native plants

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Eating Weeds: Pineapple Weed

When I wrote about pineapple weed (Matricaria discoidea) last year during the Summer of Weeds, I knew that it was edible but I didn’t bother trying it. Pineapple weed is one of my favorite native weeds (yes, it happens to be a native of northwestern North America). I enjoy its sweet fragrance, its frilly leaves, its “petal”-less flowers, and its diminutive size. I also appreciate its tough nature. Now that I have tried pineapple weed tea, I have another thing to add to this list of pros.

pineapple weed (Matricaria discoidea)

One thing about pineapple weed that always impresses me is its ability to grow in the most compacted soils. It actually seems to prefer them. It is consistently found in abundance in highly trafficked areas, like driveways, parking lots, and pathways, seemingly unfazed by regular trampling. Referring to pineapple weed in one of his books about wildflowers, botanist John Hutchinson wrote, “the more it is trodden on the better it seems to thrive.” This is not something you can say about too many other plants.

Both the leaves and flowers of pineapple weed are edible. The flowers seem to be the more common of the two to consume, generally in tea form. In his book Wild Edible and Useful Plants of Idaho, Ray Vizgirdas writes, “A delicious tea can be made from the dried flowers of the plant. The leaves are edible, but bitter. The medicinal uses of pineapple weed are identical to that of chamomile (Anthemis). Used as a tea it is a carminative, antispasmodic, and mild sedative.” In Wild Urban Plants of the Northeast, Peter Del Tredici writes, “A tea made from the leaves has been used in traditional medicine for stomachaches and colds.”

I harvested my pineapple weed at the end of a dirt parking lot and in an adjacent driveway/pathway. I noted how the pineapple weed’s presence waned as I reached the edges of the parking lot and pathway where, presumably, the ground was less compact. Maybe it has more to compete with there – other weeds – and so it shows up less, or maybe its roots simply “prefer” compact soils. Perhaps a little of both. Once I got my harvest home, I rinsed it off and left it to dry. Later, I snipped off the flower heads and made a tea.

I probably used more water than I needed to, so it was a bit diluted, but it was still delicious. It smelled and tasted a lot like chamomile. Sierra agreed. With a little honey added, it was especially nice. Sierra agreed again. The flowers of pineapple weed can be used fresh or dried. They can also be mixed with other ingredients to make a more interesting tea, like the recipe found here.

If you are hesitant to take the leap into eating weeds, a tea may be the simplest thing you can try. Pineapple weed tea is a great way to ease yourself into it. Apart from maybe having to harvest it from strange places, it probably isn’t much different from other teas you have tried, and, from my experience, it’s delightful.

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When Urban Pollinator Gardens Meet Native Plant Communities

Public concern about the state of bees and other pollinating insects has led to increased interest in pollinator gardens. Planting a pollinator garden is often promoted as an excellent way for the average person to help protect pollinators. And it is! However, as with anything in life, there can be downsides.

In many urban areas, populations of native plants remain on undeveloped or abandoned land, in parks or reserves, or simply as part of the developed landscape. Urban areas may also share borders with natural areas, the edges of which are particularly prone to invasions by non-native plants. Due to human activity and habitat fragmentation, many native plant populations are now threatened. Urban areas are home to the last remaining populations of some of these plants.

Concern for native plant populations in and around urban areas prompted researchers at University of Pittsburgh to review some of the impacts that urban pollinator gardens may have and to develop a “roadmap for research” going forward. Their report was published earlier this year in New Phytologist.

Planting a wildflower seed mix is a simple way to establish a pollinator garden, and such mixes are sold commercially for this purpose. Governmental and non-governmental organizations also issue recommendations for wildflower, pollinator, or meadow seed mixes. With this in mind, the researchers selected 30 seed mixes “targeted for urban settings in the northeastern or mid-Atlantic USA” to determine what species are being recommended for or commonly planted in pollinator gardens in this region. They also developed a “species impact index” to assess “the likelihood a species would impact remnant wild urban plant populations.”

A total of 230 species were represented in the 30 seed mixes. The researchers selected the 45 most common species for evaluation. Most of these species (75%) have generalized pollination systems, suggesting that there is potential for sharing pollinators with remnant native plants. Two-thirds of the species had native ranges that overlapped with the targeted region; however, the remaining one-third originated from Europe or western North America. The native species all had “generalized pollination systems, strong dispersal and colonization ability, and broad environmental tolerances,” all traits that could have “high impacts” either directly or indirectly on remnant native plants. Other species were found to have either high dispersal ability but low chance of survival or low dispersal ability but high chance of survival.

This led the researchers to conclude that “the majority of planted wildflower species have a high potential to interact with native species via pollinators but also have the ability to disperse and survive outside of the garden.” Sharing pollinators is especially likely due to super-generalists like the honeybee, which “utilizes flowers from many habitat types.” Considering this, the researchers outlined “four pollinator-mediated interactions that can affect remnant native plants and their communities,” including how these interactions can be exacerbated when wildflower species escape gardens and invade remnant plant communities.

photo credit: wikimedia commons

The first interaction involves the quantity of pollinator visits. The concern is that native plants may be “outcompeted for pollinators” due to the “dense, high-resource displays” of pollinator gardens. Whether pollinator visits will increase or decrease depends on many things, including the location of the gardens and their proximity to native plant communities. Pollinator sharing between the two has been observed; however, “the consequences of this for effective pollination of natives are not yet understood.”

The second interaction involves the quality of pollinator visits. Because pollinators are shared between native plant communities and pollinator gardens, there is a risk that the pollen from one species will be transferred to another species. High quantities of this “heterospecific pollen” can result in reduced seed production. “Low-quality pollination in terms of heterospecific pollen from wildflower plantings may be especially detrimental for wild remnant species.”

The third interaction involves gene flow between pollinator gardens and native plant communities. Pollen that is transferred from closely related species (or even individuals of the same species but from a different location) can have undesired consequences. In some cases, it can increase genetic variation and help address problems associated with inbreeding depression. In other cases, it can introduce traits that are detrimental to native plant populations, particularly traits that disrupt adaptations that are beneficial to surviving in urban environments, like seed dispersal and flowering time. Whether gene flow between the two groups will be positive or negative is difficult to predict, and “the likelihood of genetic extinction versus genetic rescue will depend on remnant population size, genetic diversity, and degree of urban adaptation relative to the planted wildflowers.”

The fourth interaction involves pathogen transmission via shared pollinators. “Both bacterial and viral pathogens can be transmitted via pollen, and bacterial pathogens can be passed from one pollinator to another.” In this way, pollinators can act as “hubs for pathogen exchange,” which is especially concerning when the diseases being transmitted are ones for which the native plants have not adapted defenses.

photo credit: wikimedia commons

All of these interactions become more direct once wildflowers escape gardens and establish themselves among the native plants. And because the species in wildflower seed mixes are selected for their tolerance of urban conditions, “they may be particularly strong competitors with wild remnant populations,” outcompeting them for space and resources. On the other hand, the authors note that, depending on the species, they may also “provide biotic resistance to more noxious invaders.”

All of these interactions require further investigation. In their conclusion, the authors affirm, “While there is a clear potential for positive effects of urban wildflower plantings on remnant plant biodiversity, there is also a strong likelihood for unintended consequences.” They then suggest future research topics that will help us answer many of these questions. In the meantime, pollinator gardens should not be discouraged, but the plants (and their origins) should be carefully considered. One place to start is with wildflower seed mixes, which can be ‘fine-tuned’ so that they benefit our urban pollinators as well as our remnant native plants. Read more about plant selection for pollinators here.

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Managing Spontaneous Urban Plants for Improved Aesthetics

As discussed last week, our wild, urban flora is a cosmopolitan mixture of plants that were either native to the area before it was developed, introduced from all corners of the world on purpose or by accident, or brought in by migrating wildlife. These are plants capable of establishing and sustaining themselves outside of human cultivation and management, and are found in abundance beyond the borders of our tidy gardens and manicured landscapes. They vegetate sectors of our city that have been abandoned, overlooked, or routinely neglected. Given enough time – and prolonged lack of intervention – such vegetation will proceed along the process of ecological succession in the same way that plant communities in natural areas do. And just like other plant communities within their respective ecosystems, these wild, urban plant communities provide a suite of ecological services vital to the health of our urban ecosystems.

Peter Del Tredici writes in Wild Urban Plants of the Northeast, “landscapes that include spontaneous vegetation fit the technical definition of sustainable in the sense that they are adapted to the site, require minimal maintenance, and are ecologically functional.” In an interview with Scenario Journal, Del Tredici goes on to define sustainability as “the value of the services provided by the ecosystem divided by the cost required to maintain that ecosystem.” Spontaneous urban landscapes offer “substantial ecological services at relatively low cost, or in some cases no cost,” and thus, by Del Tredici’s definition, they are “highly sustainable.”

There is one unfortunate downside – “weedy” landscapes like this are, by popular opinion, thoroughly unattractive and a sign of urban decay. This belief is held in spite of the fact that many of the plants found therein would be cherished or admired in other settings. Among deteriorating infrastructure, litter, and less attractive plants, some of our favorite plants are rendered guilty by association.

Despite their ecological benefits, abandoned areas vegetated with wild, urban plants are not favored by the public. So, to appease our aesthetic standards, sites like this can be enhanced through minimal intervention to be more attractive while retaining their ecological functions. In a paper published in a 2006 issue of Journal of Landscape Architecture, Norbert Kühn asserts that “to use spontaneous vegetation for ornamental purposes, a kind of enhancement or design work is necessary.” Species can be added and removed, and simple, infrequent maintenance measures can be implemented. Examples include extending the flowering season with spring flowering bulbs and mowing the area once or twice annually to maintain and improve the composition of the stand.

Wild bergamot (Monarda fistulosa) – one of the plants that Norbert Kühn included in his study as a candidate for improving the aesthetics of spontaneous, urban plant communities.

Favoring attractive weeds over less attractive ones and using minimal maintenance to improve aesthetics and function are the principles behind Del Tredici’s “cosmopolitan urban meadow.” In his book, he lists some criteria for plants that would be suitable for “this novel landscape form,” including: erosion control (long-lived; vegetatively spreading), stress tolerance (full sun; drought; compacted and polluted soil), aesthetic value (ornamental characteristics; not “weedy” looking), wildlife friendly (attractive to pollinators; edible seeds), and commercially available.

In an article in Harvard Design Magazine, Del Tredici and Michael Luegering describe the cosmopolitan urban meadow as “a stable assemblage of stress-tolerant, low-maintenance herbaceous perennial plants that are preadapted to harsh urban conditions and that will provide an attractive vegetation cover on vacant land.” Whether it is a “long-term landscape feature” or a placeholder until future development, it will have “the capacity to increase the aesthetic and ecological value of vacant land without the investment of large sums of money typically required for the installation and maintenance of traditional managed landscapes.”

Abandoned or undeveloped, urban lots like this one are ideal sites for “cosmopolitan urban meadows.”

In an urban context, some plant species are particularly noxious and may need to be removed from urban meadows, such as ragweed (Ambrosia spp.) for its allergens and poison ivy (Toxicodendron radicans) for its Urushiol-induced contact dermatitis. Species with a history of being invasive should also be avoided and contained, particularly in sites that are adjacent to or within a short distance from natural areas. Despite this and other minor concerns, spontaneous vegetation has great potential. In Kühn’s words it is “authentic” and a “reminder of the history of the site,” it is part of “the natural dynamic” with potential to bring us “closer to nature,” and finally, “it can be maintained for a long time [with] less care and low costs.”

Finding beauty in these urban, wild landscapes might even cause a shift in what we find appropriate for cultivated landscapes. In her book, Grow Curious, Gayla Trail reminds us that, despite all of our efforts, wildness persists even in our most earnest attempts to subdue it. Perhaps we should embrace it:

‘Wild’ and ‘cultivated’ are social constructs that we place in opposition to each other, when in reality there is a knotty labyrinth between them. We subjugate our cities and our gardens with chemicals and artifice because we are unable to see that wild and cultivated can be entwined, can be all at once tended, lyrical, surprising, domesticated, irrational, functional, and free.

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See Also: Arnold Arboretum’s Cosmopolitan Meadow at Weld Hill

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When Sunflowers Follow the Sun

Tropisms are widely studied biological phenomena that involve the growth of an organism in response to environmental stimuli. Phototropism is the growth and development of plants in response to light. Heliotropism, a specific form of phototropism, describes growth in response to the sun. Discussions of heliotropism frequently include sunflowers and their ability to “track the sun.” This conjures up images of a field of sunflowers in full bloom following the sun across the sky. However cool this might sound, it simply doesn’t happen. Young sunflowers, before they bloom, track the sun. At maturity and in bloom, the plants hold still.

What is happening in these plants is still pretty cool though, and a report published in an August 2016 issue of Science sheds some light on the heliotropic movements of young sunflowers. They begin the morning facing east. As the sun progresses across the sky, the plants follow, ending the evening facing west. Over night, they reorient themselves to face east again. As they reach maturity, this movement slows, and most of the flowers bloom facing east. Over a series of experiments, researchers were able to determine the cellular and genetic mechanisms involved in this spectacular instance of solar tracking.

Helianthus annuus (common sunflower) is a native of North America, sharing this distinction with dozens of other members of this recognizable genus. It is commonly cultivated for its edible seeds (and the oil produced from them) as well as for its ornamental value. It is a highly variable species and hybridizes readily. Wild populations often cross with cultivated ones, and in many instances the common sunflower is considered a pesky weed. Whether crop, wildflower, or weed, its phototropic movements are easy to detect, making it an excellent subject of study.

Researchers began by tying plants to stakes so that they couldn’t move. Other plants were grown in pots and turned to face west in the morning. The growth of these plants was significantly stunted compared to plants that were not manipulated in these ways, suggesting that solar tracking promotes growth.

The researchers wondered if a circadian system was involved in the movements, and so they took sunflowers that had been growing in pots in a field and placed them indoors beneath a fixed overhead light source. For several days, the plants continued their east to west and back again movements. Over time, the movements became less detectable. This and other experiments led the researchers to conclude that a “circadian clock guides solar tracking in sunflowers.”

Another series of experiments helped the researchers determine what was happening at a cellular level that was causing the eastern side of the stem to grow during the day and the western side to grow during the night. Gene expression and growth hormone levels differed on either side of the stem depending on what time of day it was. In an online article published by University of California Berkeley, Andy Fell summarizes the findings: “[T]here appear to be two growth mechanisms at work in the sunflower stem. The first sets a basic rate of growth for the plant, based on available light. The second, controlled by the circadian clock and influenced by the direction of light, causes the stem to grow more on one side than another, and therefore sway east to west during the day.”

The researchers observed that as the plants reach maturity, they move towards the west less and less. This results in most of the flowers opening in an eastward facing direction. This led them to ask if this behavior offers any sort of ecological advantage. Because flowers are warmer when they are facing the sun, they wondered if they might see an increase in pollinator visits during morning hours on flowers facing east versus those facing west. Indeed, they did: “pollinators visited east-facing heads fivefold more often than west-facing heads.” When west-facing flowers where warmed with a heater in the morning, they received more pollinator visits than west-facing flowers that were not artificially warmed, “albeit [still] fewer than east-facing flowers.” However, increased pollinator visits may be only part of the story, so further investigations are necessary.

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I’m writing a book about weeds, and you can help. For more information, check out my Weeds Poll.

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Summer of Weeds: Willowherbs and Fireweed

Last week we discussed a plant that was introduced as an ornamental and has become a widespread weed. This week we discuss some native plants that have become weedy in places dominated by humans. Similar to pineapple weed, species in the genus Epilobium have moved from natural areas into agricultural fields, garden beds, and other sites that experience regular human disturbance. Some species in this genus have been deliberately introduced for their ornamental value, but others have come in on their own. In all cases the story is similar, humans make room and opportunistic plants take advantage of the space.

Epilobium species number in the dozens and are distributed across the globe. North America is rich with them. They are commonly known as willowherbs and are members of the evening primrose family (Onagraceae). They are herbaceous flowering plants with either annual or perennial life cycles and are commonly found in recently disturbed sites, making them early successional or pioneer species. Many are adapted to wet soils and are common in wetlands and along streambanks; others are adapted to dry, open sites. Hybridization occurs frequently among species in the Epilobium genus, and individual species can be highly variable, which may make identifying them difficult.

northern willowherb (Epilobium ciliatum)

At least two North American species are commonly weedy: E. ciliatum (northern willowherb) and E. brachycarpum (panicled willowherb). Regarding these two species, the IPM website of University of California states: “Willowherbs are native broadleaf plants but usually require a disturbance to establish. Although considered desirable members of natural habitats, they can be weedy in managed urban and agricultural sites.” The field guide, Weeds of the West, refers to E. brachycarpum as a “highly variable species found mostly on non-cultivated sites, and especially on dry soils and open areas.” E. ciliatum is notorious for being a troublesome weed in greenhouses and nurseries, as discussed on this Oregon State University page.

E. ciliatum is a perennial that reproduces via both rhizomes and seeds. It reaches up to five feet tall and has oppositely arranged, lance-shaped leaves with toothed margins that are often directly attached to the stems. Its flowers are tiny – around a quarter of an inch wide – and white, pink, or purple with four petals that are notched at the tip. They sit atop a skinny stalk that is a few centimeters long, which later becomes the fruit. When dry, the fruit (or capsule) splits open at the top to reveal several tiny seeds with tufts of fine hairs.

northern willowherb (Epilobium ciliatum)

E. brachycarpum is an annual that reaches up to three feet tall and is highly branched. Its leaves are short and narrow and mostly alternately arranged. Its flowers and seed pods are similar to E. ciliatum. At first glance it can appear as one of many weeds in the mustard family; however, the tuft of hairs on its seeds distinguishes it as a willowherb.

Seeds and seed pods of panicled willowherb (Epilobium brachycarpum)

Weeds of North America by Richard Dickinson and France Royer describes one weedy species of willowherb that was introduced to North America from Europe – E. hirsutum. It is commonly referred to as great hairy willowherb, but some of its colloquial names are worth mentioning: fiddle grass, codlins and cream, apple-pie, cherry-pie, blood vine, and purple rocket. Introduced as an ornamental in the mid 1800’s, it is a semiaquatic perennial that can reach as tall as eight feet. It has small, rose-purple flowers and is frequently found growing in wetlands along with purple loosestrife (Lythrum salicaria).

Chamerion angustifolium – which is synonymously known as Epilobium angustifolium and commonly called fireweed – is distributed throughout temperate regions of the Northern Hemisphere. It is a rhizomatously spreading perennial that grows to nine feet tall; has lance-shaped, stalkless leaves; and spikes of eye-catching, rose to purple flowers. It is a true pioneer species, found in disturbed sites like clear-cuts, abandoned agricultural fields, avalanche scars, and along roadsides. It gets its common name for its reputation of being one of the first plants to appear after a fire, as John Eastman describes in The Book of Field and Roadside: “A spring fire may result in a profusion of growth as soon as 3 months afterward, testifying to fireweed’s ample seed bank in many wilderness areas.” Eastman goes on to write, “fireweed’s flush of abundance following fire may rapidly diminish after only a year or two of postburn plant growth.” This “flush of abundance” is what gives it its weedy reputation in gardens. With that in mind, it is otherwise a welcome guest thanks to its beauty and its benefit to pollinators.

fireweed (Chamerion angustifolium)

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Quote of the Week:

From the book Food Not Lawns by H.C Flores

Sometimes [weeding] feels like playing God – deciding who lives and who dies is no small matter – and sometimes it feels like war. … Take a moment to ponder the relationship of these plants to other living things around, now and in the future. Your weeds provide forage and habitat for insects, birds, and animals, as well as shelter for the seedlings of other plants. They cover the bare soil and bring moisture and soil life closer to the surface, where they can do their good work. Weeds should be respected for their tenacity, persistence, and versatility and looked upon more as volunteers than as invaders.

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Summer of Weeds: Pineapple Weed

“The spread of the fruitily perfumed pineapple weed, which arrived in Britain from Oregon in 1871, exactly tracked the adoption of the treaded motor tyre, to which its ribbed seeds clung as if they were the soles of small climbing boots.” – Richard Mabey, Weeds: In Defense of Nature’s Most Unloved Plants

Can a plant that is native to North America be considered a weed in North America? Sure. If it is acting “weedy” according to whatever definition we decide to assign to the word, then why not? Can “weeds” from North America invade Europe the same way that so many “weeds” from there have invaded here? Of course! Pineapple weed is just one such example.

Native to western North America and northeastern Asia, this diminutive but tough annual plant in the aster family can now be found around the globe. Matricaria discoidea gets its common name from the distinctive fruity scent it gives off when its leaves and flowers are crushed. Its scent is not deceptive, as it is yet another edible weed (see Eat the Weeds). Teas made from its leaves have historically been used to treat upset stomachs, colds, fevers, and other ailments.

pineapple weed (Matricaria discoidea)

Pineapple weed reaches as few as a couple centimeters to a little over a foot tall. Its leaves are finely divided and fern-like in appearance. Its flower heads are cone or egg-shaped, yellow-green, and cupped in light-colored, papery bracts. The flower heads lack ray florets and are composed purely of tightly packed disc florets. The fruits (i.e. seeds) are tiny, ribbed achenes that lack a pappus.

Compacted soils are no match for pineapple weed. It is often seen growing in hard-packed roadways and through small cracks in pavement, and it is undeterred by regular trampling. It is a master of disturbed sites and is commonly found in home gardens and agriculture fields. It flowers throughout the summer and is often confused with mayweed (Anthemis cotula); the telltale difference is that mayweed gives off a foul odor when crushed.

Meriwether Lewis collected pineapple weed along the Clearwater River during the Lewis and Clark Expedition. In their book, Lewis and Clark’s Green World, Scott Earle and James Reveal write, “There is nothing in the expedition’s journals about the plant, but it would seem that there was little reason for Lewis to collect the two specimens that he brought back other than for its ‘agreeable sweet scent.’ It is otherwise an unremarkable, rayless member of the aster family.” The authors continue their mild ribbing with this statement: “The pineapple weed deserves its appellation, for it is a common weed – although a relatively innocuous one – that grows in disturbed places, along roadsides, and as an unwanted garden guest.”

pineapple weed (Matricaria discoidea) – photo credit: wikimedia commons

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Quote of the Week:

From Weeds and What They Tell (ed. 1970) by Ehrenfried Pfeiffer

“Weeds are WEEDS only from our human egotistical point of view, because they grow where we do not want them. In Nature, however, they play an important and interesting role. They resist conditions which cultivated plants cannot resist, such as drought, acidity of soil, lack of humus, mineral deficiencies, as well as a one-sidedness of minerals, etc. They are witness of [humanity’s] failure to master the soil, and they grow abundantly wherever [humans] have ‘missed the train’ – they only indicate our errors and Nature’s corrections. Weeds want to tell a story – they are natures way of teaching [us] – and their story is interesting. If we would only listen to it we could apprehend a great deal of the finer forces through which Nature helps and heals and balances and, sometimes, also has fun with us.”

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When Alien Plants Invade – The Four Stages of Invasion, part two

In a review published in New Phytologist (2007), Kathleen Theoharides and Jeffrey Dukes examine four stages of invasion as they relate to alien (i.e. introduced or non-indigenous) plant species. In part one we discussed transport and colonization, in which species must survive being transported long distances and then take root and reach maturity in an unfamiliar location. Alien plant species don’t become invasive until they have reached the last two stages: establishment and landscape spread. Removal of the species upon reaching these stages is no easy task. Luckily, introduced species have a few barriers to overcome before this point.

An established population is one that is self-sustaining and expanding. Environmental conditions may be a limiting factor, as they were during colonization, but the main constraints at this stage are “biotic filters.” Theoharides and Dukes define these as “barriers to invasion created by the actions or presence of living organisms.” Through competition for various resources, as well as herbivory and disease, neighboring organisms affect the survival, growth, and reproduction of introduced plant species. Thus, “traits that enhance competitive performance, reduce niche overlap between [introduced species] and natives, or increase enemy resistance may be most important during establishment… Other advantageous traits include secondary chemical compounds that deter herbivores, ‘novel weapons’ such as root exudates that negatively impact other plants, fast growth, and high fecundity.”

Plants compete for light, moisture, and soil nutrients, as well as for pollinators and seed dispersers. Competition inhibits the establishment of invaders when neighboring plants consume available resources more efficiently. Introduced plants risk being outcompeted by plants that are of the same functional type (plants that are “morphologically, phenologically, and physiologically similar”). They also risk competition by a single dominant species (or group of similar species) or by “an assemblage of species with different traits.” As a general rule, plant communities with greater diversity are more resistant to invasion.

“In forests of the northeastern USA, Alliaria petiolata, an herbaceous mustard species, contains a type of phytotoxic glucosinolate that appears to disrupt the mutualism between arbuscular mycorrhizal fungi and hardwood canopy trees.” – – Theoharides and Dukes (2007) [photo credit: eol.org]

Two hypotheses postulate the success of some plant invaders in establishing themselves: the enemy release hypothesis and the evolution of increased competitive ability hypothesis. In the first hypothesis, plant species – having been removed from their native habitat – are freed from their natural enemies and are thus able to allocate more resources to growth and reproduction. The second hypothesis states that, in light of “reduced enemy pressure,” introduced species quickly evolve to allocate resources “from enemy defense to faster growth.” Escape from herbivory and diseases, however, is likely not the only factor in the success of invaders, and much still depends on the competitiveness of the plant and the availability of key resources.

After introduced plants become established, a lag phase generally occurs before landscape spread. This can be a result of a lack of genetic variation, a dearth of suitable habitat, unfavorable environmental conditions, or some combination of the three. New introductions may occur, and the population may continue to adapt and expand. Suitable habitat may be made available, and environmental conditions may shift. In time, landscape spread becomes a possibility.

Landscape spread occurs when multiple populations of a species are connected via long-distance dispersal. At this “metacommunity scale,” populations of an introduced plant species interact across a large area, with each population in a different stage of colonization and establishment. This means that transport, colonization, and establishment are all at play during the landscape spread stage.

“Abutilon theophrasti (velvetleaf) was originally introduced before 1700 in the USA. This species has only recently become an aggressive invader as a result of the evolution of different life-history strategies based on the nature of competition in its new environment.” — Theoharides and Dukes (2007) [photo credit: wikimedia commons]

Dispersal ability and habitat connectivity are key factors in determining the success of an introduced plant species during landscape spread. Long-distance dispersal can occur via wind, water, or animals. Species that depend on animals to spread their seeds rely on specific animals to be present. The seeds of Prunus serotina (black cherry), for example, are dispersed by birds. So, landscape spread is reliant on birds and “roosting trees” where the birds can perch and defacate the seeds. In many cases, “humans also play a large role in intraregional dispersal.”

Habitats vary across the landscape due to a combination of numerous geological and biological processes. The disturbance regime – “the frequency, spatial extent, severity, and intensity of killing events over time” – also helps determine landscape patterns. Natural disturbances, such as fire, weather, and natural disasters, are differentiated from disturbances caused by human activity. Large scale development and disturbance of natural areas by humans disrupts the natural disturbance regime and alters historical landscape patterns. As the authors write, “alterations of the disturbance regime that increase resource availability or change landscape patterns can promote non-indigenous plant species spread by creating favorable patches for colonization and establishment.”

Fragmented landscapes consisting of small patches of natural areas dispersed among large areas of human development are particularly prone to invasion by introduced plant species for many reasons, including increased influx of propagules and a high degree of edge effects (habitat edges have environmental conditions that are generally more prone to invasion than habitat interiors).

Habitat patches can be connected via corridors. It is through these corridors that dispersal can occur between populations in a metacommunity. Corridors connect populations of both introduced and native plant species. However, “native plants often require wide undisturbed corridors of intact habitat, while [introduced plant species] may disperse best through strips of human-disturbed habitat or ‘disturbance corridors.'” The environmental conditions in disturbance corridors and the presence of dispersal agents (including humans and domesticated animals) help facilitate the connectivity of populations of introduced plant species and promote the colonization and establishment of new populations.

In their abstract, Theoharides and Dukes write, “both research and management programs may benefit from employing multiscale and stage approaches to studying and controlling invasion.” With their conclusion they provide a list of potential management strategies for each stage, and they advise employing “natural filters in order to prevent invasion succees.” Examples include reducing habitat fragmentation and edge effects, promoting intact native communities, reducing human disturbances, promoting natural disturbance regimes, and minimizing disturbance corridors.

More Posts about Invasive Species:

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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.

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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?

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What Do Desert Tortoises Eat?

Desert conditions are not intuitively conducive to life. In many regards they are extreme. Blistering, bleak, dry, and barren. The desert is a place unsuited for the faint of heart and the ill-equipped. Broadly speaking, life in the desert is reliant on one of two things: technology or evolutionary adaptation. Like many species native to desert environments, the desert tortoise employs the latter. It is at home in the desert because it evolved there. That is not to say that life is always easy for the desert tortoise and species like it, but it is possible, thanks to hundreds of thousands of years of making it work. As John Alcock puts it in, Sonoran Desert Spring, “the tortoise will deal with its environment through evolved design rather than seek to deny the desert its due.”

Perhaps it is because the desert is such a harsh environment, requiring finely tuned adaptations for survival, that sweeping changes can put resident species in peril – threatening their long-term existence. The desert tortoise is an example of this. Since 1990, Gopherus agassizii has found itself listed as threatened under the U.S. Endangered Species Act [it is categorized as vulnerable by the IUCN] due to significant population declines and loss of habitat. Getting there did not happen overnight, and it is impossible to pinpoint a sole cause of the tortoise’s decline. Instead, a suite of things have conspired against it, making it difficult to decide on the best route towards conservation.

In an October 2012 issue of BioScience, Averill-Murray, et al. enumerate some of the human-medaited threats that act both simultaneously and synergistically against desert tortoise populations:

Habitat conversion occurs as a result of urban development, mining, waste disposal, energy development, and road construction. Habitat modification is caused by military training, off-highway vehicle use, utility corridors, livestock grazing, and the proliferation of invasive plants. … Direct losses of tortoises also occur through predation, disease, collection from the wild, and recreational killing.

Apart from climate change, which is projected to substantially reduce the historical range of the desert tortoise in the coming years, the proliferation of introduced grasses is particularly disconcerting. Such grasses tend to increase wildfire frequency in areas where wildfire is historically rare and the native flora is ill-adapted to frequent fire. This can alter plant communities in a way that favors introduced plants over plants native to the region.

Desert Tortoise (Gopherus agassizii) - photo credit: wikimedia commons

Desert Tortoise (Gopherus agassizii) – photo credit: wikimedia commons

The desert tortoise is the largest terrestrial turtle in the United States, measuring up to 15 inches long and weighing up to 15 pounds. Their carapaces are generally dull brown or gray, although those of young tortoises may have orange markings. Their limbs are stocky and elephantine, and their front legs are shovel-like and equipped with claws for digging. They reach sexual maturity between ages 15-20, generally living for at least 35 years and as many as 50-100 years.

Desert tortoises are distributed throughout the Mohave and Sonoran Deserts of southeastern United States and into the Sonaron Desert and Sinaloan foothills of northwestern Mexico. Their habitat varies widely across their range. In general, tortoises prefer sites where the soil is loamy and easy to dig as they spend much of their time in underground dens; however, they also occur in rocky foothills where shelter can be found among the rocks. In the Mohave Desert, they are commonly found in plant communities that are dominated by creosote bush (Larrea tridentata), which they use for shade and an occasional food source.

Creasote Bush (Larrea tridentata) - photo credit: wikimedia commons

Creosote Bush (Larrea tridentata) – photo credit: wikimedia commons

Recently the species known as Gopherus agassizii was determined to consist of at least two (possibly four) distinct species. Desert tortoises that occur north and west of the Colorado River have retained the scientific name G. agassizii and are commonly referred to as Agassiz’s desert tortoise. Desert tortoises occurring east of the Colorado river have been given the name G. morafkai, commonly known as Morafka’s desert tortoise. In light of this, G. agassizii may find itself uplisted to endangered, as its range has been reduced to about 30% of its former self and its southern cousins can no longer be considered a genetic reservoir.

Seeing that desert tortoises have plenty of the right foods to eat ensures their immediate survival and holds them back from the precipice of extinction. The question, “What does a desert tortoise eat?,” was what peaked my curiosity in this subject to begin with. I knew they were herbivores (for the most part), so I assumed they must have a favorite food – something that composed the majority of their diet. Finding an answer to this question led me down a rabbit hole [or should I say a tortoise hole? Some tortoise dens can extend 30 feet or more into the banks of desert dry washes.] that led me to discover the complexity of these creatures. It turns out, there is no easy answer to my initial question. What a desert tortoise eats depends on where in its expansive range it resides, what time of year it is, what plants are available in a particular year, whether or not it’s a drought year, etc.

The desert tortoise is “one of the most studied reptiles in the world,” so hundreds of observations have been made, leading to dozens of reports and studies that examine the diet of the desert tortoise; however, the results are highly variable. Due to such variability, this fact sheet from the San Diego Zoo states matter-of-factly, “an ‘average’ tortoise diet [is] hard to characterize.” But let’s try.

The desert tortoise emerges from its winter den in early spring. At the same time, annual wildflowers are also emerging, taking advantage of warming temperatures and rare soil moisture accumulated during winter precipitation. This is the desert tortoise’s preferred banquet. Because there will be little water available the rest of the year, desert tortoises hydrate themselves mainly through the plants they eat. The lush stems, leaves, and flowers of annual wildflowers provide both nutrients and the water necessary to sustain themselves throughout much of the year and aid in their growth and reproduction.

As spring turns to summer, the tortoises switch to eating herbaceous perennials and grasses. By this point, both introduced annual grasses and native perennial bunchgrasses are drying up, but tortoises are still able to extract some nutrients and moisture by eating their dry stems and leaves. Cactus pads and fruits (particularly those in the genus Opuntia) as well as young leaves of shrubs also help tortoises subsist through the long, hot summers, which are mostly spent deep in their dens away from predators and the blistering heat.

A paper published in a March 1986 issue of Herpetologica follows a group of tortoises over the period of a year and makes a number of lifestyle observations, including their diet. The authors noted that much of their diet consisted of two annual wildflowers (Camissonia munzii and Langloisia setosissima), a perennial bunchgrass (Achnatherum hymenoides), and a non-native annual grass (Bromus rubens). A paper published in a 2010 issue of Journal of Herpetology compared the nutritional quality of four plant species commonly consumed by desert tortoises: a native and non-native grass (Achnatherum hymenoides and Schismus barbatus) and a native and non-native annual forb (Malacothrix glabrata and Erodium cicutarium). They found little difference between the native and non-native species in either catagory, but determined that the forbs were significantly more nutritious than the grasses, which lead them to recommend managament practices that would increase the availability of forbs (regardless of provenance) in tortoise habitat. Numerous studies have documented the frequent consumption of introduced plant species by desert tortoises.

Redstem stork's bill (Erodium cicutarium) is an introduced species commonly consumed by desert tortoises - image credit: wikimedia commons

Redstem stork’s bill (Erodium cicutarium) is an introduced species commonly consumed by desert tortoises – image credit: wikimedia commons

For me, one of the most interesting things to learn was the variety of “non-food” items that tortoises may consume. Tortoises are often observed eating soil and rocks, and are also known to eat bones, arthropods, feces, feathers, hair, and egg shells. The rocks are thought to act as a gastrolith, aiding in digestion. The other items may help supplement minerals and nutrients the tortoises are lacking in their plant-based diet, particularly calcium which is greatly needed for growth and reproduction. Shockingly, a report that appeared in a 2007 issue of The Southern Naturalist details incidences of tortoises eating the skeletal remains of other tortoises.

Desert tortoises are an engrossing subject of study, and so much more could be said about them. For now, I leave you with this passage from Alcock’s book:

To see a tortoise with wrinkled neck and solemn eyes, moving like an animated rock, is an essential part of the experience of the desert. The removal of even a single adult extinguishes a presence that was meant to persist for years to come and snuffs out a prehistoric spark of life in a spartan environment where life, so hard-won, should be celebrated.

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