The Serotinous Cones of Lodgepole Pine

Behind the scales of a pine cone lie the seeds that promise future generations of pine trees. Even though the seeds are not housed within fruits as they are in angiosperms (i.e. flowering plants), the tough scales of pine cones help protect the developing seeds and keep them secure until the time comes for dispersal. In some species, scales open on their own as the cone matures, at which point winged seeds fall from the tree, taking flight towards their new homes. In other species, the scales must be pried open by an animal in order to free the seed. A third group of species have what are called serotinous cones, the scales of which are sealed shut with resin. High temperatures are required to soften the resin and expose the seeds.

Serotinous cones are a common trait of pine species located in regions where wildfire naturally and regularly occurs. One such species is lodgepole pine (Pinus contorta), which is found in abundance in forests across much of western North America. Lodgepole pine is a thin-barked tree species that burns easily and is often one of the first plants to recolonize after a stand-replacing wildfire. There are 3 or 4 subspecies of lodgepole pine. The one with the largest distribution and the one that most commonly exhibits serotinous cones is P. contorta subsp. latifolia, which occurs throughout the Rocky Mountains, north into the Yukon, and just west of the Cascade Range.

needles of lodgpole pine (Pinus contorta)

Lodgepole pine grows tall and straight, generally maxing out at around 80 feet tall. Its needles are about two and a half inches long, are borne in bundles of two, and tend to twist away from each other, which is one explanation for the specific epithet, contorta. Its cones are egg-shaped with asymmetrical bases, measuring less than two inches long with prickly tips at the ends of each scale. The seeds of lodgepole pine are tiny with little, papery wings that aid in dispersal. The cones can remain attached to the tree for 15-20 years (sometimes much longer), and the seeds remain viable for decades. In non-serotinous cones, the scales start opening on their own in early autumn. Serotinous cones require temperatures of 45-50°C (113-122°F), to release the resin bond between the scales. Some cones that happen to fall from the tree can open when exposed to particularly warm temperatures on the ground. Otherwise, it takes fire to free the seeds.

Serotinous cones aren’t a guarantee, and the percentage of trees with serotinous cones compared to those with non-serotinous cones varies widely across the range of lodgepole pine, both in space and in time. One reason for this is that trees with serotinous cones don’t develop them until they reach a certain age, generally around 20-30 years old, or perhaps as old as 50 or 60. The cones of young trees are all non-serotinous. But some trees never develop serotinous cones at all. Serotiny is a genetic trait, and there are various factors that either select for or against it. A number of factors are at play simultaneously over the life of a tree and across a population of trees, so it is difficult to determine exactly why the percentage of serotinous cones is so variable across the range of the species. What follows are a few potential explanations for this phenomenon.

closed cone of lodgepole pine (Pinus contorta)

As a fire-adapted, pioneer species, lodgepole pine has evolved to live in environments where fire is predictably common. Serotinous cones help ensure that a population won’t be wiped out when a massive wildfire comes through. After the fire has passed and the seeds are released, lodgepole pine can quickly repopulate the barren ground. As long as fire occurs within the lifespan of a population of similarly aged trees, it is advantageous for the majority of individuals to maintain their serotinous trait. If the population is located in an area that historically does not see much fire, serotinous cones may be a disadvantage and can have adverse effects on the longevity of that population.

A study published in Ecology in 2003 looked at the influence that the frequency of fire has on lodgepole pine stands found at low and high elevations in Yellowstone National Park. At lower elevations, where summer temperatures are warmer and precipitation is relatively minimal, fires occur more frequently compared to higher elevations, which tend to be cooler and wetter. The researchers found that at lower elevations when fires occurred at short intervals (less than 100 years between each fire), lodgepole pine was slower to repopulate compared to longer intervals. This suggests that the percentage of serotiny found in stands that experienced short fire intervals was low, and that stands with long fire intervals exhibit a higher percentage of serotiny. After all, as mentioned above, lodgepole pines don’t start developing serotinous cones until later in life.

At higher elevations, where fire occurs less frequently, lodgepole pines were found to have a low percentage of serotinous cones regardless of the age of the stand. Because the trees at high elevations are more likely to die of old age rather than fire, maintaining serotinous cones would be a disadvantage. Open cones are preferred. Thus, at least in this study, a greater percentage of serotinous cones was found in lodgepole pines at lower elevations compared to those at higher elevations. Latitude, elevation, mountain pine beetle attacks, and other environmental factors have all been used to explain differences in serotiny. However, the factor that seems to have the greatest influence is the frequency of fire. As James Lotan writes in a 1976 report: “A high degree of cone serotiny would be expected where repeated, high-intensity fires occur. Where forest canopies are disrupted by factors other than fire, open cones annually supply [seed] for restocking disturbances such as windfalls.”

That being said, one other factor does appear to play a critical role in whether or not lodgepole pines produce serotinous cones, and that is seed predation by squirrels. In a paper published in Ecology in 2004, researchers wondered why the percentage of serotinous cones wasn’t even higher in populations where fire reliably occurred during the lifetime of the stand. To help answer this question they looked at the activities of pine squirrels, which are the main seed predator of lodgepole pine seeds. Pine squirrels visit the canopy of lodgepole pines and consume the seeds found in serotinous cones. Because non-serotinous cones quickly shed their seeds, serotinous cones are a more reliable and accessible food source, and because pine squirrels are so effective at harvesting the seeds of serotinous cones, the researchers concluded that, “in the presence of pine squirrels, the frequency of serotiny is lower and more variable, presumably reflecting,” among a variety of other factors, “the strength of selection exerted by pine squirrels.”

A study published in PNAS in 2014 added evidence to this conclusion. While acknowledging that fire plays a major role in the frequency of serotinous cones, the researchers asserted that “squirrels select against serotiny and that the strength of selection increases with increasing squirrel density.” However, despite making it easier for squirrels to access their seeds, lodgepole pines maintain a degree of serotinous cones, since clearly their main advantage is retaining a canopy-level seed bank from which seeds are released after a fire and by which a new generation of lodgepole pines is born.

open cones of lodgepole pine (Pinus contorta)

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Meet Erigeron linearis

Erigeron is a genus of herbaceous, flowering plants consisting of between 390 and 460 species and is a member of the aster/sunflower family (Asteraceae). Plants in this genus are annuals, biennials, or perennials and are mainly found in temperate regions around the world. At least 163 species occur in the contiguous United States. Erigeron diversity is particularly high in western states; however, each state is home to at least one Erigeron species.

A common name for plants in this genus is fleabane. This name comes from an outdated belief that the plants can be used to repel or poison fleas, flies, gnats, and other tiny insects, a belief for which there is no evidence. In Ancient Greek, the name Erigeron is said to mean something akin to “old man in the early morning,” likely referring to the appearance of the seed heads which look like little tufts of white hair. Some Erigeron species are also commonly referred to as daisies.

desert yellow fleabane (Erigeron linearis)

One species of Erigeron that I would like you to meet is Erigeron linearis. While most of the plants in this genus have flowers that are white, pink, or various shades of purple, E. linearis is a yellow-flowered species, hence the common name, desert yellow fleabane. Another common name for this plant is narrow leaved fleabane, a reference to its linear leaves. E. linearis is a small plant with a prominent taproot that reaches up to 20 centimeters tall and forms a leafy, rounded mat or cushion of whitish or gray-green, alternately arranged leaves. The white appearance is due to numerous, fine, appressed hairs on the leaves and stems. Flower stalks are produced in abundance in late spring through early summer and are mostly leafless. They reach above the mound of leaves and are each topped with at least one flower head, which nods at first, but then straightens out as the flowers open. Each flower head is about 2 centimeters wide and is typical of plants in the sunflower family, with a cluster of deep yellow disc florets in the center, surrounded by ray florets that are lighter in color. Both disc and ray florets are fertile; however, the disc florets have both “male” (stamens) and “female” (pistils) flower parts, while the ray florets have only “female” parts. The involucre, which sits at the base of the flowers, is egg-shaped or hemispheric and made up of a series of tiny, fuzzy bracts called phyllaries.

the flower head of desert yellow fleabane (Erigeron linearis)

The fruit of Erigeron linearis is called a cypsela, an achene-like fruit that is characteristic of plants in the sunflower family. The fruits are miniscule and topped with a pappus composed of short outer bristles and longer, pale, inner bristles. The two types of pappus bristles (or double pappus) must be the reason for the scientific name this species was originally given in 1834, Diplopappus linearis. While the seeds of more than 80% of flowering plant species found in dryland regions exhibit some form of dormancy, a study published in Plant Biology (2019), found that E. linearis is one of the few species with non-dormant seeds. This means that for those of us interested in growing plants native to the Intermountain West, E. linearis is a pretty easy one to grow and is a great addition to water-wise gardens, pollinator gardens, and rock gardens.

Erigeron linearis seedling

Erigeron linearis is distributed across several western states and into Canada. It is found in northern California, eastern Oregon and Washington, southern British Columbia, across Idaho and east into southern Montana, western Wyoming and northwestern Utah. It is found at low to moderate elevations in open, rocky foothills, grasslands, sagebrush steppe, and juniper woodlands. It prefers well-drained soils and full sun. It is one of many interesting plants found on lithosols (also known as orthents), which are shallow, poorly develop soils consisting of partially weathered rock fragments. In the book Sagebrush Country, Ronald Taylor calls lithosols “the rock gardens of the sagebrush steppe,” and refers to E. linearis and other members of its genus as “some of the more colorful components of the lithosol gardens.” E. linearis is a food source for pronghorn, mule deer, and greater sage-grouse, and the flowers are visited by several species of bees and butterflies. The plant is also a larval host for sagebrush checkerspots.

desert yellow fleabane (Erigeron linearis)

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Book Review: In Defense of Plants

Many of us who are plant obsessed didn’t connect with plants right away. It took time. There was a journey we had to go on that would ultimately bring us to the point where plants are now the main thing we think about. After all, plants aren’t the easiest things to relate to. Not immediately anyway. Some of us have to work up to it. Once there, it’s pretty much impossible to go back to our former lives. What was once just a background of green hues is now a rich cast of characters, each with their own name, unique features, and distinct story to tell. Essentially, we went through what Matt Candeias refers to as our ” green revolution.” Candeias – author and host of the long-running blog and podcast, In Defense of Plants – shares his story of learning to love plants and offers a convincing arguement for why you should love them too in his new book, aptly titled, In Defense of Plants.

It’s hard to picture Candeias as anything but a plant lover. If you’ve been following his work, you’ll know he makes it a point to put plants at center stage. It seems that much of the popular content available about plants focuses on the usefulness of plants as they pertain to humans. In many cases it can be easier to find out how to grow a certain plant species than to learn about where it’s from and what it’s like in the wild. Candeias let’s the plants speak for themselves by giving them a voice through his blog, podcast, and now his book. Through the stories he shares we get a peek into the way Candeias sees plants, with the hope being that others might also “be bitten by the botanical bug.”

One of the first plants that captured the attention of Candeias was perennial blue lupine (Lupinus perennis). While assisting with a habitat restoration project at a sand and gravel quarry, Candeias was tasked with improving the establishment of lupine, which is the host plant for the caterpillars of an endangered species of butterfly called Karner blue. The work he did at the quarry and the botanical research that went into it helped Candeias realize that plant’s aren’t at all boring, but are “incredibly interesting organisms worthy of respect and admiration” and that “plants can be both surprisingly relatable and incredibly alien all at once.” His “green revolution” had begun.

The seeds of lupine are dispersed ballistically. As the seed pods dry, tension builds. Then, as Matt Candeias writes in In Defense of Plants, “with an audible pop, the pods eventually explode, catapulting the seeds out into the environment.”

In each chapter of In Defense of Plants we get a peak into the experiences that brought Candeias to where he is now as he discovers the wonder of plants. His personal stories help introduce the main topic of each chapter. Topics include plant sex, plant dispersal, plant defenses, carnivorous plants, and parasitic plants. From countless possible examples, Candeias selects a few of his favorite plant species to help illustrate each topic. Along the way, the reader is presented with various other interesting plant-related facts as Candeias discusses the behaviors of some of the world’s most fascinating plants. In the chapter on dispersal, for example, unlikely agents of seed dispersal (like catfish!) are introduced, as well as phenomena like geocarpy, in which plants are essentially planting themselves.

Carnivorous plants provide an excellent gateway into convincing people who claim to have no interest plants that they actually do. It’s difficult to deny the impressive nature of a meat-eating plant. In the carnivorous plant chapter, Candeias introduces us to the various ways such plants capture and consume their prey, and even wonders if some of these plants should be considered omnivores. After all, certain butterworts digest pollen that falls onto their sticky leaves, and some bladderworts suck in plenty of algae and possibly gain nutrients from the act. If capturing insects inside leaves modified to look like pitchers or on leaves covered in digestive enzyme-producing glands doesn’t impress you, consider the carnivorous actions of corkscrew plants, which drill their leaves into the soil to go after soil-dwelling organisms like protozoans and worms.

Parasitic plants should also excite a reluctant plant lover. These are plants that take all or most of what they need to survive from another plant or host organism. Mistletoes are one of the more familiar parasitic plants, and Candeias describes several, including one that lives almost entirely within the stems of cacti. In fact, “you would never know a cactus had been infected until the mistletoe living within decides to flower,” at which point the flowers push their way out through the sides of the cactus. Dodder is another fairly common, highly specialized, and easy to identify parasitic plant. It basically looks like “a tangled pile of orange spaghetti tossed over the surrounding vegetation.” Orchids, a favorite of Candeias, are known for being mycoheterotrophs, which essentially means they parasitize fungi. Their seeds come unequipped with the energy stores needed to get going, so they borrow resources from mycorrhizal fungi in order to get their start. Years pass before the orchid can offer anything in return.

Datura is a genus of plants that produces toxic compounds like scopolamine and atropine. In his book, In Defense of Plants, Matt Candeias warns, “it would only take a small amount of these chemicals to completely ruin your week and slightly more to put you in a grave.”

After spending more than 200 pages celebrating plants and their amazing abilities and diversity, it’s fitting that Candeias spends the final chapter of his book mourning some of the ways the actions of humans threaten the existence of so many plants. He remarks how unfortunate it is that “plants with their unseeing, unhearing, unfeeling ways of life usually occupy the lowest rung of importance in our society.” Many of us barely notice the loss, yet “plants are the foundation of functioning ecosystems.” Due to that fact, “destroying plant communities causes disastrous ripples that reverberate throughout the entire biosphere of our planet.” Everything suffers when plants are lost. Fortunately, the book doesn’t end on this dark note. Candeias’s overall message is hopeful. When we learn to understand, appreciate, and care about plants, we will want to do everything we can to protect and restore them. With any luck, after reading this book, you too will want to offer your time, energy, and resources in defense of plants.

Listen to Matt talk about his new book on this episode of his podcast.

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Eating Weeds: Japanese Knotweed

When I first learned that Japanese knotweed was edible, I had my doubts. Sure, lots of plants may be edible, but are they really something you’d want to eat? I know Japanese knotweed as one of the most notorious weeds on the planet. Its destructive, relentless, and prolific nature has landed it on the world’s 100 worst invasive species list, right up there with black rats, Dutch elm disease, and killer algae. Having encountered a fair number of Japanese knotweed stands, the first thing to come to mind has never been, “that looks delicious.” Mature stalks stand as tall as 3 meters with broad, leathery leaves and hollow, bamboo-like stems. Their late summer flowers – a mess of tiny white florets on sprawling flower stalks – are a pollinator’s delight and favored by beekeepers for their abundant nectar. I don’t doubt that the honey produced from such an encounter is tasty, but the plant itself? I needed convincing.

Finally, I looked into it. I came across recipes of Japanese knotweed pickles and learned that it was the young, early emerging shoots that were sought after. That changed my perspective. Certainly you wouldn’t want to gnaw on a woody, 4 foot tall Japanese knotweed stalk, but the tender stems as they’re just beginning to re-emerge from the ground in the spring? Now those might be worth trying.

emerging stems of Japanese knotweed (Reynoutria japonica)

Japanese knotweed (Reynoutria japonica) was introduced to Europe from Japan in the 1800’s, arriving at Royal Botanic Gardens Kew by 1850. At that point, it was a prized ornamental. Its thick stems spotted with reds and purples, its broad, shiny leaves, and its showy flower heads all gave it garden appeal. It was also found to be useful for stabilizing hillsides and reducing erosion, honey production, and as a rhubarb substitute (it’s in the same plant family as rhubarb after all). Not long after that, it made its way to North America. Certainly people must have been aware of its propagative prowess as they moved the plant around. It readily roots from root and stem fragments, plus it produces extensive rhizomes, working their way as deep as 3 meters into the soil and as far as 7 meters away from the parent plant. Perhaps that should have been cause for alarm, but how could anyone have predicted just how aggressive and widespread it would soon become?

Thanks to the plant’s rhizomes, Japanese knotweed grows in thick, many-stemmed stands, pushing out, shading out, and leaving very little room for other plants. The rhizomes are also tough and can push up through gravel, cement, and asphalt. They are notorious for damaging foundations, pipes, and even pushing their way through floorboards. If that’s not enough, Japanese knotweed is pretty much impossible to kill. Herbicides may set it back, but they generally don’t take it out. Cutting it down repeatedly can slow it down, but it may also encourage it to grow more thickly and spread out more widely. Smothering it can work, but you have to be prepared to keep it smothered for quite a while. The deep rhizomes are slow to die, and they may eventually find their way outside of the smothered area, popping up to destroy your efforts to contain it. You can try to dig it out, but the amount of dirt you’d have to dig to get every last root and rhizome really isn’t feasible in most circumstances.

But hey, you can eat it, and perhaps you should. A quick internet search reveals a number of ways the plant can be consumed – purees, chutneys, compotes, sorbets. I chose to go with pickled Japanese knotweed. It seemed simple and approachable enough and a good way to determine if I was going to like it or not. Room temperature brine fermentation is pretty easy. You basically put whatever you’re wanting to pickle in a jar, add whatever spices and things you’d like, fill the jar with salty water, then seal it shut and let it sit there for a few days. Before you know it, you’ve got pickles.

There are several recipes for pickled Japanese knotweed to choose from. I went with this one. The seasonings I used were a bit different, and the stalks I had weren’t as “chubby” as recommended, but otherwise my approach was the same. After a few days, I gave them a try. I was pleasantly surprised. I thought they tasted a little like nopales. Sierra reluctantly tried them and was also surprised by how good they were. They reminded her of pickled asparagus. Other sources describe them as lemony, crunchy, tart and suggest serving them with fish, ramen, or even adding them to a cocktail made with purslane. Many of the weeds I’ve tried have been a fun experience, but not necessarily something I need to repeat. Japanese knotweed pickles, on the other hand, could become a spring tradition for me, and since we’re pretty much stuck with this plant, I’m sure to have a steady supply.

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When Acorn Masts, Rodents, and Lyme Disease Collide

“‘Mast years’ is an old term used to describe years when beeches and oaks set seed. In these years of plenty, wild boar can triple their birth rate because they find enough to eat in the forestes over the winter… The year following a mast year, wild boar numbers usually crash because the beeches and oaks are taking a time-out and the forest floor is bare once again.” — The Hidden Life of Trees by Peter Wohlleben

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When a plant population’s annual production of seeds is highly variable and synchronous, it is considered a masting or mast seeding species. Why and how masting happens is a bit of a mystery, and efforts are underway to better understand this phenomenon. One thing is clear, boom and bust cycles can have dramatic effects on animals that use the fruits and seeds of these plants for food. Acorn production in oaks provides a stark example. As Koenig, et al. describe in Ecology (2015), a “variable acorn crop initiates a ‘chain reaction’ of responses that cascades through the ecosystem, affecting densities of deer, mice, ground-nesting birds, gypsy moths, and the tick vectors of Lyme disease.” The connection between mast seeding oaks and the prevalence of tick-borne pathogens is of particular interest considering the risks posed to humans.

Lyme disease is an infectious diesease caused by a bacterium vectored by ticks in the genus Ixodes. The life-cycle of a tick is generally 2 to 3 years, beginning after a larva hatches from an egg. From there the larva develops into a nymph and later an egg-laying adult, taking a blood meal each step of the way. Tick larvae feed on the blood of small rodents and birds, which is where they can pick up the bacterium that causes Lyme disease. After feeding, they develop into nymphs and go in search of another blood meal, perhaps another rodent or maybe something larger like a deer or a human. It is in their nymphal and adult stages that ticks transmit Lyme disease to humans. Nymphs tend to transmit the disease more frequently, partly because they go undetected more easily.

The risk to humans of being infected with Lyme disease varies year to year and is dependent largely on how many infected ticks are present. For this reason, it is important to understand the factors affecting the density of infected nymphs. In a study published in PLoS Biology (2006), Ostfeld, et al. collected data over a 13 year period in plots located in deciduous forests in the state of New York, a hotspot for Lyme disease. The predictors they considered included temperature, precipitation, acorn crop, and deer, white-footed mouse, and chipmunk abundances. Deer abundance and weather conditions had long been considered important in predicting the prevalence of ticks, but little attention had been paid to small mammals – the larval hosts for ticks – and the variability of acorn crops – an important food source for rodents.

deer tick (Ixodes scapularis) — via PhyloPic; user Mathilde Cordellier

The results of their study revealed a clear pathway – more acorns leads to more rodents which leads to more Lyme disease carrying nymphs. The process takes a couple of years. First, oak trees experience a mast year, flooding rodent populations with food. In the following year, the numbers of mice and chipmunks is unusually high. The year after that, there are lots and lots of nymphal ticks infected with Lyme-disease. The relationship is so direct that Richard Ostfeld claims, based on his research, that he can predict the incidence of Lyme disease among residents of New York and Connecticut based on when a mast year occurs. In a summer when there is an abundance of 2 -year-old oak seedlings in the surrounding forests, expect the infection rate of Lyme disease to be high.

Lyme disease also occurs in regions where oak trees are not present or are uncommon, so variability in acorn crops isn’t always the best predictor. The researchers acknowledge that acorn abundance is not going to be “a universal predictor of risk;” instead, anything that leads to an increase in rodent populations, whether it is some other food source or a lack of predators, may be a key indicator since rodents are reservoir hosts of Lyme disease.

A study published in Parasites and Vectors (2020) looked at the effects of rodent density on a number of tick-borne pathogens. They confirmed that an “increase in rodent density positively affects populations of nymphal ticks in the following year;” yet, they could not confirm that rodent density is the sole predictor of disease risk. Other factors come into play depending on the disease in question, and further research is needed to improve models that predict tick-borne diseases. They did, however, confirm that, by flooding the food supply with acorns, mast years can boost populations of a variety of rodents.

white-footed mouse (Peromyscus leucopus) — via wikimedia commons; USGS

A fear of ticks is justified. They suck your blood after all, and besides that, they can transmit some pretty serious diseases. Arm yourself by educating yourself. One place to do that is with The Field Guides podcast. Their tick two-parter is well worth the listen (part one and part two). Not only will it give you valuable information in protecting yourself against ticks, it may also give you an appreciation for their prowess. Just maybe. See also their You Tube video demonstrating how to sample for ticks.

To Fruit or Not to Fruit – The Story of Mast Seeding

Perennial plants that are able to reproduce multiple times during their lifetime don’t always yield the same amount of seeds each time they reproduce. For some of these plants, there is a stark difference between high-yield years and low-yield years, with low-yield years outnumbering the occasional high-yield years. In years when yields are high, fruit production can seem excessive. This phenomenon is called masting, or mast seeding, and it takes place at the population level. That is, during a mast year, virtually all individuals in a population of a certain species synchronously produce a bumper crop of seeds.

Plants of many types can be masting species. Bitterroot milkvetch (Astragalus scaphoides) and a tussock grass known as Chionochloa pallens are masting species, for example. However, this behavior is most commonly observed in trees, notably nut producing trees like oaks, beeches, and pecans. As you might imagine, the boom and bust cycles of mast seeding plant populations can have dramatic ecological effects. Animals that eat acorns, for example, are greeted with a veritable buffet in a mast year, which can increase their rate of reproduction for a spell. Then, in years when acorns are scarce, the populations of those animals can plummet.

How and why masting happens is not well understood. It is particularly baffling because masting populations can cover considerably large geographic areas. How do trees covering several square miles all “know” that this is the year to really go for it? While a number of possible explanations have been explored, there is still much to learn, especially since so many different species growing in such varied environments exhibit this behavior.

A popular explanation for mast seeding is predator satiation. The fruits and seeds of plants are important food sources for many animals. When a population of plants produces fruit in an unusually high abundance, its predators won’t possibly be able to eat them all. At least a few seeds will be left behind and can sprout and grow into new plants. By satiating their predators they help ensure the survival of future generations. However, even if a plant species has evolved to behave this way, it still doesn’t explain how all the plants in a particular population seem to know when it’s time for another mast year.

Predator satiation is an example of an economy of scale, which essentially means that individual plants benefit when the population acts as a whole. Another economy of scale that helps explain masting is pollen coupling. This has to do with the timing of flowering in cross pollinating species. If individuals flower out of sync with one another, the opportunities for cross pollination are limited. However, if individuals in a population flower simultaneously, more flowers will be pollinated which leads to increased fruit and seed production.  For this to happen, there are at least two factors that come into play. First, the plants have to have enough resources to flower. Making flowers is expensive, and if the resources to do so (like carbon, nitrogen, and water) aren’t available, it won’t happen. Second, weather conditions have to work in their favor. Timing of flowering depends, not only on daylength, but on temperature, rainfall, and other local weather conditions. If individuals across a population aren’t experiencing similar weather, the timing of their flowering may be off.

pollen-producing (male) flowers of pecan (Carya illinoinensis) — via wikimedia commons; Clemson University

Resource matching and resource budgeting are other proposed explanations for masting. Since plants can only use the resources available to them for things like growth and reproduction, they vary each year in how much growing or reproducing they do. Theoretically, if plants in a population are all going to flower in the same year, they all have to have access to a similar amount of resources. Often, the year following a mast year, there is a significant drop in fruit production, as though the plants have used up all of their available resources for reproduction and are taking a break. Some hypothesize that masting is a result of resource storage, and that plants save up resources for several years until they have what they need for yet another big year.

Another thing to consider is how plant hormones might play a role in masting. Gene expression and environmental cues both result in hormonal responses in plants. As Bogdziewicz, et al. write in Ecology Letters (2020), “if hormones and the genes that control them are hypersensitive to an environmental signal, masting can be at least partially independent of resource- and pollen-based mechanisms.” This and other potential explanations for masting are, at this point, largely theoretical. In their paper, Bogdziewicz, et al. propose a number of ways that theoretical predictions can be experimentally tested. If the “research agenda” outlined in their paper is carried out, they believe it will “take the biology of masting from a largely observational field of ecology to one rooted in mechanistic understanding.”

In her book, Braiding Sweetgrass, Robin Wall Kimmerer proposes an additional explanation for the mechanisms behind masting – the trees are talking to one another. Not in the way that you and I might converse, but rather by sending signals through the air via pheromones and underground via complex fungal networks. There is already evidence for this behavior when it comes to plants defending themselves from predators and in sharing resources, so why not in planning when to reproduce? As Kimmerer writes regarding masting, “the trees act not as individuals, but somehow as a collective.” The question now is how.

seedlings of European beech (Fagus sylvatica), a mast-seeding species — via wikimedia commons; user: Beentree

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Winter Trees and Shrubs: Northern Catalpa

The names of plants often contain clues that can either help with identification or that tell something about the plant’s history or use. The name, catalpa, is said to be derived from the Muscogee word, katałpa, meaning “winged head,” presumably referring to the tree’s winged seeds. Or maybe, as one writer speculates, it refers to the large, heart-shaped, floppy leaves that can make it look like the tree is “ready to take flight.” Or perhaps it’s a reference to the fluted, fused petals of the tree’s large, tubular flowers. I suppose it could mean any number of things, but I’m sticking with its seeds, which are packed by the dozens in the tree’s long, slender, bean-like fruits. The seeds are flat, pale brown, and equipped with paper thin, fringed appendages on either side that assist in wind dispersal – wings, in other words.

winged seeds of northern catalpa (Catalpa speciosa)

Catalpa speciosa, or northern catalpa, is a relatively fast growing, short-lived tree native to the Midwest and one of only two species in the genus Catalpa found in the United States. Its distribution prior to the arrival of Europeans appears to have been restricted to a portion of the central Mississippi River valley, extending west into Arkansas, east into Tennessee, and north into Illinois and Indiana. It has since been widely planted outside of its native range, naturalizing in areas across the Midwest and eastern US. Early colonizers planted northern catalpa for use as fence posts, railroad ties, and firewood. Its popularity as an ornamental tree is not what it once was a century ago, but it is still occasionally planted in urban areas as a shade tree. Its messiness – littering the ground below with large leaves, flowers, and seed capsules – and its tendency to spread outside of cultivation into natural areas are reasons why it has fallen out of favor with some people.

The oval to heart-shaped, 8 to 12 inch long leaves with long petioles rotting on the ground below the tree are one sure sign that you’ve encountered a catalpa in the winter time. The leaves are some of the first to fall at the end of the growing season, briefly turning an unmemorable yellow before dropping.

leaf of northern catalpa (Catalpa speciosa) in the winter with soft hairs on the underside still visible

The leaf arrangement on northern catalpa is whorled and sometimes opposite. The twigs are easy to identify due to several unique features. They are stout, round, and grayish brown with prominent lenticels. The leaf scars are large, rounded, and raised up on the twig, looking a bit like little suction cups. They are arranged in whorls of three, with one scar considerably smaller than the other two. A series of bundle traces inside the scar form an ellipse. The leaf buds are tiny compared to the scar and are protected by loose, pointed, brown bud scales. Northern catalpa twigs lack a terminal bud. In the winter, seed capsules or the stalk of an old inflorescence often remain attached to the terminal end of the twig. The pith inside of the twig is thick, white, and solid.

twig of northern catalpa (Catalpa speciosa)

pith inside twig of northern catalpa (Catalpa speciosa)

Another common name for Catalpa speciosa is cigar tree, a name that comes from its up to 18 inch long, cigar-like seed capsules that hang from the otherwise naked tree throughout the winter. The sturdy, cylindrical pod starts out green in the summer and turns dark brown by late fall. Seed pods that haven’t fallen or already split open will dehisce in the spring time, releasing their papery seeds to the wind.

fruits of northern catalpa (Catalpa speciosa) hanging from the tree in the winter

The young bark of northern catalpa is thin and easily damaged. As it matures, it becomes furrowed with either scaly ridges or blocky plates. Mature trees are generally twisted at the base but otherwise grow straight, reaching 30 to 60 feet tall (sometimes taller) with an open-rounded to narrow-oval crown.

maturing bark of northern catalpa (Catalpa speciosa)

Northern catalpa is one of the last trees to leaf out in the spring. In late spring or early summer, 10 inch long clusters of white, tubular flowers are produced at the tips of stems. Before the flowers open, they look a bit like popped popcorn, reminding me of a song from my childhood (which I will reluctantly leave right here). The margins of its trumpet-shaped petals are ruffled and there is yellow, orange, and/or purple spotting or streaking on the inside of the tubes.

flower of northern catalpa (Catalpa speciosa) just before it opens

More Winter Trees and Shrubs on Awkward Botany:

A Few More Snags Near Ketchum

Nearly a year has passed since Sierra and I took a trip to Ketchum, Idaho and I reported on some of the snags we encountered there. After months without a break, we finally had the chance to get away for a few days, and since we were desperate for some time off and a change of scenery, we couldn’t turn it down. Plus, we were heading back to Ketchum, so I knew I’d get to check out a few more snags. I was stoked.

I’m obsessed with trees, and my preference is for live ones (generally speaking), but dead trees are certainly gaining in popularity. After all, a dead tree isn’t truly dead. As its corpse slowly rots, it continues to harbor and support life inside and out in a substantial way. Forests need dead trees just as much as they need live trees. Plus, ecology aside, dead trees are no less photogenic than any other tree.

Death isn’t all bad. New life springs from decay. Given our current state of affairs, we need this reminder, and snags offer it in spades. As Sierra and I pulled up to the Apollo Creek trailhead, we looked out onto a section of forest that had clearly been ravaged by fire in the not too distant past. Acres of standing and fallen burned out trees bore witness to this fact. Yet among the dead, life flourished, as dozens of songbirds actively foraged on and around the charred trees. They were there for the insects that were feeding on the dead wood, fueling themselves for fall migration. In the spring, when the birds return, some of them may even nest in the cavities of the dead trees. They will feed again on the insects and raise up a new generation of songbirds that will do the same. In and among snags there are myriad examples just like this, showing us the countless ways in which death supports new life.

What follows is a small sampling of the snags we encountered this time around on our trip to Ketchum.

post-fire snag among many other snags

a series of cavities in a post-fire snag

snag surrounded by live trees

three new snags

fallen snag

broken snag

new tree emerging from a nurse stump

not a snag, but one of many lupines we saw flowering along Apollo Creek Trail

The Hidden Flowers of Viola

Violas keep a secret hidden below their foliage. Sometimes they even bury it shallowly in the soil near their roots. I suppose it’s not a secret really, just something out of sight. There isn’t a reason to show it off, after all. Showy flowers are showy for the sole purpose of attracting pollinators. If pollinators are unnecessary, there is no reason for showy flowers, or to even show your flowers at all. That’s the story behind the cleistogamous flowers of violas. They are a secret only because unless you know to look for them, you would have no idea they were there at all.

Cleistogamy means closed marriage, and it describes a self-pollinating flower whose petals remain sealed shut. The opposite of cleistogamy is chasmogamy (open marriage). Most of the flowers we are familiar with are chasmogamous. They open and expose their sex parts in order to allow for cross-pollination (self-pollination can also occur in such flowers). Violas have chasmogamous flowers too. They are the familiar five-petaled flowers raised up on slender stalks above the green foliage. Cross-pollination occurs in these flowers, and seed-bearing fruits are the result. Perhaps as a way to ensure reproduction, violas also produce cleistogamous flowers, buried below their leaves.

an illustration of the cleistogamous flower of Viola sylvatica opened to reveal its sex parts — via wikimedia commons

Flowers are expensive things to make, especially when the goal is to attract pollinators. Colorful petals, nectar, nutritious pollen, and other features that help advertise to potential pollinators all require significant resources. All this effort is worth it when it results in the ample production of viable seeds, but what if it doesn’t? Having a method for self-pollination ensures that reproduction will proceed in the absence of pollinators or in the event that floral visitors don’t get the job done. A downside, of course, is that a seed produced via self-pollination is essentially a clone of the parent plant. There will be no mixing of genes with other individuals. This isn’t necessarily bad, at least in the short term, but it has its downsides. A good strategy is a mixture of both cross- and self-pollination – a strategy that violas employ.

The cleistogamous flowers of violas generally appear in the summer or fall, after the chasmogamous flowers have done their thing. The fruits they form split open when mature and deposit their seeds directly below the parent plant. Some are also carried away by ants and dispersed to new locations. Seeds produced in these hidden flowers are generally superior and more abundant compared to those produced by their showy counterparts. People who find violas to be a troublesome lawn weed – expanding far and wide to the exclusion of turfgrass – have these hidden flowers to blame.

That being said, there is a defense for violas. In the book The Living Landscape by Rick Darke and Doug Tallamy, Tallamy writes: “Plants such as the common blue violet (Viola sororia), long dismissed by gardeners as a weed, can be reconstituted as desirable components of the herbaceous layer when their ecosystem functionality is re-evaluated. Violets are the sole larval food source for fritillary butterflies. Eliminating violets eliminates fritillaries, but finding ways to incorporate violets in garden design supports fritillaries.”

sweet violet (Viola odorata)

In my search for the cleistogamous flowers of viola, I dug up a sweet violet (Viola odorata). I was too late to catch it in bloom, but the product of its flowers – round, purple, fuzzy fruits – were revealed as I uprooted the plant. Some of the fruits were already opening, exposing shiny, light brown seeds with prominent, white elaiosomes, there to tempt ants into aiding in their dispersal. I may have missed getting to see what John Eastman calls “violet’s most important flowers,” but the product of these flowers was certainly worth the effort.

Fruits formed from the cleistogamous flowers of sweet violet (Viola odorata)

Up close and personal with the fruit of a cleistogamous flower

The seeds (elaiosomes included) produced by the cleistogamous flower of sweet violet (Viola odorata)

See Also:

Winter Interest in the Lower Boise Foothills

The Boise Foothills, a hilly landscape largely dominated by shrubs and grasses, are a picturesque setting any time of the year. They are particularly beautiful in the spring when a wide array of spring flowering plants are in bloom, and then again in late summer and early fall when a smaller selection of plants flower. But even when there aren’t flowers to see, plants and other features in the Foothills continue to offer interest. Their beauty may be more subtle and not as immediately striking as certain flowers can be, but they catch the eye nonetheless. Appeal can be found in things like gnarled, dead sagebrush branches, lichen covered rocks, and fading seed heads. Because the lower Boise Foothills in particular have endured a long history of plant introductions, an abundance of weeds and invasive plants residing among the natives also provide interest.

This winter has been another mild one. I was hoping for more snow, less rain, and deeper freezes. Mild, wet conditions make exploring the Foothills difficult and ill-advised. Rather than frozen and/or snow covered, the trails are thick with mud. Walking on them in this state is too destructive. Avoiding trails and walking instead on trail side vegetation is even more destructive, and so Foothills hiking is put on hold until the ground freezes or the trails dry out. This means I haven’t gotten into the Foothills as much as I would like. Still, I managed to get a few photos of some of the interesting things the lower Boise Foothills have to offer during the winter. What follows is a selection of those photos.

snow melting on the fruit of an introduced rose (Rosa sp.)

fading seed heads of hoary tansyaster (Machaeranthera canescens)

samaras of box elder (Acer negundo)

snow on seed heads of yarrow (Achillea millefolium)

gall on introduced rose (Rosa sp.)

sunflower seed heads (Helianthus annuus)

sunflower seed head in the snow (Helianthus annuus)

snow falling in the lower Boise Foothills

fading seed heads of salsify (Tragopogon dubius)

lichen on dead box elder log

seed head of curlycup gumweed (Grindelia squarrosa)

lichen and moss on rock in the snow

fruits of poison ivy (Toxicodendron radicans)

See Also: Weeds and Wildflowers of the Boise Foothills (June 2015)

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