Douglas-fir seeds: a trick question

It's winter. It's raining outside. What a perfect time to take apart a Douglas-fir cone on the kitchen table.

Douglas-fir cones are emblems of the Pacific NW. Above each cone scale, distinctive "mousetails" stick out. Kindergartners learn to identify Douglas-fir from these "mousetails."

In this photo, the cone scales are wide open. Why? Because the the cone has been indoors, drying out. If you were to dunk the cone in a glass of water for a day, the scales would close.

This is the process that causes the cones on almost all types of Pacific NW conifers to open in late-summer or early-fall to release their seeds: The scales open as the cone dries.


What is inside?

Using a scissors, I managed to cut one scale off. This view is of the inside of the scale. You can easily see the imprint (like a fossil) of where two winged seeds used to be.

On most Pacific NW conifers, two seeds grow behind each cone scale.

On this cone, almost all of the scales have imprints of seeds but no actual seeds. The actual seeds fell out months ago like they are supposed to.

Here are some abnormalities. These cone scales contain seeds that failed to fall out. Something went wrong in the reproductive process; I do not know what.

Here are some of the few seeds that I found in this particular cone. All the rest are gone, having fallen out.

Douglas-firs, like most Pacific NW conifers, have winged seeds. To extract an actual seed from the winged part requires a razor blade, care, and patience. The seeds themselves are tiny.

There are companies that collect seeds for their own use or to sell for timber plantations or re-vegetation projects. One example of such a company is Silvaseed in Roy, Washington. To extract the actual seeds from the winged "chaff," they use machines that resemble a clothes dryer with a myriad of razors positioned just outside the holes in the drum. The razors cut off the chaff.

Here's the trick question:

In a steady-state environment, out of all the seeds that a single Douglas-fir tree produces in its lifetime, how many will grow into another mature Douglas-fir tree?

First, let's estimate how many seeds we're talking about. On the cone in the photo above, I tried to count the number of cone scales. There were less than 50. Let's round up to 50. Behind each cone scale are 2 seeds, so we'll estimate 100 seeds per cone.

I've looked at Douglas-firs in the summer and tried to estimate how many cones are on each tree. It can be well into the thousands, and probably well into the ten thousands. Let's round down to 1,000 cones per tree.

Douglas-firs, like many other Pacific NW conifers, go through cycles of cone productivity. A Douglas-fir is highly productive (a "mast" year) only once every 5-to-7 years, with a cycle of moderate or even no cone production in between. For the purpose of estimating, let's assume a tree has 1,000 cones once every 5 years and zero cones in all other years.

Douglas-firs can live to 1,000 years or more. For the sake of estimating, let's assume that they last 500 years.

How many seeds is that?

  • 100 seeds per cone

  • 1,000 cones per tree

  • every 5 years

  • for 500 years

That's 100,000 seeds per tree, 100 times during the tree's life. The total is 10 million seeds. That's a lot!

This total may be off by a factor of 10 or 100 or even 1,000: The point is that a single Douglas-fir produces a great many seeds during its life.

So, in a steady-state environment, how many of those myriad of seeds should germinate and grow into another mature Douglas-fir?

Your thoughts?

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