swept out to sea. 

Sean thought that the combination of these two conditions would influence how much carbon is stored in mangrove soils along the coast and inland. Larger trees are generally more productive than smaller ones, meaning they might contribute more plant material to soils. This led Sean to two possible predictions. The first was that there might be more carbon in soils along the coast because taller mangroves would add more carbon to the soil compared to shorter inland mangroves. However, Sean thought he might also find the opposite pattern because the mangroves along the coast have more disturbance from storms that could release carbon from the soils. 

To test these competing hypothesis, the team of scientists set out into the Everglades in the Biscayne National Park in Homestead, Florida. Their mission was to collect surface soils and measure mangrove tree height. To collect soils, they used soil cores, which are modified cylinders that can be hammered into the soil and then removed with the soil stuck in the tube. Tree height was measured using a clinometer, which is a tool that uses geometry to estimate tree height. They took these measurements along three transects. The first transect was along the coast where trees had an average height of 20 meters. The second transect between the coast and inland wetlands where trees were 10 meters tall, on average. The final transect was inland, with average tree height of only 1 meter tall.  With this experimental design Sean could compare transects at three distances from the coast to look for trends. 

Once Sean was back in the lab, he quantified how much carbon was in the soil samples from each transect by heating the soil in a furnace at 500 degrees Celsius. Heating soils to this temperature causes all organic matter, which has carbon, to combust. Sean measured the weight of the samples before and after the combustion. The difference in weight can be used to calculate how much organic material combusted during the process, which can be used as an estimate of the carbon that was stored in the soil. 

Featured scientist: Sean Charles from Florida International University

Flesch–Kincaid Reading Grade Level = 9.6

Additional teacher resources related to this Data Nugget:

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