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:

Awkward Botanical Sketches #5: Leaves of Yellowstone Edition

Earlier this month, I met up with Eric LoPresti and others at Yellowstone National Park to help take a census of Abronia ammophila, a rare plant endemic to the park and commonly referred to as Yellowstone sand verbena. Abronia (a.k.a. the sand-verbenas) is a small genus of plants in the family Nyctaginaceae that is native to western North America. Several species in the genus have fairly limited distributions, and as the common name implies, members of this genus generally occur in sandy soils. A. ammophila is no exception. A report written by Jennifer Whipple and published in 2002 described it as “restricted to stabilized sandy sites that lie primarily just above the maximum splash zone along the shoreline of [Yellowstone Lake].” Despite the large size of the lake, A. ammophila is not widespread. Most individuals are found along the north shore of the lake, and even there it has been declining. According to Whipple’s report, “Yellowstone sand verbena has been extirpated from a significant portion of its original range along the shoreline of the lake due largely to human influences.”

Like other sand verbenas, A. ammophila has sticky leaves to which sand particles easily adhere, a phenomenon known as psammophory and an act that may help in defense against herbivory. The plant grows prostrate across the sand and produces attractive, small, white, trumpet-shaped flowers in groups of up to 20 that open wide when light levels are low, such as in the evening and in times of heavy cloud cover. The flowers are self-fertile, but insects may also play a role in pollination. It is imperative that questions surrounding its pollination biology, seed dispersal, and other factors regarding its life history are answered in order to halt any further decline of the species and ensure its survival for generations to come.

While in Yellowstone, I enjoyed looking at the all plants, several of which were new to me. I decided to sketch a few of the leaves that I found common around our campsite. I was particularly interested in discolored, diseased, drought-stressed, and chewed-on leaves, since they are more interesting to sketch and color. While I was at it, I attempted to draw a Yellowstone sand verbena seedling as well.

wild strawberry (Fragaria sp.)
Richardson’s geranium (Geranium richardsonii)
lodgepole pine (Pinus contorta)
veiny dock (Rumex venosus)
cinquefoil (Potentilla sp.)
seedling of Yellowstone sand verbena (Abronia ammophila)

More Awkward Botanical Sketches

Field Trip: Chico Hot Springs and Yellowstone National Park

Thanks to an invitation from my girlfriend Sierra and her family, I spent the first weekend in May exploring Yellowstone National Park by way of Chico Hot Springs in Pray, Montana. The weather was perfect, and there were more plants in bloom than I had expected. During our hikes, my eyes were practically glued to the ground looking for both familiar and unfamiliar plant life. Most of the plants in bloom were short and easily overlooked. Many were non-native. Regardless, the amateur botanist in me was thrilled to be able to spend time with each one, whether I was able to identify it or not. I tried to remind myself to look up as often as I was looking down. Both views were remarkable.

On our first day there, we hiked in the hills above Chico Hot Springs. The trail brought us to a place called Trout Pond. There were lots of little plants to see along the way.

Trout Pond (a.k.a. Chico Pond) near Chico Hot Springs in Pray, Montana

mountain bluebells (Mertensia longifolia)

shooting star (Dodecantheon pulchellum)

western stoneseed (Lithospermum ruderale)

western wallflower (Erysimum capitatum)

The next day we drove into Yellowstone. From the north entrance we headed east towards Lamar Valley. Wildlife viewing was plentiful. Elk, bison, bighorn sheep, pronghorn, black bears, red foxes, and even – if you can believe it – Canada geese.

Sierra looks through the binoculars.

Perhaps she was looking for this red-tailed hawk.

Daniel looks at a tiny plant growing in the rocks.

Still not sure what this tiny plant is…

On our third day there, we headed south to see some geysers. We made it to the Norris Geyser Basin and then decided to head east to see the Grand Canyon of the Yellowstone. This was our geology leg of the tour. But that doesn’t mean we didn’t stop to look at a plant or two along the way.

Nuttall’s violet (Viola nuttallii) near the petrified tree in Yellowstone National Park

Wild strawberry (Fragaria sp.) at Norris Geyser Basin in Yellowstone National Park

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Photos of Lamar Valley, red-tailed hawk, Daniel looking at a tiny plant, mystery plant, and Grand Canyon of the Yellowstone were taken by Sierra Laverty. The rest were taken by Daniel Murphy.

Speaking of Sierra, she is the founder and keeper of Awkward Botany’s Facebook page and Instagram account. Please check them out and like, follow, etc. for Awkward Botany extras.