Plant Species of San Jacinto Mountain:
Variation of Yellow Pine Cones With Moisture Content

This page was written in 2007. The information on this page is still correct for the cones I had seen in 2007, but with further experience I have learned much more about the variation in these species. In particular, the prickles of the two species are quite variable, and I've found a number of Jeffrey cones that are prickly even when closed. Further, the angle of the rows of scales primarily depends on the size of the cone, which determines how many rows of scales it has, which in turn determines how steep the angle of the rows are. See Plants of Southern California: Pinus jeffreyi and P. ponderosa var. pacifica for much more information about these two species and those characteristics.

The following text is from 2007.

This page discusses the effect of cone moisture on two distinguishing characteristics of Jeffrey pine, Pinus jeffreyi and ponderosa pine, P. ponderosa: the prickliness of the cones, and the angle of the rows of the scales.

Cone Prickliness

The prickliness of the cones is embodied in the simple mnemonic gentle Jeffrey; prickly ponderosa. This mnemonic is probably the main cause of botanists confusing the two species, even though it comes directly from botanical floras, and has some truth to it. For example, Munz (1974) says for ponderosa: the spreading scales with short prickles protruding outward; and for Jeffrey: the scales with long mostly deflexed prickles, the points seldom protruding outward.

I was very confused about this distinguishing characteristic for years, and eventually concluded in 2006 and 2007 that it simply didn't work in southern California; both species seemed to almost always have gentle cones, with prickles not protruding outward.

Enlightenment came serendipitously. In order to analyze the difference between these two species, I had collected cones from both in early October 2007. Solely to keep specimens from different days separate, I had left cones collected on 4 October 2007 outside, and cones collected on 10 October 2007 inside. On 13 October 2007 we received 0.25 inches of rain. I was astonished when I happened to walk by the cones left outside; they had both become prickly!

The following pictures compare the dry cones left inside with the moist cones left outside.

The leftmost two cones are Jeffreys from two different trees; the rightmost two cones are ponderosas from the same tree, the first specimen on the Devils Slide Trail. In both sets, the dry cone is at left and the moist cone is at right.

Closeup of Jeffrey cones from the side and from above; dry cone at left and moist cone at right:

Closeup of ponderosa cones from the side and from above; dry cone at left and moist cone at right:

It is very clear from the above photographs, and even clearer when you pick up the cones, that both species are gentle when dry, and prickly when wet.

If you look closely at the prickles in the above pictures, there is no difference in orientation in dry cones, but there is a difference in moist cones. The following pictures show closeups of the prickles, again with the dry cone on the left and the moist cone on the right:

Jeffrey pine conesPonderosa pine cones

Aha! Now it is clear that the Munz statement is correct, but only for moist cones. The ponderosa prickles are indeed short, and they do protrude outward. The Jeffrey prickles are indeed long, and they are mostly deflexed, seldom protruding outward.

But note again in the dry cones on the left, there is absolutely no difference in the orientation of the prickles.

The mnemonic thus has a grain of truth to it, as long as you understand that it applies only to moist cones, and the meaning of gentle and prickly has nothing to do with how the cones feel in your hand. This mnemonic probably works a lot more often in wetter northern California than in drier southern California.

Angle Of The Rows Of The Scales

The Flora of North America key to separate these two species includes this characteristic:

41. ....; seed cones with steep spirals of 5-7 scales per row as viewed from side;... .. P. ponderosa
41'. ...; seed cones with low spirals of 8 or more scales per row as viewed from side;... .. P. jeffreyi

I was excited when I read this, since it seemed here was a clear-cut difference between the two species, with even a numeric difference quoted!

But when I looked at cones to try to verify this, I was totally mystified. I could neither get the numbers of scales per row to work out, nor could I see any difference at all in the angles of the spirals. Try it yourself on the dry cones pictures above, on other pictures here, or using the cones shown in the Jepson Manual illustration on p. 123. It is hard even to see well-defined rows in many cones.

Once again, enlightenment came from the same serendipitous chain of events as for the prickliness. The following picture shows the moist cones of the two species side by side, with several spirals on each cone indicated with black lines:

I was shocked! Very clear spiral rows had appeared! And they had the angles given in the Flora of North America key!

Both cones have missing scales that I ripped out in order to test another distinguishing characteristic, the difference in color above and below; in the picture immediately above, I failed to rotate the cone on the left to put the missing scales on the back side. The missing scales have no affect on the spiral angle of the spirals above the rows with missing scales, and other cones with no missing scales show similar angles. See the topmost pictures above for a view of the other side of this cone.

There is no doubt that the spirals for the ponderosa cone are much steeper than those for the Jeffrey cone. For the Jeffrey cone, the average angle is 14° for the top line and 24° for the bottom line. For the ponderosa, the average angle is 47° for the top line and 42° for the bottom line.

I still have a problem with the counts of the number of scales, however. The Jeffrey cone has 7 scales in the top marked spiral, and just barely squeezes in 8 in the bottom marked spiral. The ponderosa cone has 9(!) scales in its top marked spiral, and 8 in the bottom marked spiral. So that aspect of the Flora of North America key doesn't seem to work.

But I'm happy; two out of three ain't bad, especially when I've been 0 for 3 for such a long time. (:-)


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Copyright © 2007-2014 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
http://tchester.org/sb/plants/species/yellow_pine_cones_moisture.html
Comments and feedback: Tom Chester
Updated 30 October 2014.