Plants of Southern California:
Pinus jeffreyi and P. ponderosa var. pacifica

Fig. 1. In southern California, Jeffrey needles are usually dull blue-gray-green (left); ponderosa needles are always shiny green (right). This difference is most apparent in full sunlight with the sun behind you. At other angles, and/or without direct sunlight, the difference in color from a distance may not be so apparent. Some Jeffreys have greener colors than others, and hence having two colors side by side is not proof by itself that one is a ponderosa. However, a fresh needle sample viewed in direct sunlight, compared to a fresh ponderosa needle sample, will always distinguish the two species. Photograph from the former Pine Hollow Picnic Area on the Angeles Crest Highway, 1.4 miles east of Islip Saddle, taken on 10 October 2014.
Click on the picture for a larger version showing more of the trees.


Jeffrey pine at mile 1.83 of the Devils Slide TrailPonderosa pine at mile 0.00 of the Devils Slide Trail

Fig. 2. In southern California, in all except very low rainfall years, Jeffrey cones (left) are roughly 50% larger than ponderosa cones (right). Shown are typical cones from the Devils Slide Trail at San Jacinto Mountain, photographed on 10 October 2007. Once you get familiar with the size difference, you can usually identify the trees from the cones lying on the ground on the roadside from a car driving at 30 mph.


Fig. 3. The scales on the Jeffrey cone are close to being the same color on both sides (left). The scales on the ponderosa cone are almost always darker, usually blackish, on the prickle side (right). The scales of both species darken with age, but the difference between Jeffrey and ponderosa usually remains quite apparent. Note the wicked spines, most of which are outcurved, on what is erroneously referred to as the "gentle Jeffrey" cone. (That misleading mnemonic is the source of most of the confusion in identifying these species.)
The Jeffrey cone was photographed on 5 October 2014 from the Heartbar Area at San Bernardino Mountain. The ponderosa cone was photographed on the Big Horn Mine Trail in the San Gabriel Mountains on 26 September 2014.
Click on the pictures for larger versions.


Table of Contents


Introduction
Distinguishing Characteristics
Non-distinguishing Characteristics
Summary of Haller's findings on hybrids


Introduction

Pinus jeffreyi and P. ponderosa are two species that cause fits for most southern California botanists who have not studied the two species closely.

These two are undoubtedly completely-separate species, separated by a number of characteristics, that rarely hybridize; see the definitive work by Haller summarized below. However, many botanists do not know how to reliably tell them apart. Probably as a consequence, the rumor persists that they hybridize frequently, which is definitely not the case. See Figs. 1 to 3 above for examples of three of the clear differences between these species.

Hybrids are rare for a number of excellent reasons. For example, the 1990 update of the Silvics Manual, in the entry for Ponderosa Pine says:

Natural crosses of ponderosa pine with Jeffrey pine have been observed in California where their ranges overlap, but they are rare. Where the two species grow in the same stand, different flowering times and other reproductive barriers restrict crossing.

These quite-different species, as expected, have problems in producing fertile seeds even when a successive cross-pollination is achieved. The Silvics Manual entry on Jeffrey Pine says:

Observed crossing barriers include failure of viable pollen to germinate on the nucellus, failure of pollen tubes to maintain normal growth rates in the nucellus, lack of exact timing between tube growth and egg formation, collapse of the proembryo, and failure of the embryo to develop. In both crosses, slow growth of the pollen tube appears to be the most common barrier to development of hybrid seeds (29).

See also Pinus jeffreyi and P. ponderosa on the Devils Slide Trail for a discussion about how people have made an explosive mistake in trying to make turpentine from Jeffrey pines instead of ponderosa pines. The chemistry of the sap is completely different between the two species, with Jeffrey sap containing heptane, a highly explosive compound, and ponderosa sap containing non-explosive terpenes, used for making turpentine.

These compounds, possibly along with others, are part of the chemical defense of each species against predators. Many insects have evolved defenses against the defense chemicals unique to a single species, and thus can attack only that single species, or closely related ones with the same chemicals. Different species of bark beetles infest ponderosas and Jeffries, and they are never confused about which species is which; it would be a deadly mistake for them to make, too.

These two bark beetle species make differently-shaped egg galleries, and this is yet another way to tell the difference between the species. The western pine beetle attacks ponderosa and Coulter pines, and the Jeffrey pine beetle attacks only Jeffrey pine.

If bark beetles, with brains smaller than your thumbnail, can tell the difference between ponderosa and Jeffrey pines, with a little attention humans ought to be able to do the same. (:-)
 

The rumor also persists that these two species are hard to tell apart, which of course contributes to people thinking that they hybridize. If you can't tell them apart, it is only natural to conclude that they must be the same species, or they must frequently hybridize. In fact, our experience is that at least 95% of the time, it is easy to discriminate the species simply by glancing at the size of the cones on the ground, without doing any other observations. If the cone size falls into the fairly-rare intermediate zone, an examination of difference in the color of the scales on top and bottom, and/or the needle color, will usually quickly resolve the determination.

This is not to say that the determination of every tree of these species is always easy. Some trees try to fool you, just like many specimens of other species try to do as well, due to intrinsic variation within a species. It sometimes takes a committee of 4-5 trained observers a few minutes of study before arriving at a determination that everyone is then happy with.

With a little practice in an area where both species actually occur, you can learn to distinguish the species readily and reliably. Harlow and Harrar, in their 1953 Checklist of native and naturalized trees of the United States (including Alaska), state Trained observers rarely experience any difficulty in distinguishing between Jeffrey and Ponderosa Pines in the field. We strongly recommend you visit an area where both species occur to try to learn how to identify them, since it is all too easy to try to make two species out of one, due to the intrinsic variability of each species, if you are in an area where only one species occurs.

Three areas with both species in close proximity are:

Complicating the problem for botanists is that a widely-known mnemonic, "Prickly ponderosa, gentle Jeffreyi", that claims to separate the species by how prickly the cone is, is largely useless in southern California unless the cones are closed from recent rainfall, and even then is not a highly-reliable characteristic. Tom has measured a lot of cones for the length measurements reported below, and picking up the cones of either species can be a prickly experience. Tom has actually been prickled more in picking up Jeffrey cones than by picking up ponderosa cones. Jeffrey cones often have a much more wicked prickle, which is easy to contact in picking up open cones.

Tom led a CNPS field trip one time that began in the Humber Park area of SnJt, where there are only ponderosas. Tom had the group collect yellow-pine cones, and decide whether there were one or two species here. The group concluded that there were two types of cones here, prickly and not prickly, and consequently thought both species were here. In fact, the group had separated the cones based on age and/or how open the cones were. When they were shown some moistened cones Tom had brought along of both species, they were amazed at the true gentle / prickly difference between the cones of the species.

By the way, Tom had previously misdetermined these ponderosas as Jeffries, when he was a rookie botanist. Learning how to reliably separate these species took considerable work.
 

Now that we've established that these are clearly-different species, the rest of this page will discuss how to tell them apart.

There is only one variety of P. ponderosa in southern California, var. pacifica , so we will drop the mention of the variety in the following. All the following pertains only to the southern California plants.

More extensive discussions of some characteristics can be found on these older pages (written prior to this page, and mostly not revised since then) specific to San Jacinto Mountain:

and in these newer pages:

In all of the photographs on this page, and tables, Jeffrey is always on the left, and ponderosa on the right. Most, but not all, of the photographs can be clicked to obtain a larger version of the picture, which often shows a larger field.

Distinguishing Characteristics

The distinguishing characteristics to separate these two species are listed in Table 1, in declining order of their reliability (see next paragraph). Comments are given about the ease of use in the field for many of these. These characteristics are often discussed in more depth in links in Table 1.

Our definition of reliability is the percentage of time that a tree of a given species will exhibit the characteristic it is "supposed to have" in the field. It is not an estimate of how reliable that characteristic alone would be to differentiate the species; see below for a example of the difference.

The reliability for each characteristic is usually estimated from our use in the field, once we became familiar with how to use that characteristic to separate the species, and are mostly qualitative numbers. In general, beginners will not have anywhere near that reliability in use of the characteristics until they have seen both species a few times, and become knowledgeable in their differences.

Table 1. Characteristics that distinguish P. jeffreyi and P. ponderosa, in descending order of reliability when used by a trained observer

ReliabilityCharacteristicP. jeffreyiP. ponderosaComments / Ease of Use in the Field
98%Sap chemistryContains heptanesContains terpenesThis can be done in the field, but it requires stabbing the tree with a knife, waiting an hour to get a fresh drop of resin, putting that drop of resin in a container, and then covering the drop with Schiff's reagent. After 15 to 60 minutes, depending on ambient temperature, Schiff's reagent will turn a deep purple color for Jeffrey resin, and retains its clear yellowish color for ponderosa resin.

The reliability number is from Haller 1962 who used this test extensively, and found only a single tree with all the external characteristics of P. ponderosa but which had the P. jeffreyi resin reaction color.

Smelling the bark crevices (see below) is possibly an easier method of detecting these compounds.

98%Needle colorgray blue green, dull, glaucous-appearing (but sometimes appearing greenish)green, shiny, not glaucous-appearingSee Fig. 1 and Needle Color.

The reliability of a fresh needle color is very close to 100% if you have an example of needles of both species side by side, viewed in direct sunlight.

We give a number of 98% for the reliability in the field, since this is somewhat tricky to use in the field.

The apparent needle color of an entire tree seen from a distance changes dramatically depending on illumination and the observing angle to the illumination, as well as how many year's of leaves are on the tree at once, since the color of ponderosa needles changes with age. The color difference is most apparent in full sunlight with the sun behind you.

Worse, in some locations, some Jeffreys have greener colors than others, and hence having two colors side by side is not proof by itself that one is a ponderosa. In those cases, a needle sample compared to a ponderosa needle will still reveal the difference.
Interestingly, lighting conditions (e.g. indoor lights, such as a microscope light) can make the needles appear to be the same color!

95%Vegetative budsnot-resinous, with abundant white membranous-looking hairs on the edges of the bracts. Opening buds have prominent bracts with acute tipsresinous, with smaller numbers of brownish hairs on the edges of the bracts. Opening buds have much-less-obvious bracts with rounded tipsSee pictures.

This difference is quite reliable in fresh specimens using a hand lens, with ponderosa buds often appearing so resinous that you wouldn't even think of touching them, and Jeffrey buds appearing entangled in white hairs without significant obvious resin.

In our experience, this characteristic is nearly 100% reliable, but Haller gives it as just 87 to 90% reliable in sympatric populations, with only 66% of Jeffrey buds in sympatric populations having abundant white hairs. Hence we have knocked down our estimate of 100% reliability to 95%, in case populations we haven't seen in southern California are not as reliable as those we have seen.

This difference is used in the Jepson Manual and Flora of North America keys.

95%Cone scale colorbrown on both top and bottomgen darker (blackish) on prickle side than on the other sideSee Fig. 3.

This is quite easy to use in the field, and simply requires picking up a cone and turning it from top to bottom. Jeffrey cones will usually have the same color no matter the orientation. The color of ponderosa cones usually will change almost as dramatically as changing the slat angle of a venetian blind in daylight.

The actual coloration varies with cone age, but the difference or not of the two sides generally remains apparent.

We estimate the reliability at 95% since infrequently we come across cones that are not obvious at a glance whether the observed color difference is clearly one species or the other.

90%Cone lengthgen 10-20 cmgen 7-12 cmSee Fig. 2 and cone lengths.

If you observe a number of cones of length 12 cm and greater, the probability is nearly 100% that you have found a Jeffrey. In non-drought years, if you observe a number of cones smaller than 12 cm in length, the probability is nearly 100% that you have found a ponderosa. This difference works for most trees in severe drought years as well, but in those years a small minority (1%?) of Jeffries, in drier locations, will have ponderosa-size cones.

90%wing length / seed length1.5-2.42.9-4.5We've only checked this characteristic in seeds from a single cone of each species, since seeds are not always readily available. For those, we measured a ratio of 1.4 to 1.5 for Jeffrey; 2.8-3.7 for ponderosa. See Fig. 6. The reliability score is from Haller 1962.
85%cone shapemore deltoid - relatively wider near their fat endmore cylindric - more slender at their fat endThis takes some experience to get 85% reliability, and it much more apparent in closed cones than in open cones. It is hard to describe the difference, but the gestalt of the cone is different. Studying pictures of closed cones is perhaps the best way to pick up on this. The Jeffrey cones in that picture have more volume for a given length since those cones are relatively wider near their fat end. The ponderosa cones are more slender, closer to the same width throughout their fat end.

It is much harder to see the shape difference in open cones, but with a practiced eye, it is sometimes so obvious that when you glance at the cones on the ground, you immediately know which species they are. This is especially true when you have been seeing one species, and then see the other species for the first time that day.

80%Bark chip inside colorgen red-brown, sometimes with some yellow-browngen yellow-brown, sometimes with some red-brownSee Bark Chip Color. In most cases, when you check the inside color of a bark chip freshly removed from a tree, it will be quite yellowish for ponderosas, and often red-brown for Jeffries. However, Jeffries can have yellow-brown chips that can easily be confused with ponderosa chips. Experience helps a lot in using this difference reliably.
70%Bark crevice odorsweet smell described as vanilla, pineapple, banana, and/or butterscotchnon-sweet smell (turpentine odor), or sweet smell not {pineapple or banana or butterscotch}, but possibly vanillaOdor is not apparent during low-moisture times of year, or on cold days, or on shaded part of trunk. Under most decent conditions, about 70% of the time, if you smell the bark of a Jeffrey, you'll get an odor of vanilla, etc., and 30% of the time you'll get very little odor. About 90% of the time, if you smell the bark of a ponderosa, you'll get a turpentine smell, or no smell, and 10% of the time you may get a faint sweet smell that might remind you of vanilla, etc. The reliability number quoted is the minimum of those two numbers.
50%?Bark beetle chamber straightnessmostly straight, sometimes with J-shapehighly curvedWorks only on dead trunks without bark that had bark beetle infestations. Other beetles can carve chambers that confuse the issue after the tree is dead.
30%Bark appearanceExtreme Jeffrey bark for old specimens tend to be dark purplish red, with deeper fissures that are closer together, producing narrower plates.Extreme ponderosa bark for old specimens tend to have shallower fissuers that are farther apart, producing wider plates Bark changes markedly with age of the tree; in youth, both species have dark-colored bark without plates. Haller 1962 says in a typical stand of mature Ponderosa pine only 60% of the individuals have broad plates and yellowish color, and that individual trees often have a great difference in bark apperance on opposite sides of the same tree. The bark of both taxa tend to be the same at lower elevation. Haller concludes Bark fissures and color obviously leave much to be desired as indicator characteristics.
30%Cone scale tip pricklePrickle often as long as its pyramidal base, sometimes recurvedPrickle often shorter than its pyramidal base, gen straight"Prickly ponderosa, gentle Jeffreyi" is probably responsible for most misidentifications of these species. See the example given above of how unreliable this mnemonic is.

There is a grain of truth in this mnemonic if you observe mature cones closed from recent rainfall. However, immature closed Jeffrey cones on the tree, or ones that fell prematurely, are very prickly.

10%Bark "hardness", measured by tapping the plate of an old tree with a fingernailsmall pieces gen not falling freesmall pieces gen falling freeThis is mentioned by Peattie, and is "kind of true" for very mature specimens. The bark "puzzle pieces" of ponderosa do fall off more readily for very mature specimens, but this simply is not very reliable. The puzzle pieces do not fall off very readily for most trees of both species, and very old Jeffries sometimes have puzzle pieces that fall off leaving great piles on the ground around the tree.
Characteristics given below need more work by us to estimate their reliability and ease of use
?Immature cone colorbrown; light green to reddish-purpleyellow; light yellowish-greenThe first-mentioned colors for each species are given by Minnich and Everett 2001; the second-mentioned set is given by Haller and Vivrette 2011. We have not yet tried to verify this.
?Pollen cone color before anthesisyellow or with a purplish tingered or magentaColors from Farjon 2010. We have not yet tried to verify this.
?Separation of scales in cone just prior to and after cone falllower scales not very spreading and reflexed, thus not well separated from adjacent scaleslower scales spreading and reflexed, thus well separated from adjacent scalesMentioned in Flora of North America key. Can only be seen in freshly-fallen cone, or one about to fall.

Discussion of the reliability numbers. An example of the difference between the reliability of the characteristic for each species, versus the reliability of the characteristic for separating the species. About 70% of the time, if you smell the bark of a Jeffrey, you'll get an odor of vanilla, etc., and 30% of the time you'll get very little odor. About 90% of the time, if you smell the bark of a ponderosa, you'll get a turpentine smell, or no smell, and 10% of the time you may get a faint sweet smell that might remind you of vanilla, etc. The reliability number quoted below is the minimum of those two numbers.

The reliability of that characteristic to determine the species can be computed as follows, if you are in an area with equal numbers of both species. If you smell vanilla, etc., you have a ~88% probability that you have smelled a Jeffrey. If you don't smell vanilla, you have a 75% probability that you have smelled a ponderosa.

Comment on how accurate determinations are made from just four of the best characteristics that can be used easily in the field. If one uses just the best four characteristics other than sap chemistry, the reliability numbers imply an error rate of just 0.0000002 in the determination if all four characteristics point to one species. (The error rate is arrived at by multiplying {1 minus the reliability} for each of the four characteristics, which equals (0.02)* (0.02) * (0.05) * (0.10) = 2 x 10-6.)

Non-distinguishing Characteristics

Table 2 gives characteristics that references have claimed distinguishes the species, but which in fact do not distinguish the species reliably enough to be trusted.

Table 2. Characteristics that are not useful to distinguish P. jeffreyi and P. ponderosa

CharacteristicP. jeffreyiP. ponderosaComments
Tree morphologyoften a more conical shape, more slender, elongate limbs, distinctly upturned at endsoften a more cylindric shape, with stout limbs, gen not upturned at endsThese traits are mentioned by Peattie.

Minnich and Everett used the shape in aerial photographs, with the needle color, to reliably distinguish these species. The morphology difference probably is useful in examining stands of these species in aerial photographs, since the results of Minnich and Everett seem reliable. But it is problematic for individual trees seen from the ground; we have seen far too much variability for this to be useful from the ground. For example, compare the shapes of the trees and how upturned the branches are in the larger picture used for Fig. 1; both species look the same to us in that picture for those traits.

Bark chip inside presence of resin pitsnot presentpresentThis is mentioned by Peattie, but we have found that both species have resin pits inside most bark chips. It does seem that ponderosa bark chips have a greater density of resin pits, but since the number of resin pits varies from zero to ~10 for ponderosa, and zero to ~5 for Jeffries, this simply isn't reliable.
The following two characteristics are all related to the cone scale arrangement, and are not reliable discriminants
# of scales per row in cone as viewed from side8 or more in low spirals5-7 in steep spiralsMentioned in Flora of North America key
# of spiral rows in cone8 in one direction; 13 in the other5 in one direction, 8 in the otherMentioned in Haller and Vivrette 2011.

Discussion of distinguishing characteristics

Bark chip color

The bark of both species forms plates that flake into puzzle-size pieces that can be easily broken off. The inside of a fresh flake is often yellowish-brown for ponderosa pine, and often reddish-brown for Jeffrey pine. However, ponderosa pine can also have reddish-brown on the inside of its bark, so this is a somewhat tricky characteristic to use. Nonetheless, it can help in determining the identity of a given tree. See Pinus jeffreyi and P. ponderosa bark chip colors.


Needle color

Fig. 4 shows the same needles under different lighting conditions, and sometimes with the needles in switched positions to make sure the positions of the needles isn't important. The P. ponderosa needles were from a tree on the Big Horn Mine Trail at SnGb. The P. jeffreyi needles were from a tree on the Table Mountain access road at SnGb. They were photographed the next day.

The color difference appears quite different depending on illumination and how the camera exposed the picture. One photograph makes the difference appear very dramatic, with the Jeffrey needles being strongly blue-glaucous and the ponderosa needles green. Five photographs show a clear difference, but not as strong as the one showing a dramatic difference, with the Jeffrey needles now appearing somewhat gray-green-glaucous. The two photographs using microscope lighting show the least difference, with the Jeffrey needles now appearing just barely more gray-green-glaucous than the ponderosa needles.

We have also found that ponderosa needles become more similar in color and shininess to Jeffrey needles as they age on the tree. Nonetheless, a side by side comparison in sunlight still shows a clear difference.

In the field, the needles of both species weather to the same brown color. In the one voucher specimen of each species we examined, the needle color difference was not apparent to us, but we examined it under indoor lighting plus diffuse lighting from a north-facing window, which minimizes the difference even of fresh needles.

Outdoor in somewhat diffuse sunlight.
Jeffrey on left.
Outdoor in direct sunlight
Jeffrey on left.
Outdoor in direct sunlight
Jeffrey on right.
Outdoor in direct sunlight
Jeffrey on left.
Direct photograph of needles on microscope stage, using only microscope light
Jeffrey on left.
Photograph through the microscope eyepiece, using only microscope light
Jeffrey on left.
Indoor diffuse light from skylight, black background
Jeffrey on left.
Indoor direct sunlight through sliding glass door, black background, sunlight from base of needles
Jeffrey #1 and #3 from left.

Fig. 4. Photographs of the same Jeffrey and ponderosa needles under different lighting conditions. The ponderosa needles came from the Big Horn Mine Trail in the San Gabriel Mountains; the Jeffrey needles from the lower part of Table Mountain Road, collected on 26 September 2014 and photographed the next day.
Click on the pictures for larger versions.

Vegetative Buds

The vegetative buds appear quite different between the two species, see Fig. 5. The floras only mention that ponderosa buds are resinous, and Jeffrey buds are not. We were surprised to see a much bigger difference, with prominent acute bracts in the Jeffrey buds as they are breaking dormancy, and not prominent at all in the ponderosa buds as they are breaking dormancy.

The vegetative buds change with time as they break dormancy. They first appear as a single large bud (see the first row in Fig. 5), and as they open appear as a cluster of individual needle buds (see the second row in Fig. 5).

P. jeffreyi

 

P. ponderosa

 
 

Fig. 5. Photographs of vegetative buds for Jeffrey (left) and ponderosa (right). Top row: dormant buds. Bottom row: buds that begun to open.
The Jeffrey buds have non-resinous bracts with margins with abundant white membranous hairs. The copious resin is apparent in the ponderosa buds instead of the white membranous hairs of the Jeffrey buds. In the second row, the bracts are very prominent around each needle bud for the Jeffrey buds, but the bracts are insignificant next to each needle bud for the ponderosa buds. The ponderosa resin is now in strings.
Note that the color difference in the needles is readily apparent in these pictures, even though the species are not side by side in the same photograph.
The first row of buds were photographed on the north side of Mt. Islip in the San Gabriel Mountains on 10 October 2014. The second row of buds were photographed on the Seven Pines Trail at San Jacinto Mountain on 30 September 2014.
Click on the pictures for larger versions.

Seeds

We have only checked this characteristic in seeds from a single cone of each species, since seeds are not always readily available. The difference was immeidately obvious in the field, with no measurement required. For those, we measured a ratio of 1.4 to 1.5 for Jeffrey; 2.8-3.7 for ponderosa, consistent with the ranges reported by Haller 1962 of 1.5-2.4 for Jeffrey and 2.9-4.5 for ponderosa. The seeds are shown in Fig. 6.

Fig. 6. Seeds of Jeffrey (left) and ponderosa (right) from San Gabriel Mountains near Pine Hollow Picnic Area, 1.4 miles east of Islip Saddle, taken on 10 October 2014..

Summary of Haller's findings on hybrids

Haller did an absolutely beautiful study of these two species for his Ph.D., which he published in UC Publications in Botany, 34, No. 2, pp. 126-166, 1962. The title of his paper was Variation and hybridization in ponderosa and Jeffrey pines.

Haller sampled 25 pines each at a number of locations throughout California, including a number of transects through areas that had both species in one part of the transect, with three or four sampling locations in each transect. Haller scored each individual tree for 15 traits that can distinguish the species, with the extreme ponderosa trait scored as "0", the extreme Jeffrey trait scored as "2", and a dead intermediate trait scored as "1". He then added up the scores. A perfect ponderosa would score zero; a perfect Jeffrey would score 30 (= 15 x 2).

Haller does not give the total number of trees he measured, but from the number of locations he mentioned, the total must have been well over 1,000 trees.

Haller found that in areas where both species live, on average 24 of those 25 trees scored as clearly a ponderosa (scores 0 to 6) or clearly a Jeffrey (scores 24 to 30). These are the same scores as he found in areas that had only one of the two species.

On average, one tree per population (remember, only in the areas where both species live), scored within a few points of 15, which would be an F1 hybrid, or an intermediate value (~7 or ~23) indicating a backcross of an F1 hybrid with one of the other parents. Haller stated that he would probably not have been able to detect a second generation backcross with the same other parent, since it would then be close to one of the parents.

A summary of his findings are:

The first two statements are not tautological. If these two species really were just extreme forms of a single species, one would not expect that all the specimens in most areas would score as clearly just one of the species.

The following computation tries to put the hybrid numbers in perspective for the probability of finding a hybrid in a randomly-selected location in California.

We don't know exact statistics of how many areas in all of California are pure Jeffrey, etc. A rough guess is that something like 80% of California (that has one or both of these species) are pure stands, with 20% being mixed stands. If that is the case, considering all areas, less than 1% of the total population of these yellow pines in California would be a hybrid.


We thank Rudi Schmid for providing copies of a number of key references, especially the Haller 1962 paper and the Haller and Vivrette 2011 paper, as well as comments on this page.

We thank Adrienne Ballwey, Mike Crouse, Walt Fidler, and Jane Tirrell for help in the field on several field trips to work on the separation of these species, and Nancy Accola, James Dillane, Bob Smith and Elize Van Zandt for help in the field on one field trip each.


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Copyright © 2014 by Tom Chester and Jane Strong
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Last update: 25 November 2014