Learning Lessons from Invaded Forests

In 1946, North American beavers were introduced to the archipelago of Tierra del Fuego at the tip of South America in an attempt to start an industry based on beaver fur. Although this industry has not thrived, beavers have multiplied enormously. By cutting trees and building dams, they have transformed forests into meadows and also fostered the spread of introduced ground cover plants. Now numbering in the tens of thousands in both Chilean and Argentinian parts of the archipelago, beavers are the target of a binational campaign to prevent them from spreading to the mainland of these two nations. — Invasive Species: What Everyone Should Know by Daniel Simberloff

Beavers in South America are just one example of the series of effects a species can have when it is placed in a new environment. Prior to the arrival of beavers, there were no species in the area that were functionally equivalent. Thus, through their felling of trees and damning of streams, the beavers introduced novel disturbances that have, among other things, aided the spread of non-native plant species. Ecologists call this an invasional meltdown, wherein invasion by one organism aids the invasion of another, making restoration that much more difficult.

Complicated interactions like this are explored by David Wardle and Duane Peltzer in a paper published last month in Biological Invasions. Organisms from all walks of life have been introduced to forests around the world, and while many introductions have had no discernable impact, others have had significant effects both above and below ground.

The authors selected forest ecosystems for their investigation because “the imprint of different invaders on long-lived tree species can often be observed directly,” even when the invading organisms are doing their work below ground. Moreover, a greater understanding of “the causes and consequences of invasions is essential for reliably predicting large-scale and long-term changes” in forest ecosystems. Forests do not regenerate quickly, so protecting them from major disturbances is important. Learning how forests respond to invasions can teach us how best to address the situation when it occurs.

The authors begin by introducing the various groups of organisms that invade forests and the potential impacts they can have. This is summarized in the graphic below. One main takeaway is that the effects of introduced species vary dramatically depending on their specific attributes or traits and where they fall within the food chain. If, like the beaver, a novel trait is introduced, “interactions between the various aboveground and belowground components, and ultimately the functioning of the ecosystem” can be significantly altered.

Wardle, D.A., and D.A. Peltzer. Impacts of invasive biota in forest ecosystems in an aboveground-belowground context. Biological Invasions (2017).  doi:10.1007/s10530-017-1372-x

After highlighting some of the impacts that invasive species can have above and below ground, the authors discuss basic tenets of invasion biology as they relate to forest ecosystems. Certain ecosystems are more vulnerable to invasions than others, and it is important to understand why. One hypothesis is that ecosystems with a high level of species diversity are more resistant to invasion than those with low species diversity. This is called biotic resistance.  When it comes to introduced plants, soil properties and other environmental factors come in to play. One species of plant may be highly invasive in one forested ecosystem, but completely unsuccessful in another. The combination of factors that help determine this are worth further exploration.

When it comes to restoring invaded forests, simply eliminating invasive species is not always enough. Because of the ecological impacts they can have above and below ground, “invader legacy effects” may persist. As the authors write, this requires “additional interventions to reduce or remove [an invader’s] legacy.” Care also has to be taken to avoid secondary invasions, because as one invasive species is removed another can take its place.

Nitrogen-fixing plants (which, as the authors explain, “feature disproportionately in invasive floras”) offer a prime example of invader legacy effects. Introducing them to forest ecosystems that lack plants with nitrogen fixing capabilities “leads to substantially greater inputs of nitrogen … and enhanced soil fertility.” Native organisms – decomposer and producer alike – are affected. Simply removing the nitrogen fixing plants does not at once remove the legacy they have left. Examples include Morella faya invasions of forest understories in Hawaii and invasions by Acacia species in South Africa and beyond.

“It has been shown that co-invasion by earthworms enhances the effects that the invasive nitrogen fixing shrub Morella faya has on nitrogen accretion and cycling in a Hawaiian forest, by enhancing burial of nitrogen-rich litter.” – D.A. Wardle and D.A. Peltzer (2017) – photo credit: wikimedia commons

The authors conclude with a list of “unresolved issues” for future research. A common theme among at least a couple of their issues is the need for observing invasive species and invaded environments over a long period. Impacts of invasive species tend to “vary across both time and space,” and it is important to be able to predict “whether impacts are likely to amplify or dampen over time.” In short, “focus should shift from resolving the effects of individual invasive species to a broader consideration of their longer term ecosystem effects.”

This paper does not introduce new findings, but it is a decent overview of invasion biology and is worth reading if you are interested in familiarizing yourself with some of the general concepts and hypotheses. It’s also open access, which is a plus. One thing that is clear after reading this is that despite our growing awareness of the impacts of invasive species, there is still much to be learned, particularly regarding how best to respond to them.

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Poisonous Plants: Yews

Wildfires last summer followed by a particularly harsh winter has driven herds of elk, deer, antelope, and other ungulates closer to urban and suburban areas in southern Idaho. This has resulted in several of the animals making a meal out of a particularly poisonous plant and then promptly dying. The plant is a yew, an ornamental shrub or tree that is commonly used in residential and commercial landscapes. Seven elk died after eating Japanese yew in the Boise Foothills. Fifty pronghorn antelope died after eating the same plant species in the small city of Payette. Eight more elk were found dead in North Fork and Challis, poisoned by yew; eight others were found dead outside of Idaho Falls having suffered a similar fate. And this is just a sampling. Needless to say, such tragedies have spawned a greater awareness of this and other deadly poisonous plants – plants that were purposely planted in our yards, thought benign, but lying in wait to kill.

Japanese yew (Taxus cuspidata) - photo credit: wikimedia commons

Japanese yew (Taxus cuspidata) – photo credit: wikimedia commons

Yews, plants in the genus Taxus, are in the family Taxaceae, a coniferous family that consists of around 5-7 genera and up to 30 species (sources vary). Taxus is one of the largest genera in the family with between 9 and 11 species. The genus occurs across three continents, with at least four species naturally occurring in North America (T. canadensis, T. brevifolia, T. globosa, and T. floridana). The species most commonly grown as ornamentals include Japanese yew (T. cuspidata), English yew (T. baccata), and a hybrid of the two (T. x media).

Generally speaking, yews are evergreen shrubs or trees with inch long, dark green needles that come to a sharp point. Branches are alternately arranged and the bark is scaly and reddish-brown. As trees they can reach heights of more than 60 feet, but in a garden setting the plants are usually hedged into more managable-sized shrubs. Taxus species are dioecious, which means that individuals are either male or female. The females produce fleshy, round, cup-shaped fruits that are pink, red, or green. This structure is called an aril and is produced by the swelling of the stem around a single seed. All parts of the plant are poisonous, with only one exception – the aril. This is problematic because the bright-colored aril can appear quite appetizing. And it is edible; however, when the seed is consumed along with it, the plant’s poison makes its way into the body.

The fruits of yew (Taxus sp.)

The fruits of yew (Taxus sp.)

Yew poisoning is unfun. Death can occur in a matter of a few hours, depending on the parts of the plant and amount consumed. The North American Guide to Common Poisonous Plants and Mushrooms lists these symptoms: “nausea, dry throat, severe vomiting, diarrhea, rash, pallor, drowsiness, abdominal pain, dizziness, trembling, stiffness, fever, and sometimes allergy symptoms.” Symptoms of severe poisoning include, “acute abdominal pain, irregular heartbeat, dilated pupils, collapse, coma, and convulsions, followed by a slow pulse and weak breathing.” The cause of death is respiratory and heart failure.

Yews contain a number of toxic compounds, including volatile oils and a cyanogenic glycoside. The compound responsible for yew’s high toxicity is taxine, a potent cardiotoxin and, as it turns out, an effective drug against certain types of cancer. Very small doses of this poison can be deadly. One or two yew seeds can kill a small child, and a handful or two of the needles can kill an animal, depending on its size. Even dried branches and leaves remain toxic, so wreaths made with yew should be disposed of in a landfill rather than tossed into a yard or field where domestic animals and livestock can find them. Yew consumption should be promptly addressed by visiting an emergency room or calling the Poison Control Center.

Yew’s deadly reputation is not something to take lightly. They are a popular ornamental because of their attractive fruits and evergreen foliage, their tolerance of shade, and their low maintenance requirements, but homeowners with children, pets, or proximity to horses, cows, or wild animals should consider removing them. If a decision is made to keep them, the shrubs can be wrapped in burlap during the winter to prevent hungry animals from coming in for a bite, particularly on properties that are adjacent to natural areas.

For more information about yew identification and removal, check out this article in the Idaho Statesman. Also, consider this wise counsel by Amy Stewart from her book, Wicked Plants:

Do not experiment with unfamiliar plants or take a plant’s power lightly. Wear gloves in the garden; think twice before swallowing a berry on a trail or throwing a root into the stew pot. If you have small children, teach them not to put plants in their mouths. If you have pets, remove the temptation of poisonous plants from their environment. The nursery industry is woefully lax about identifying poisonous plants; let your garden center know that you’d like to see sensible, accurate labeling of plants that could harm you. Use reliable sources to identify poisonous, medicinal, and edible plants.

More Poisonous Plant Posts on Awkward Botany:

In Praise of Poison Ivy

This is a guest post by Margaret Gargiullo. Visit her website, Plants of Suburbia, and check out her books for sale on Amazon.

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No one seems to like Toxicodendron radicans, but poison ivy is an important plant in our urban and suburban natural areas. Poison ivy (Anacardiaceae, the cashew family) is a common woody vine, native to the United States and Canada from Nova Scotia to Florida, west to Michigan and Texas. It is also found in Central America as far south as Guatemala. It is all but ubiquitous in natural areas in the Mid-Atlantic United States. It has been recorded in over 70 wooded parks and other natural areas in New York City.

Leaflets of three? Let if be. Poison ivy (Toxicodendron radicans). photo credit: wikimedia commons

Leaflets of three? Let if be. Poison ivy (Toxicodendron radicans) – photo credit: wikimedia commons

Poison ivy does have certain drawbacks for many people who are allergic to its oily sap. The toxins in poison ivy sap are called urushiols, chemicals containing a benzene ring with two hydroxyl groups (catechol) and an alkyl group of various sorts (CnHn+1).

These chemicals can cause itching and blistering of skin but they are made by the plant to protect it from being eaten by insects and vertebrate herbivores such as rabbits and deer.

Poison ivy is recognized in summer by its alternate leaves with three, shiny leaflets and by the hairy-looking aerial roots growing along its stems. In autumn the leaves rival those of sugar maple for red and orange colors. Winter leaf buds are narrow and pointed, without scales (naked). It forms extensive colonies from underground stems and can cover large areas of the forest floor with an understory of vertical stems, especially in disturbed woodlands and edges. However, It generally only blooms and sets fruit when it finds a tree to climb. When a poison ivy stem encounters a tree trunk, or other vertical surface, it clings tightly with its aerial roots and climbs upward, reaching for the light (unlike several notorious exotic vines, it does not twine around or strangle trees). Once it has found enough light, it sends out long, horizontal branches that produce flowers and fruit.

Flowers of poison ivy are small and greenish-white, not often noticed, except by the honeybees and native bees which visit them for nectar and exchange pollen among the flowers. Honey made from poison ivy nectar is not toxic. Fruits of poison ivy are small, gray-white, waxy-coated berries that can remain on the vine well into winter. They are eaten by woodpeckers, yellow-rumped warblers, and other birds. Crows use poison ivy berries as crop grist (instead of, or along with, small stones) and are major dispersers of the seeds.

The fruits of poison ivy (Toxicodendron radicans) - photo credit: Daniel Murphy

The fruits of poison ivy (Toxicodendron radicans) – photo credit: Daniel Murphy

It is as a ground cover that poison ivy performs its most vital functions in urban and suburban woodlands. It can grow in almost any soil from dry, sterile, black dune sand, to swamp forest edges, to concrete rubble in fill soils, and along highways. It enjoys full sun but can grow just fine in closed canopy woodlands. It is an ideal ground cover, holding soil in place on the steepest slopes, while collecting and holding leaf litter and sticks that decay to form rich humus. It captures rain, causing the water to sink into the ground, slowing runoff, renewing groundwater, filtering out pollutants, and helping to prevent flooding.

Poison ivy is usually found with many other plants growing up through it – larger herbs, shrubs, and tree seedlings that also live in the forest understory. It seems to “get along” with other plants, unlike Japanese honeysuckle or Asian bittersweet, which crowd out or smother other plants. Poison ivy is also important as shelter for birds and many invertebrates.

While those who are severely allergic to poison ivy have reason to dislike and avoid it, Toxicodendron radicans has an important place in our natural areas. No one would advocate letting it grow in playgrounds, picnic areas, or along heavily used trail margins, but it belongs in our woods and fields and should be treated with respect, not hatred. Recognize it but don’t root it out.

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Further Reading: Uva, R. H., J.C. Neal and J. M. DiTomaso. 1997. Weeds of the Northeast. Comstock Publishing. Ithaca, NY.

This piece was originally published in the New York City Dept. of Parks & Recreation, Daily Plant.

Alien Plant Invasions and the Extinction Trajectory

One of the concerns about introduced species becoming invasive is that they threaten to reduce the biodiversity of the ecosystems they have invaded. They do this by spreading rampantly, using up resources and space, altering ecosystem functions, and ultimately pushing other species out. In the case of certain invasive animals, species may be eliminated via predation; but plants don’t eat each other (generally), so if one plant species is to snuff out another plant species it must use other means. Presently, we have no evidence that a native plant species has been rendered extinct solely as a result of an invasive plant species. That does not mean, however, that invasive plants are not doing harm.

In a paper published in AoB Plants in August 2016, Paul O. Downey and David M. Richardson argue that, when it comes to plants, focusing our attention on extinctions masks the real impact that invasive species can have. In general, plants go extinct more slowly than animals, and it is difficult to determine that a plant species has truly gone extinct. Some plants are very long-lived, so the march towards extinction can extend across centuries. But the real challenge – after determining that there are no above-ground signs of life – is determining that no viable seeds remain in the soil (i.e. seed bank). Depending on the species, seeds can remain viable for dozens (even hundreds) of years, so when conditions are right, a species thought to be extinct can emerge once again. (Consider the story of the Kankakee mallow.)

On the other hand, there is plenty of evidence that invasive plant species have had significant impacts on certain native plant populations and have placed such species on, what Downey and Richardson call, an extinction trajectory. It is this trajectory that deserves our attention if our goal is to save native plant species from extinction. As described in the paper, the extinction trajectory has six steps – or thresholds – which are defined in the infographic below:

6-threshold-extinction-trajectory

Downey and Richardson spend a portion of the paper summarizing research that demonstrates how invasive plants have driven native plants into thresholds 1-3, thereby placing them on an extinction trajectory. In New Zealand, Lantana camara (introduced from the American tropics) creates dense thickets, outcompeting native plants. Researchers found that species richness of native plants declined once L. camara achieved 75% cover in the test sites. In the U.S., researchers found reduced seed set in three native perennial herbs as a result of sharing space with Lonicera mackii (introduced from Asia), suggesting that the alien species is likely to have a negative impact on the long-term survivability of these native plants. Citing such research, Downy and Richardson conclude that “it is the direction of change that is fundamentally important – the extinction trajectory and the thresholds that have been breached – not whether a native plant species has actually been documented as going extinct due to an alien plant species based on a snapshot view.”

Introduced to New Zealand from the American tropics, largeleaf lantana (Lantana camara) forms dense thickets that can outcompete native plant species. (photo credit: wikimedia commons)

Introduced to New Zealand from the American tropics, largeleaf lantana (Lantana camara) forms dense thickets that can outcompete native plant species. (photo credit: wikimedia commons)

In support of their argument, they also address problems with the way some research is done (“in many instances appropriate data are not collected over sufficiently long periods,” etc.), and they highlight the dearth of data and research (“impacts associated with most invasive alien plants have not been studied or are poorly understood or documented”). With those things in mind, they make recommendations for improving research and they encourage long-term studies and collaboration in order to address the current “lack of meta analyses or global datasets.” A similar recommendation was made in American Journal of Botany in June 2015.

The language in this report makes it clear that the authors are responding to a certain group of people that have questioned whether or not the threat of invasive plants has been overstated and if the measures we are taking to control invasive plants are justified. The following cartoon that appeared along with a summary of the article way oversimplifies the debate:

04_figure2

Boy: There are no studies that show weeds cause native plants to go extinct, thus we should not control them. Plant: If we wait until then, we’ll all be gone!!! Girl: Just because no one has demonstrated it does not mean that extinctions do not occur. The problem is not overstated!

It seems to me that a big part of why we have not linked an invasive plant species to a native plant species extinction (apart from the difficulty of determining with certainty that a plant has gone extinct) is that extinctions are often the result of a number of factors. The authors do eventually say that: “it is rare that one threatening process in isolation leads to the extinction of a species.” So, as much as it is important to fully understand the impacts that invasive plant species are having, it is also important to look at the larger picture. What else is going on that may be contributing to population declines?

Observing invaded plant populations over a long period seems like our best bet in determining the real effects that invasive species are having. In some cases, as Downey and Richardson admit, “decreased effects over time” have been documented, and so “the effects [of invasive species] are dynamic, not static.” And speaking of things that are dynamic, extinction is a dynamic process and one that we generally consider to be wholly negative. But why? What if that isn’t always the case? Extinctions have been a part of life on earth as long as life has been around. Is there anything “good” that can come out of them?

Introducing Invasive Species

The terms “invasive” or “invasive species” get thrown around a lot. They are frequently used to describe anything that is “misbehaving,” or acting in a way that doesn’t fit our idealized vision for how a landscape should look and function. Oftentimes a species that is introduced (by humans) or is not native to an area automatically gets labeled invasive, even if it isn’t acting aggressively or having any sort of dramatic impact on the ecosystem. It is an alien species in an alien environment; it has invaded, therefore it is invasive.

image credit: cartoon movement

image credit: cartoon movement

Determining what is actually invasive in what location and at what time is much more complex than that. We do our best to understand the natural features and functions of ecosystems, and we single out any species, whether introduced or not, that is acting to upset things. That species is considered invasive and, if the goal is to restore the natural balance, it must be controlled. To what degree a species should be controlled depends on the degree that it is upsetting things. Ultimately, it comes down to human judgement. Hopefully that judgement is based on the best available evidence, but that isn’t always the case.

But we are getting ahead of ourselves. What I mostly want to accomplish with this post is to introduce the concept of invasive species and point you to a selection of resources to learn more about them. I defined invasive species in a post I wrote back in August 2015, so I will repeat myself here:

“Invasive species” is often used inappropriately to refer to any species that is found outside of its historic native range (i.e. the area in which it evolved to its present form). More appropriate terms for such species are “introduced,” “alien,” “exotic,” “non-native,” and “non-indigenous.” The legal definition of an invasive species (according to the US government) is “an alien species that does or is likely to cause economic or environmental harm or harm to human health.” Even though this definition specifically refers to “alien species,” it is possible for native species to behave invasively.

These terms refer not just to plants but to all living organisms. The term “noxious weed,” on the other hand, is specific to plants. A noxious weed is a plant species that has been designated by a Federal, State, or county government as “injurious to public health, agriculture, recreation, wildlife, or property.” A “weed” is simply a plant that, from a human perspective, is growing in the wrong place, and any plant at any point could be determined to be a weed if a human says so.

Invasive species are easily one of the most popular ecological and environmental topics, and resources about them abound – some more credible than others. Here is a list of places to start:

That should get you started. There are, of course, numerous books on the subject, as well as a number of peer-reviewed journals dedicated to biological invasions. You should also be aware that IUCN maintains a list of the Top 100 World’s Worst Invasive Species and that there is a National Invasive Species Awareness Week, which is quickly approaching. This episode of Native Plant Podcast with Jamie Reaser (executive director of National Invasive Species Council) offers an informative discussion about invasive species, and a search for “invasive species” on You Tube brings up dozens of results including this brief, animated video:

 

I want to believe that we are doing the right thing when we make concerted efforts to remove invasive species and restore natural areas, but I’m skeptical. The reason why I have chosen to spend an indefinite amount of time exploring the topic of invasive species is because I truly want us to get it right. Yet I don’t even know that there is a “right.” It seems to me that there are endless trajectories – each one of them addressing different objectives and producing different outcomes. In a way we are playing God, regardless of which approach we take. We are making decisions for nature as if we know what’s best for it or that there even is a “best.”

Humans have had major impacts on virtually every square inch of the planet and have been placing our fingerprints on every ecosystem we touch since long before we became the humans we are today, and so it is difficult for me to envision a planet sans humans. It is also difficult for me to buy into the idea that our planet should look as though humans haven’t touched it (i.e. pristine). Because we have been touching it – for hundreds of thousands of years. Efforts to rewind time to before introductions occurred or to hold an ecosystem in stasis, securing life for only those species that “belong” there, seem noble yet fanciful at best and misguided, arrogant, and fruitless at worst.

To be the best conservationists we can be, we probably need to find a middle ground regarding invasive species – not a deter and eliminate at all costs approach, but also not a complete surrender/all are welcome and all can stay stance. Somewhere in between seems reasonable, acknowledging that the strategy taken will be different every time based on the location, the species in question, and our objectives. Of course, none of my beliefs or opinions on this topic (or any topic for that matter) are fully formed. I am trying to do my best to maintain an open mind, seeking out the best information available and following the evidence where it takes me. A topic as complex as invasion biology, however, is never going to be easy to finalize one’s opinions on, and so this journey will be boundless. I hope you will join me.

tea-bag

Last but not least, here are two articles that discuss updating our approach to dealing with invasive species:

 

What Is a Plant, and Why Should I Care? part four

What Is a Plant?

Part one and two of this series have hopefully answered that.

Why should you care?

Part three offered a pretty convincing answer: “if it wasn’t for [plants], there wouldn’t be much life on this planet to speak of.”

Plants are at the bottom of the food chain and are a principle component of most habitats. They play major roles in nutrient cycling, soil formation, the water cycle, air and water quality, and climate and weather patterns. The examples used in part three of this series to explain the diverse ways that plants provide habitat and food for other organisms apply to humans as well. However, humans have found numerous other uses for plants that are mostly unique to our species – some of which will be discussed here.

But first, some additional thoughts on photosynthesis. Plants photosynthesize thanks to the work accomplished by very early photoautotrophic bacteria that were confined to aquatic environments. These bacteria developed the metabolic processes and cellular components that were later co-opted (via symbiogensis) by early plants. Plants later colonized land, bringing with them the phenomena of photosynthesis and transforming life on earth as we know it. Single-celled organisms started this whole thing, and they continue to rule. That’s just something to keep in mind, since our focus tends to be on large, multi-cellular beings, overlooking all the tiny, less visible beings at work all around us making life possible.

Current representation of the tree of life. Microorganisms clearly dominate. (image credit: nature microbiology)

Current representation of the tree of life. Microorganisms clearly dominate. (image credit: nature microbiology)

Food is likely the first thing that comes to mind when considering what use plants are to humans. The domestication of plants and the development of agriculture are easily among the most important events in human history. Agricultural innovations continue today and are necessary in order to both feed a growing population and reduce our environmental impact. This is why efforts to discover and conserve crop wild relatives are so essential.

Plants don’t just feed us though. They house us, clothe us, medicate us, transport us, supply us, teach us, inspire us, and entertain us. Enumerating the untold ways that plants factor in to our daily lives is a monumental task. Rather than tackling that task here, I’ll suggest a few starting points: this Wikipedia page, this BGCI article, this Encylopedia of Life article, and this book by Anna Lewington. Learning about the countless uses humans have found for plants over millennia should inspire admiration for these green organisms. If that admiration leads to conservation, all the better. After all, if the plants go, so do we.

Humans have a long tradition of using plants as medicine. Despite all that we have discovered regarding the medicinal properties of plants, there remains much to be discovered. This one of the many reasons why plant conservation is so important. (photo credit: wikimedia commons)

Humans have a long tradition of using plants as medicine. Despite all that we have discovered regarding the medicinal properties of plants, there remains much to be discovered. This is one of the many reasons why plant conservation is imperative. (photo credit: wikimedia commons)

Gaining an appreciation for the things that plants do for us is increasingly important as our species becomes more urban. Our dense populations tend to push plants and other organisms out, yet we still rely on their “services” for survival. Many of the functions that plants serve out in the wild can be beneficial when incorporated into urban environments. Plants improve air quality, reduce noise pollution, mitigate urban heat islands, help manage storm water runoff, create habitat for urban wildlife, act as a windbreak, reduce soil erosion, and help save energy spent on cooling and heating. Taking advantage of these “ecosystem services” can help our cities become more liveable and sustainable. As the environmental, social, and economic benefits of “urban greening” are better understood, groups like San Francisco’s Friends of the Urban Forest are convening to help cities across the world go green.

The importance of plants as food, medicine, fuel, fiber, housing, habitat, and other resources is clear. Less obvious is the importance of plants in our psychological well being. Numerous studies have demonstrated that simply having plants nearby can offer benefits to one’s mental and physical health. Yet, urbanization and advancements in technology have resulted in humans spending more and more time indoors and living largely sedentary lives. Because of this shift, author Richard Louv and others warn about nature deficit disorder, a term not recognized as an actual condition by the medical community but meant to describe our disconnect with the natural world. A recent article in BBC News adds “nature knowledge deficit” to these warnings – collectively our knowledge about nature is slipping away because we don’t spend enough time in it.

The mounting evidence for the benefits of having nature nearby should be enough for us to want to protect it. However, recognizing that we are a part of that nature rather than apart from it should also be emphasized. The process that plants went through over hundreds of millions of years to move from water to land and then to become what they are today is parallel with the process that we went through. At no point in time did we become separate from this process. We are as natural as the plants. We may need them a bit more than they need us, but we are all part of a bigger picture. Perhaps coming to grips with this reality can help us develop greater compassion for ourselves as well as for the living world around us.

Tomato vs. Dodder, or When Parasitic Plants Attack

At all points in their lives, plants are faced with a variety of potential attackers. Pathogenic organisms like fungi, bacteria, and viruses threaten to infect them with diseases. Herbivores from all walks of life swoop in to devour them. For this reason, plants have developed numerous mechanisms to defend themselves against threats both organismal and environmental. But what if the attacker is a fellow plant? Plants parasitizing other plants? It sounds egregious, but it’s a real thing. And since it’s been going on for thousands of years, certain plants have developed defenses against even this particular threat.

Species of parasitic plants number in the thousands, spanning more than 20 different plant families. One well known group of parasitic plants is in the genus Cuscuta, commonly known as dodder. There are about 200 species of dodder located throughout the world, with the largest concentrations found in tropical and subtropical areas. Dodders generally have thread-like, yellow to orange, leafless stems. They are almost entirely non-photosynthetic and rely on their host plants for water and nutrients. Their tiny seeds can lie dormant in the soil for a decade or more. After germination, dodders have only a few days to find host plants to wrap themselves around, after which their rudimentary roots wither up. Once they find suitable plants, dodders form adventitious roots with haustoria that grow into the stems of their host plants and facilitate uptake of water and nutrients from their vascular tissues.

A mass of dodder (Cuscuta sp.) - photo credit: wikimedia commons

A mass of dodder (Cuscuta sp.) – photo credit: wikimedia commons

Some plants are able to fend off dodder. One such instance is the cultivated tomato (Solanum lycopersicum) and its resistance to the dodder species, Cuscuta reflexa. Researchers in Germany were able to determine one of the mechanisms tomato plants use to deter dodder; their findings were published in a July 2016 issue of Science. The researchers hypothesized that S. lycopersicum was employing a similar tactic to that of a microbial invasion. That is, an immune response is triggered when a specialized protein known as a pattern recognition receptor (PRP) reacts with a molecule produced by the invader known as a microbe-associated molecular pattern (MAMP). A series of experiments led the researchers to determine that this was, in fact, the case.

The MAMP was given the name Cuscuta factor and was found “present in all parts of C. reflexa, including shoot tips, stems, haustoria, and, at lower levels, in flowers.” The PRP in the tomato plant, which was given the name Cuscuta receptor 1 (or CuRe 1), reacts with the Cuscuta factor, triggering a response that prohibits C. reflexa access to its vascular tissues. Starved for nutrients, the dodder perishes. When the gene that codes for CuRe 1 was inserted into the DNA of Solanum pennellii (a wild relative of the cultivated tomato) and Nicotiana benthamiana (a relative of tobacco and a species in the same family as tomato), these plants “exhibited increased resistance to C. reflexa infestation.” Because these transgenic lines did not exhibit full resitance to the dodder attack, the researchers concluded that “immunity against C. reflexa in tomato may be a process with layers additional to CuRe 1.”

photo credit: wikimedia commons

photo credit: wikimedia commons

A slew of crop plants are vulnerable to dodder and other parasitic plants, so determining the mechanisms behind resistance to parasitic plant attacks is important, especially since such infestations are so difficult to control, have the potential to cause great economic damage, and are also a means by which pathogens are spread. It is possible that equivalents to CuRe 1 exist in other plants that exhibit resistance to parasitic plants, along with other yet to be discovered mechanisms involved in such resistance, so further studies are necessary. Discoveries like this not only help us make improvements to the plants we depend on for food, but also give us a greater understanding about plant physiology, evolutionary ecology, and the remarkable ways that plants associate with one another.

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