Why Seeds From Some Trees Pop In High Heat - Bay Nature (2024)
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.
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.
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.
This cone can remain in the tree's branches for decades, until the heat of a passing fire melts the resin that seals it and allows the cone to open, dropping its seeds. To the left is a mature lodgepole pine forest.
On sunny winter days, tree bark will warm from solar energy. This causes cells to expand in the bark and the sapwood under the bark. When the sun goes down, the temperature drops very quickly causing the bark to contract. However, the sapwood under the bark doesn't cool as fast, causing the bark to crack.
If you are in luck, you may hear the snapping sound of the cones popping open naturally to release the seeds. Pinecone seeds must be released from their cones before the keys can be transported on the winds. Big Idea: Plants have distinct characteristics.
Aside from weather-related stresses, botanists have speculated that heavy pinecone production may also be a defense mechanism that the tree uses to prevent insect infestations. Insects prey on the seeds dropped by trees and will “camp out” beside a dubious source of sustenance.
A white pine seed looks a bit like a maple seed lodged inside the cone, with a papery tail extending the length of the scale. This tail acts like a kite. During rainy weather, the moisture-sensitive scales of a pine cone close to keep the seeds dry and protect them from rot.
The inner layers of a pine cone expand like a sponge, causing the scales to close. When the weather is warm, dry, and right for growing, pine cones open their scales to release their seeds. The inner layers of the pine cone shrink, causing the scales to open.
Step 5: Preheat your oven to 200 degrees on a convect setting if you have it. If not, you can raise the temperature to 225 degrees. The pine cones will take several hours to completely dry out and reopen. Just stop by the oven every 45 minutes to an hour to rotate them.
It's simply so that the seeds are free to disperse when conditions are best for winds to carry them far and wide which is when it is dry. Wet weather dampens dispersal and so the cone closes up when wet to protect the seeds.
The serotinous cone of a lodgepole pine, opened by the flames of the Maple Fire. Lodgepole pine is an evergreen conifer tree that ranges from Alaska to Mexico. Generally, it grows in dry montane and subalpine forests in the Rocky Mountains to near the Pacific Ocean.
A tree that has lost part or all of its leaves or needles because of heat from a fire may or may not recuperate depending on the species and degree of the fire damage. Healthy, deciduous trees can be resilient after being partially burned and may produce new leaves and stems, as well as sprouts at the base of the tree.
But, fire is a part of the lodgepole pine ecosystem. Fire removes old trees, trees affected by insects or disease and opens cones to release seeds for a new forest. Lodgepole pine likes open sunny areas and reseeds quickly in burned-over areas.
After seeds have developed, pine cones open in dry weather, and seeds are dispersed by wind. If you hold a freshly dropped female cone with the pointy bit facing up, you'll see the seeds sitting close to the stem, just above the pine cone scales – those individual woody pieces that give the cone its shape.
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.
It's simply so that the seeds are free to disperse when conditions are best for winds to carry them far and wide which is when it is dry. Wet weather dampens dispersal and so the cone closes up when wet to protect the seeds.
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