Plant Species of San Jacinto Mountain

Quercus X morehus, oracle oak

Fig. 1. Each picture shows one leaf of Quercus wislizeni on the left, one leaf of Q. kelloggii on the right, and two leaves of the hybrid between these two species, Q. X morehus, in the middle. The leaves came from plants found on SR243 between Idyllwild and Mountain Center on 31 December 2011. Left: upper surface of leaves. Right: lower surface of leaves. Click on pictures for larger versions.


Q. X morehus, oracle oak, is in most cases an F1 hybrid resulting from a direct cross of Q. kelloggii, black oak, with Q. wislizeni, interior live oak. It is pretty clearly a hybrid, being found only in small numbers where both parents occur, with traits intermediate to those of its parents.

It was originally described as a species, with no mention of its relation to its parents. Kellogg named it Quercus Morehus, and gave it the common name of Abram's Oak, in 1859, despite it being known from a single specimen. That specimen was collected by Mr. A.A. Veatch, and we thought perhaps Mr. Veatch's first name might be Abram, but his first name was Andrew.

When none of us could figure out how it became known as the Oracle Oak, Bob Allen supplied the answer. He wrote this species was named for the biblical Oak of Moreh (Hebrew, moreh, derived from yarah = to teach or direct, also one who directs or gives oracular answers). Jane Strong then pointed out to us that Abram was short for Abraham.

Although we still don't know why Kellogg gave it this name, this origin explains how it eventually became known as the oracle oak, since oracle oak is another name for "Abraham's oak of Moreh".

It was treated as a species in Jepson's 1925 Manual, but in the 1930s, Carl Wolff performed a progeny test that established the oracle oak as a hybrid. He produced about 900 young plants from several large collections of acorns from a number of trees of the oracle oak. He wrote:

Of these, many were so weak that they only produced a few pairs of leaves and survived only a few months. Others closely resembled seedlings of Q. kelloggii and Q. wislizeni, while a few from nearly every lot were typical Q. morehus.
In 1935 Munz wrote the oracle oak was supposedly a hybrid, and in 1959 he wrote:
At scattered locations and usually of 1 to few individuals, below 5000 ft.; usually near Q. kelloggii and Q. wislizeni, of which spp. it is undoubtedly a hybrid and with which some retrogression is shown. Munz and Keck 1959, p. 903.
The leaf properties are usually exactly what one would expect from a hybrid. The leaf size is about halfway between the small leaf of interior live oak and the large leaf of black oak. The leaf lobes are about halfway between the non-lobed leaf of interior live oak and the lobed leaf of black oak. The leaves are partly and/or tardily deciduous, in between the persistent leaves of interior live oak and the 100% deciduous leaves of black oak.

The partly and/or tardily deciduous property of the leaves make early winter the best time to look for specimens, since they stand out very clearly at that time of year when the black oaks have lost all their leaves. The interior live oaks all have green leaves then, since their leaves are not deciduous. The oracle oak will either still have its leaves, which are much different from interior live oak leaves, or will have lost some of its leaves, with some leaves still present that are yellow to brown. The latter produces a tree with a very open look that has some green leaves and some yellow-brown leaves, making its identity obvious at a glance, as shown in Fig. 2.

Fig. 2. Oracle oak along SR243 just north of the Road to the Grinding Station. An interior live oak is on the extreme upper right. Click on the picture for a larger version. See also this oak resprouting from the base, and from some of its main trunks, one year after the 2018 Cranston Fire (and a close-up of the resprouting branches).

Dave found the above oracle oak, along with three other specimens, just by watching for them as he was driving to and from Idyllwild along SR243 on two days. His discovery of these oaks prompted the authors to make a special survey for them on 26 January 2012.

We have seen considerable variation in the size of leaves for oracle oaks, similar to the variation in the size of leaves for their parents, often caused by the amount of sun received by the individual tree or individual branch (sun vs. shade leaves). It is also possible that backcrossing to the parents is responsible for some of the size variation. See photo of an assortment of oracle leaves, collected from a number of individual plants, demonstrating the variation in both the size of the leaves and the lobes.

Distribution at San Jacinto Mountain from Survey along SR74 and SR243

All the authors took advantage of a warm day in Idyllwild on 26 January 2012 to survey more closely for oracle oaks from Lake Hemet to Idyllwild along SR74 and SR243, at elevations from 4500 to 5300 feet. We began our survey along SR74 just east of Keen Camp Summit, to look for a specimen of Q. X morehus that Kate Kramer kindly told us about.

Fig. 3 gives a map showing our survey route, voucher locations of Q. kelloggii, and very rough areas that are dominated by each of these species. To avoid making the map in Fig. 3 too busy, voucher locations of Q. wislizeni were not shown, but are shown in a map with the same scale.

Fig. 3. Map of Idyllwild / Lake Hemet Area showing the Q. X morehus survey route on 26 January 2012 in purple; voucher locations for Q. kelloggii as filled red squares; the location where Q. kelloggii is dominant compared to Q. wislizeni as the red diagonal hatched area within the solid red curve; and the location where Q. wislizeni is dominant compared to Q. kelloggii as the black diagonal hatched area within the solid black curve. Voucher locations for Q. wislizeni fall mostly within that black diagonal hatched area along SR74, with a small number of vouchers in the area dominated by Q. kelloggii.

We drew the areas dominated by each of these species from our survey results from this trip, from our extensive surveys at the higher elevations in this area, and from voucher locations. The separation is primarily on elevation, as detailed below.

In this area of Riverside County, vouchers of Q. wislizeni range from 3000 to 5900 feet elevation, with just one pair of duplicate vouchers above 5600 feet elevation, at 5900 feet. Our extensive San Jacinto fieldwork confirm that Q. wislizeni becomes scarce above 5600 feet elevation, being increasingly replaced by Q. chrysolepis above about 5000 feet. Of these two species, in our 26 January 2012 survey, we found only Q. wislizeni below 5000 feet, with the first Q. chrysolepis found at 5200 feet elevation, and rapidly becoming more abundant above that elevation.

Q. wislizeni dominated the area of our 26 January 2012 survey, which ranged from 4500 to 5300 feet elevation. We found large stands of it in a number of places. James commented during the survey that he had never seen such dense concentrations of Q. wislizeni anyplace else.

In this area of Riverside County, vouchers of Q. kelloggii range from 4000 to 6800 feet elevation, with few vouchers below 5000 feet. Our 26 January 2012 survey found few Q. kelloggii specimens. Several times, after we found a Q. X morehus specimen, we had to look around to find a nearby Q. kelloggii. We estimated at the end of the survey that we saw fewer than 50 individuals of Q. kelloggii in our entire survey, compared to hundreds of Q. wislizeni.

For species that mostly flower at different times, with only a little overlap, hybrids are most likely to be created when there is a small population of one species amidst a large population of the other. The small population of one species receives a large amount of pollen from the other species. Two properties of many oak species encourage hybridization in such conditions. First, at least some oak species are primarily out-crossers and do not pollinate themselves. Second, oaks share a large amount of genetic material with other oak species, and easily hybridize with others in the same subsection of the family.

As a result, an isolated tree of one species surrounded by many individuals of another species will perforce produce hybrid offspring.

It is also possible that the nearest members of the abundant species can be pollinated preferentially by an isolated tree of the other species. This can occur if the female flowers of the abundant species are viable before the abundant species produces its own pollen, and if the isolated tree produces pollen at that time.

Figs. 4 and 5 show the results of our survey.

Fig. 4. All the locations checked for oracle oaks are show. Blue crosses represent areas where we found oracle oaks; red crosses where we did not.

Fig. 5. Locations checked along SR243 for oracle oaks. Blue crosses represent areas where we found oracle oaks; red crosses where we did not.

We were very surprised by how many oracle oaks we saw, since we had previously thought that this hybrid was quite rare. We saw 12 separate trees in our survey. Of those 12, four trees were found previously by Dave; one was Kate Kramer's tree; and two were shown to us by two people we happened to meet when we discovered one specimen in Mountain Center. The five newly discovered trees found in our survey came from three stops, and we stopped three other places where we found no specimens. Thus we found previously unknown oracle oaks at half of our "new" survey stops!

Distribution at San Jacinto Mountain from all Surveys

Dave has done a number of further surveys in late 2012 and late 2013, and we now have 67 locations where we have observed oracle oaks.

Fig. 6 shows the geographic location of all known oracle oak locations as of 7 January 2014, including 67 of our observations and 11 voucher observations:

Fig. 6. Geographic plot of all known oracle oak locations as of 7 January 2014.

A histogram of the elevations for our 67 locations is given in Fig. 7.

Fig. 7. Histogram of the elevations of our 67 known oracle oak locations as of 7 January 2014.

Table 1 lists our oracle oak locations, with the observation date and source.

Areas we have surveyed specifically for oracle oak, where we have found few or none, are:

It may be that at elevations above 5600 feet there is less overlap in blooming time between the parent species.

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Copyright © 2012-2019 by Dave Stith, Tom Chester and James Dillane.
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Updated 3 June 2019.