Seed Dispersal via Caching – The Story of Antelope Bitterbrush

Generally speaking, individual plants produce an enormous amount of seeds. This may seem like a huge waste of resources, but the reality is that while each seed has the potential to grow into an adult plant that will one day produce seeds of its own, relatively few may achieve this. Some seeds will be eaten before they get a chance to germinate. Others germinate and soon die from lack of water, disease, or herbivory. Those that make it past the seedling stage continue to face similar pressures. Reaching adulthood, then, is a remarkable achievement.

Antelope bitterbrush is a shrub that produces hundreds of seeds per individual. Each seed is about the size of an apple seed. Some seeds may be eaten right away. Others fall to the ground and are ignored. But a large number are collected by rodents and either stored in burrows (larder hoarding) or in shallow depressions in the soil (scatter hoarding). It is through caching that antelope bitterbrush seeds are best dispersed. When rodents fail to return to caches during the winter, the seeds are free to sprout in the spring. Some of the seedlings will dry out and others will be eaten, but a few will survive, making the effort to produce all those seeds worth it in the end.

Fruits forming on antelope bitterbrush (Purshia tridentata)

Antelope bitterbrush (Purshia tridentata) is in the rose family and is often simply referred to as bitterbrush. It occurs in grasslands, shrub steppes, and dry woodlands throughout large sections of western North America. It is a deciduous shrub that generally reaches between three and nine feet tall but can grow up to twelve feet. It has wedge-shaped leaves that are green on top, grayish on bottom, and three-lobed. Flowers are yellow, strongly fragrant, and similar in appearance to others in the rose family. Flowering occurs mid-spring to early summer. Fruits are achenes – single seeds surrounded by papery or leathery coverings. The covering must rot away or be removed by animals before the seed can germinate.

Bitterbrush is an important species for wildlife. It is browsed by mule deer, pronghorn antelope, bighorn sheep, and other ungulates, including livestock. It provides cover for birds, rodents, reptiles, and ungulates. Its seeds are collected by harvester ants and rodents, its foliage is consumed by tent caterpillars and other insects, and its flowers are visited by a suite of pollinators. For all that it offers to the animal kingdom, it also relies on it for pollination and seed dispersal. The flowers of bitterbrush are self-incompatible, and if it wasn’t for ants and rodents, the heavy seeds – left to rely on wind and gravity – would have trouble getting any further than just a few feet from the parent plant.

Antelope bitterbrush (Purshia tridentata) in full bloom – photo credit: wikimedia commons

In a study published in The American Naturalist (February 1993), Stephen Vander Wall reported that yellow pine chipmunks were the primary dispersal agents of bitterbrush seeds in his Sierra Nevada study area. The optimal depth for seedling establishment was between 10-30 millimeters. Seeds that are cached too near the surface risk being pushed out of the ground during freeze and thaw cycles where they can desiccate upon germination. Cached bitterbrush seeds benefit when there are several seeds per cache because, as Vander Wall notes, “clumps of seedlings are better able to push through the soil and can establish from greater depths than single seedlings.”

Another study by Vander Wall, published in Ecology (October 1994), reiterated the importance of seed caching by yellow pine chipmunks in the establishment of bitterbrush seedlings. Seed caches, which consisted of anywhere from two to over a hundred seeds, were located as far as 25 meters from the parent plant. Cached seeds are occasionally moved to another location, but Vander Wall found that even these secondary caches produce seedlings. Of course, not all of the seedlings that sprout grow to maturity. Vander Wall states, “attrition over the years gradually reduces the number of seedlings within clumps.” Yet, more than half of the mature shrubs he observed in his study consisted of two or more individuals, leading him to conclude that “they arose from rodent caches.”

A study published in the Journal of Range Management (January 1996) looked at the herbivory of bitterbrush seedlings by rodents. In the introduction the authors discuss how “rodents [may] not only benefit from antelope bitterbrush seed caches as a future seed source, but also benefit from the sprouting of their caches as they return to graze the cotyledons of germinating seeds.”  In this study, Ord’s kangaroo rats, deer mice, and Great Basin pocket mice were all observed consuming bitterbrush seedlings, preferring them even when millet was offered as an alternative. The two species of mice also dug up seedlings, possibly searching for ungerminated seeds. Despite seed dispersal via caching, an overabundance of rodents can result in few bitterbrush seedlings reaching maturity.

A cluster of antelope bitterbrush seedlings that has been browsed. “Succulent, young seedlings are thought to be important in the diets of rodents during early spring because of the nutrients and water they contain.” — Vander Wall (1994)

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Photos of antelope bitterbrush seedling clusters were taken at Idaho Botanical Garden, where numerous clusters are presently on display along the pathways of the native plant gardens and the adjoining natural areas. 

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.

The Making of a Kill Jar

I often hear stories from plant lovers about their initial nonchalance concerning plants. The common refrain seems to be that they were fascinated by wildlife and largely ignored plant life until they came to the realization that plants were integral in the lives of animals and play a major role in shaping the environments that support all life. Such an epiphany spawns an insatiable obsession with botany, at least for some people.

I seem to be on the opposite trajectory. It’s not like I have ever really been disinterested in animals; I’ve just been significantly more interested in plants and haven’t bothered to learn much about the animal kingdom (with the exception of entomology). My growing fascination with pollination biology (see last year’s Year of Pollination series) isn’t much of a stretch because insects have always appealed to me, and their intimate interactions with plants are hard to ignore. Ultimately, it is my interest in urban ecology and wildlife friendly gardening that is driving me to learn more about animals.

I started this year off by finally reading Doug Tallamy’s popular book, Bringing Nature Home. Tallamy wrote a lot about birds in his book, which got me thinking more about them. I then discovered Welcome to Subirdia, a book by John Marzluff that explores the diversity of birds that live among us in our urban environments. I then found myself paying more attention to birds. Many bird species rely on insects for food at some point in their lives. Plants regularly interact with insects both in defending themselves against herbivory and in attracting insects to assist in pollination. It’s all connected, and it seems I wouldn’t be much of a botanist then if I didn’t also learn about all of the players involved in these complex interactions.

So, now I’m a birdwatcher and an insect collector. Or at least I’m learning to be. Insects are hard to learn much about without capturing them. They often move quickly, making them hard to identify, or they go completely unnoticed because they are tiny and so well hidden or camouflaged. With the help of a net and a kill jar, you can get a closer look. This not only allows you to determine the species of insects that surround you, but it can also help give you an idea of their relative abundances, their life cycles, where they live and what they feed on, etc.

insect net 2_bw

As the name implies, if you’re using a kill jar, your actions will result in the death of insects. Some people will be more pleased about this than others. If killing insects bothers you, don’t worry, insect populations are typically abundant enough that a few individuals sacrificed for science will not hurt the population in a serious way.

Kill jars can be purchased or they can be made very simply with a few easy to find materials. Start with a glass jar with a metal lid. Mix up a small amount of plaster of paris. Pour the wet plaster in the jar, filling it to about one inch. Allow the plaster to dry completely. This process can be sped up by placing the jar in an oven set on warm. When the plaster is dry, “charge” the jar by soaking the plaster with either ethyl acetate, nail polish remover, or rubbing alcohol. I use nail polish remover because it is cheap and easily accessible. It doesn’t work as quickly as pure ethyl acetate, but it is less toxic. Place a paper towel or something soft and dry in the jar. This keeps the insects from getting beaten up too much as they thrash about. Once the insect is dead, it can be easily observed with a hand lens or a dissecting microscope. It can also be pinned, labeled, and added to a collection.

There are several resources online that describe the process of collecting and preserving insects, including instructions for making an inexpensive kill jar, which is why I am keeping this brief and will instead refer you to a couple of such sites. Like this one from Purdue University’s extension program. It’s directed toward youth, but it includes great information for beginners of any age. This post by Dragonfly Woman is a great tutorial for making a kill jar, and there are several other posts on her blog that are very informative for insect collectors of all experience levels.

I guess you could consider this part of my journey of becoming a naturalist. Perhaps you are on a similar journey. If so, share your thoughts and experiences in the comment section below.

Book Review: Hellstrip Gardening, part two

Hellstrip Gardening by Evelyn J. Hadden is a book intended to help transform roadside beds (or any neglected or hard to garden spot) into a verdant and productive green space. A “paradise,” if you will. Last week, I introduced the concept of hellstrips and briefly discussed the first section of Hadden’s book. This week we are looking at the second section, which is all about the unique challenges and obstacles that hellstrip gardening entails. Hadden has divided this section into 8 main areas of focus. She provides a ton of great information that is sure to be incredibly useful for anyone seriously engaged in improving a hellstrip. If you are one of those people, I highly recommend referring to the book. For simplicity’s sake, this post will include a quick overview of each of the main themes, detailing a few of the things that stood out to me.

Working with Trees

Trees offer many benefits to urban and suburban areas; however, it is not uncommon to see hellstrips with trees that are much too large for the space. Hellstrips are often surrounded by paved surfaces and are heavily trafficked. This leads to soil compaction which results in roots being starved of oxygen and water. Where there are power lines overhead, oversized trees must be heavily pruned to make room for them. Consider planting small or medium sized trees in these spaces. Make sure the soil is well aerated and that there is enough space for the roots to expand out beyond the canopy. Hadden advises avoiding growing turfgrass below trees because it is shallow rooted and uses up much of the available water and oxygen; instead plant deep rooted perennials that naturally grow in wooded environments.

Working with Water

Depending on where you are located, your hellstrip is either going to be water limited or water abundant. Water availability also varies depending on the time of year. If you are mostly water limited, include plants that can tolerate drought conditions. Avoid planting them too close to each other so that they aren’t competing for water. Increase your soil’s water holding capacity by adding organic matter and mulching bare ground. Strategically placed boulders can create cool, moist microclimates where plants can endure hot, dry stretches. If you are dealing with too much water, you can “increase the absorption power” of your property by ensuring that your soil is well aerated and high in organic matter. Plant high water use perennials, grasses, shrubs, and trees with extensive root systems. Replace impermeable surfaces with ground covers and permeable pathways to reduce runoff, and reshape beds so that they collect, hold, and absorb excess runoff.

Working with Poor Soil

Curbside beds in urban areas are notorious for having soil that is compacted, contaminated, and depleted of nutrients. This issue can be addressed by removing and replacing the soil altogether or by heavily amending it. Another solution is to only include plants that can tolerate these harsh conditions. Most likely you will do something in between these two extremes. Adding organic matter seems like the best way to improve soil structure and fertility. Because contaminants from paved surfaces are regularly introduced to curbside gardens, there is a good chance that the soil may contain high levels of lead and other heavy metals. It is a good idea to test the soil before planting edibles. Contaminated soils can be remediated by growing certain plants like annual sunflowers, which take up heavy metals into their tissues. These plants must then be disposed of as hazardous waste.

Common sunflower (Helianthus annuus) is one of several plants that can be used to remediate polluted soils. (photo credit: www.eol.org)

Common sunflower (Helianthus annuus) is one of several plants that can be used to remediate polluted soil (photo credit: www.eol.org)

Working with Laws and Covenants

Regulations and restrictions may prohibit you from creating the hellstrip garden you dream of having. Start by informing yourself of your areas laws and covenants. Some restrictions may be based on public safety (such as restrictions on street trees) while others may be based on outdated ways of thinking. Hadden advises not to assume that a regulation can’t be reversed; however, first you must prepare a well reasoned argument based on facts and evidence. Will your landscape design conserve resources, provide ecological services, improve property values, enhance the neighborhood in some way? Perhaps “your property can model a new landscaping strategy.” Prepare to state your case respectfully, intelligently, and convincingly, and you might just find yourself at the forefront of a new movement.

Living with Vehicles

A garden growing along a roadway is sure to be confronted by vehicles. Hadden suggests using “easily replaceable plants for vulnerable areas.” You can also protect your garden by installing a low fence or wall or by planting sturdy shrubs, prickly plants, or plants that are tall and/or brightly colored. If parking is a regular occurrence, leave room for people to exit their vehicles without trampling the garden. A garden surrounded by paved surfaces will be hotter than other areas on your property, so plant heat tolerant plants or shade the garden with trees and shrubs. A hedge, trellis, fence, or berm can act as a wind and dust break and can help reduce noise. Aromatic plants can help combat undesirable urban smells, and noise can be further masked by water features and plantings that attract songbirds.

Living with Wildlife

Wildlife can either be encouraged or discouraged depending on your preferences. Discouraging certain wildlife can be as simple as “learn[ing] what they need in terms of food and shelter, and then eliminat[ing] it.” A garden full of diverse plant life can help limit damage caused by leaf-eating insects. Encouraging birds and bats can also help control insects. Herbivory by mammals can be reduced by growing a wide array of plants and not over fertilizing or overwatering them. Conversely, encouraging wildlife entails discovering what they like and providing it. For example, to encourage large populations of pollinators, plant a diversity of plants that flower throughout the year and provide nesting sites such as patches of bare ground for ground nesting bees. Keep in mind that your property can be part of a wildlife corridor – a haven for migrating wildlife in an otherwise sea of uninhabitable urban space.

Living with Road Maintenance and Utilities

Curbsides gardens are unique in that they are directly affected by road maintenance and they often must accommodate public utility features like electrical boxes, fire hydrants, street signs, and telephone poles. In areas where salts are applied to roads to reduce ice, hellstrips can be planted with salt tolerant plants and can be deeply watered in order to flush salts down into the soil profile. In areas that receive heavy snowfall, avoid piling snow directly on top of plants. Always call utility companies before doing any major digging to find out where underground pipes and electrical cables are located. Utility features can be masked using shrubs, trellises, and vining plants (especially annual vines that are easily removed and replaced); just be sure to maintain access to them. If your hellstrip consists of “unsightly objects,” Hadden recommends “composing a riveting garden scene to divert attention from an uninspiring view.”

Fire hydrant decorated with ivy (photo credit: wikimedia commons)

Fire hydrant decorated with ivy (photo credit: wikimedia commons)

Living with the Public

Your hellstrip is the most public part of your yard, so you are going to have to learn to share. In order to keep trampling to a minimum and contained to certain areas, make it obvious where pathways are and use berms to raise up the beds. Keep the paths clear of debris and avoid messy fruit and nut trees that can make pathways unfriendly to walk on. Avoid planting rare and valuable plants in your curbside garden. Remember that your hellstrip is typically the first part of your property that people see, so make a good first impression. Also, consider the potential that your public hellstrip garden has for building community and inspiring others.

There is so much more in this section; it is impossible to discuss it all here. Again, if you are serious about improving a hellstrip, get your hands on this book. All hellstrips are different and will have unique challenges. Hadden does a great job of touching on nearly any issue that may arise. Now that we’ve covered challenges and obstacles, next week we will look at designing, building, and managing hellstrip gardens.