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Local Knowledge: It’s the witch’s work

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By Paul Swenson EBS COLUMNIST

Remember back in June and early July when the air was full of yellow clouds blowing around in the trees? I’m sure for those of you with seasonal allergies, you do. What were those clouds, where do they come from and why are they important?

Many of you may know that those clouds are the pollen from different evergreen trees in our area, with the largest contributor being the lodgepole pine tree (Pinus contorta). The lodgepole pine is the one tree in our area that grows in compromised, unfertile soil, which makes it a hero tree in my book.

Lodgepole pines help repair ecosystems after forest fires, and grow easily in disturbed soils after logging, or other man induced degradation to the landscape. If you have visited the areas burned by fires, in and around Yellowstone National Park, you have seen the regenerative power of the lodgepole. 

Immature lodgepoles repair damage from the 2012 fire. PHOTO BY PAUL SWENSON

Lodgepole pines, as with most evergreen trees, have flowers, but not like the ones we all think of from high school biology class where you were probably taught all the male and female parts: stamen (the male) and the carpal (the female). If a flower has both parts, it is called “perfect” and usually uses a pollinator like a bee to transfer pollen to fertilize the seed in the ovary. But the lodgepole has an “imperfect” flower, meaning a flower contains either the male or female parts of the flower.

A female lodgepole flower. PHOTO BY PAUL SWENSON
A male lodgepole flower. PHOTO BY PAUL SWENSON

The female, seed producing, flower looks like a tiny pinecone. The pointy ends of the segments are the ends of tubes that capture pollen that will fertilize the seeds in the cone. To get the pollen to the female flower without the use of a pollinator, lodgepoles depend on the wind to carry it between the male and female flowers. Once the seeds in the cone are fertilized, the tree spends the rest of the next year photosynthesizing sugars for producing new growth and maturing the seeds in the cones. 

Like several other species of pine trees that have evolved with forest fires, lodgepoles produce two types of cones: an open cone, one that disperses winged seeds once they are mature, and closed or serotinous cones.

A closed or serotinous cone. PHOTO BY PAUL SWENSON

The serotinous cones are held together by pine resin for many years until it melts at a temperature between 120-140 degrees Fahrenheit. These temperatures are only seen in our locale during a forest fire. Therefore, the lodgepole seeds from these closed cones are some of the first seeds that germinate in the soil after a fire.

For such heroic trees, they have their own Achilles’ heels: parasites, insects, drought, fungi, and time. One such parasitic infection is called witches’ broom (Arceuthobium americanum) also known as American dwarf mistletoe. It is a very interesting infection that is quite apparent as one hikes through the surrounding forest. 

Witches’ broom is a parasitic plant that hijacks the lodgepole’s carbohydrate manufacturing process for its own gain. In doing so, the witches’ broom causes the branches of the pine to grow without regulation and causes a structure that looks like a “broom.” As the infection spreads the host tree becomes more and more contorted and eventually dies. 

Witches’ broom itself has an interesting life cycle. Once it infects a tree, it takes several years for the telltale signs of an infection to be apparent. The fascinating part is how the infection spreads.  

In late summer it produces very small stems that emerge from the extensive stem system that grows under the bark of the lodgepole, exposing male or female flowers to the environment. Just like the pine, they are incomplete. The female flower does not have petals but does have 2-5 sepals, the green outer part of a bud that protects the flower inside.  

Once fertilized each flower grows one berry and, when it is mature, contains a fluid under pressure. There is enough pressure that when the berry bursts it can launch the seed up to 35 feet from its host tree. The seed is covered in a sticky mucous so when it lands on a neighboring tree, it sticks to its needles. The coating dissolves in rainwater and the seed slides down the needle to infect the branch. Sometimes the seeds stick to a bird’s feathers so it can be spread far and wide.

Of course, the witches’ broom is bad for the lodgepole, but it does make for some interesting viewing now that you know what to look for out on your hikes. See if you can be the first in your party to spot one. They provide protection for many songbirds, grouse, squirrels, Pine Martins, and other organisms that if you look closely, you might be surprised at what you find.

Paul Swenson has been living in and around the Big Sky area since 1966. He is a retired science teacher, fishing guide, Yellowstone guide and naturalist. Also an artist and photographer, Swenson focuses on the intricacies found in nature. 

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