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Why Did I Hear Popcorn Sounds In the Recent Extreme Heat Wave?

by Trent Pearce on September 26, 2017

Cones are opening to release seeds of this Monterey pine. (Photo by Forest and Kim Starr)
Cones are opening to release seeds of this Monterey pine. (Photo by Forest and Kim Starr)

I was standing under a big Monterey Pine on Friday at Stinson Beach when the temperature was 100 degrees. I heard a continuous popping sound, like popcorn…could it have been the cones popping open in the extreme heat? I didn’t see any evidence of seeds drifting down, or cones falling off the tree. — Liz, Oakland

Standing on the back lawn of Tilden Nature Area Environmental Education Center on Labor Day weekend when the temperature soared to 104 degrees, I heard a similar sound coming from the crowns of our nearby Monterey pines. Although you and I didn’t see any seeds falling to the ground accompanying these “pops,” the cones were in fact opening, and the seeds were waiting for their attachment points to dry, which releases them to the wind.

Monterey pine, Pinus radiata, shares an adaptive trait with two other Californian conifers that evolved long ago to live with fire. Rather than opening their cones each year and dropping seeds into a crowded forest understory, these trees keep their cones glued shut with a sticky resin. Seeds can remain stored in these cones for many years, protected from predation or disease, until conditions below the tree become favorable for seed dispersal and germination. Conifers that employ this reproductive strategy are known as “closed-cone,” since their cones neither open nor detach from the tree when the seeds are ripe. The technical term for this ecological adaptation is serotiny.

Monterey pine cones before serotiny. (Photo by Forest and Kim Starr)

Monterey pine cones before serotiny. (Photo by Forest and Kim Starr)

After a forest fire, the understory is barren, full of freshly available nutrients and ideal for seed germination. Closed-cone pines have evolved to use the heat from a forest fire as a trigger to opening their cones. The extreme heat melts the resinous sap, allowing the cone to open and exposing the seeds to the outside world. The seeds then quickly dry and detach from inside the cone scales and are dispersed by wind, gravity, and wildlife. In contrast, “open cone” conifers rely on ambient moisture inside the cone to hold the seeds in place until weather cues signal that conditions are right for dispersal.

Unlike our other two closed-cone conifers (bishop and knobcone pines), Monterey pine is not strictly dependent on forest fires for cone opening and seed dispersal. Extremely hot weather can also melt the resin and allow the cones to open, permitting a slower drying, detachment, and dispersal of some of the seeds. Cones opened this way can reportedly close again when temperatures drop. Time can take its toll on the serotinous cones as well: the tissues within the cone lose their strength with age and the cones will eventually open unprompted by heat, although this can take over a decade.

Why do Monterey pine need these different ways to open cones and disperse seeds? Think of it as built-in redundancy to give them a better chance at reproducing. While a forest fire creates the ideal conditions for seed dispersal, opening the cones on hot days allows these trees to release small amounts of seeds, slowly replenishing the seed bank and encouraging regeneration without relying on fire as a catalyst. This is fortuitous for a tree whose native range has seen not only a massive decrease in fire frequency over the past hundred years, but also unprecedented high temperatures this summer.

Today, Monterey pine is widely planted for timber, pulp, and aesthetics. It is commonly quoted as being the most widely planted tree in the world. Growing from Australia to Chile, it has vastly outgrown its small native range on the California coast and Baja islands. But its adaptations remain, and on very hot days it may just pop a few cones open and slowly shower us with ripe seeds.

trent-pearceTrent Pearce is an interpretive naturalist with the California Center for Natural History and the East Bay Regional Park District. Currently residing in Berkeley, California, he enjoys documenting the Bay Area’s flora, fauna, and fungi on www.iNaturalist.org, and can be seen prowling the dusty trails of the East Bay, camera in hand.

Ask the Naturalist is a reader-funded bimonthly column with the California Center for Natural History that answers your questions about the natural world of the San Francisco Bay Area. Have a question for the naturalist? Fill out our question form or email us at atn at baynature.org!

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