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)

Further Reading and Viewing:

Pine Cones and the Fibonacci Sequence

While we’re on the topic of pine cones, have you ever considered their scales and the spirals they form? Nature is replete with spirals, so perhaps it’s no surprise that they are found in pine cones. The more interesting thing is that the number of spirals found on pine cones are almost always Fibonacci numbers. But maybe that’s not that surprising either, as Fibonacci numbers are also pretty common in nature.

Add 1 plus 1 and you get 2. Add 2 plus 1 and you get 3. 3 + 2 = 5, 5 + 3 = 8, and 8 + 5 = 13. One, two, three, five, eight, and thirteen are Fibonacci numbers. Continue adding the sum to the number that came before it, and that’s the Fibonacci Sequence. The ratio of two neighboring Fibonacci numbers is an approximation of the golden ratio (e.g. 8/5 = 1.6). This is commonly represented by drawing a series of squares on graph paper and then drawing a spiral across the squares. Each square drawn is larger than the last in accordance with the Fibonacci sequence, and the spiral drawn through the squares is a logarithmic spiral.

So, what does this have to do with pine cones? Well if you count the number of spirals that are going to the right, then count the number of spirals going to the left, you usually end up with two adjacent numbers in the Fibonacci sequence. Most often it’s either 5 and 8 or 8 and 13. You can find this same pattern in lots of other plant parts, including the aggregate fruits of pineapples, the disc flowers of sunflowers (and other plants in the aster family), the bracts of artichoke flowers, florets on a cauliflower, and leaf arrangements of all sorts of other plants.

The arrangement of leaves is called phyllotaxis, and when the leaves on a stem form a spiral pattern it’s called a phyllotactic spiral. The benefit the plant receives from having its leaves grow in a spiral formation down the length of its stem is actually quite simple – it keeps them from shading each other out and thereby maximizes their exposure to the sun. If you measure the angle between each leaf, the angle should be the same between each adjacent leaf on the stem. In order for the number of spirals to be a Fibonacci number, the leaves have to be oriented at a specific angle from each other. But this isn’t always the case. Depending on the angle, the number of spirals could be part of some other number sequence, like Lucas numbers perhaps.

While the specifics of plant growth can be quite complex, the reason for the patterns that result is actually quite simple. As plants grow new parts, they are put in a spot where there is room for them to grow, which is at some angle from the part that grew before it. Once that angle is “chosen,” it generally doesn’t change, and as more plant parts grow, a spiral forms (or no spiral forms at all, depending on the pattern of growth). If plant parts are oriented at a specific angle (~ 137.5o), the numbers of spirals end up being Fibonacci numbers. For a more thorough and entertaining explanation of all this, check out this three part video series from Khan Academy. It’s well worth the watch.

And now an example:

Count the number of right-hand spirals on this ponderosa pine cone. There are 8. That’s a Fibonacci number!

Count the number of left-hand spirals on this ponderosa pine cone. To make it easier to count, you can start or end with the top left spiral that has alternating red and green scales. There are 13. That’s another Fibonacci number!

And now your mission, should you choose to accept, is to find a pine cone (or some other conifer cone) in which the number of right and left-hand spirals are not Fibonacci numbers. They’re definitely out there, so let me know what you find in the comment section below.

Further Reading:

Pine Cones Are Like Hangars for Pine Tree Seeds

Over the past year I’ve written about the making of pine tar and the drinking of pine needle tea. But why stop there? Pines are a fascinating group of plants, worthy of myriad more posts, and so my exploration into the genus continues with pine cones and the seeds they bear.

Pines are conifers and, more broadly, gymnosperms. They are distinct from angiosperms (i.e. flowering plants), with the most obvious distinction being that they don’t make flowers. Since they are flowerless, they are also fruitless, as fruits are seed-bearing structures formed from the ovary or ovaries of flowering plants. Pines do make seeds though, and, as in angiosperms, pollen is transported from a “male” organ to a “female” organ in order for seeds to form. Rather than being housed in a fruit, the seeds are essentially left out in the open, which is why the term “naked seeds” is frequently used in reference to gymnosperms.

seed cone of Scots pine (Pinus sylvestris ‘Glauca Nana’)

In the case of pines and other conifers, the seeds may be naked, but they’re not necessarily homeless. They have the protection of cones, which is where the female reproductive organs are located. Male, pollen cones are separate structures and are smaller and less persistent than the cones that house the seeds. A cone, also known as a strobilus, is a modified branch. A series of scales grow in a spiral formation along the length of the branch, giving the cone its shape. On the inside of these scales is where the seeds form, two per scale. First they are egg cells, and then, after pollination and a period of maturation, they become seeds. The scales protect them throughout the process and then release them when the time is right.

With more than 120 species in the genus Pinus, there is great diversity in the size, shape, and appearance of pine cones. While at first glance they don’t appear all that different from one another, the cones of each species have unique characteristics that can help one identify the pine they fell from without ever having to see the tree. Pine cones are also distinct from the cones of other conifers. For one, pine cones take at least two or, in some cases, three years to reach maturity, whereas the cones of other conifers develop viable seeds in a single year. Pine cones are also known to remain on the tree for several years even after the seeds are mature – in some species up to 10 years or more – and they don’t always part with their seeds easily. Lodgepole pines (Pinus contorta) require high temperatures to melt the resin that holds their scales closed, the cones of jack pine (P. banksiana) generally only open in the presence of fire, and the seeds of whitebark pine (P. albicaulis) are extracted with the aid of birds (like Clark’s nutcracker) and other animals.

immature seed cone of lodgepole pine (Pinus contorta)

Every pine cone is special in its own right, but some stand out in particular. The largest and heaviest pine cones are found on Coulter pine (P. coulteri), measuring up to 15 inches long and weighing as much as 11 pounds with scales that come to a sharp point. It’s understandable why the falling cones of this species are frequently referred to as widowmakers. Longer cones, but perhaps less dangerous, are found on sugar pine (P. lambertiana). The tallest trees in the genus, the cones of sugar pine consistently reach 10 to 20 inches long and sometimes longer.

Pine tree seeds are a food source for numerous animals, including humans. Most are so small they aren’t worth bothering with, however, several species have seeds that are quite large and worth harvesting. Most commercially grown pine nuts come from stone pine (P. pinea) and Korean pine (P. koraiensis). In North America, a wild source for pine nuts is found in the pinyon pines, which have a long history of being harvested and eaten by humans.

immature seed cone of ponderosa pine (Pinus ponderosa)

The seeds of many pines come equipped with little wings called samaras, which aid them in their dispersal. Upon maturity, pine cone scales open and release the seeds. Like little airplanes leaving the hangar, the seeds take flight. Wind dispersal is not an effective means of dispersal for all pines though. A study published in Oikos found that seeds weighing more than 90 milligrams are not dispersed as well by wind as lighter seeds are. When it comes to long distance dispersal, heavier seeds are more dependent on animals like birds and rodents, and some pines rely exclusively on their services. The author of the study, Craig Benkman, notes that “bird-dispersed pines have proportionately thinner seed coats than wind-dispersed pines,” which he points out in reference to Japanese stone pine (P. pumila) and limber pine (P. flexilis), whose seeds weigh around 90 milligrams yet rely mostly on birds for dispersal. Benkman suspects that the seeds of these two species “would probably weigh over 100 milligrams if they had seed coats of comparable thickness as wind-dispersed seeds.”

Whitebark pine, as mentioned above, holds tightly to its seeds. Hungry animals must pry them out, which they do. Pine seeds are highly nutritious and supplement the diets of a wide range of wildlife. Some of the animals that eat the seeds also cache them for later. Clark’s nutcrackers are particularly diligent hoarders, harvesting thousands more seeds than they can possibly consume and depositing them in small numbers in locations suitable for sprouting.

Even large seeds that naturally fall from their cones have a chance to be dispersed further. As the seeds become concentrated at the base of the tree, ground-foraging rodents gather them up and cache them in another location, which Benkman refers to as secondary seed dispersal.

Particularly in pine species with wind dispersed seeds, what the weather is like helps determine when the hangar door will open to release the flying seeds. When it is wet and rainy, the scales of pine cones close up. The seeds wouldn’t get very far in the rain anyway, so why bother? When warm, dry conditions return, the scales open back up and the seeds are free to fly again. You can even watch this in action in the comfort of your own home by following the instructions layed out in this “seasonal science project.”

immature seed cones of limber pine (Pinus flexilis)

mature seed cones of limber pine (Pinus flexilis)

Further Reading:

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Photos of pine cones were taken at Idaho Botanical Garden in Boise, Idaho