Nascent Inflorescences In Arctostaphylos pringlei ssp. drupacea

Arctostaphylos, manzanitas, are one of a handful of genera with species that produce their inflorescence buds months before the buds develop into blooms. At the Santa Rosa Plateau Ecological Reserve, species in just four genera do this: Arctostaphylos (A. rainbowensis, A. glandulosa); Ceanothus (C. crassifolius); Rhus (R. ovata, R. trilobata); and Xylococcus (X. bicolor).

In the following, buds usually refers to inflorescence buds; vegetative buds will be used to discriminate such buds when needed.

Having dormant buds ready to burst into bloom when conditions are favorable gives these species one advantage. With the exception of the Rhus species, these species are among the first ones to bloom each year. Perhaps the development of the buds the previous year gives them a head start so that they can be ready to bloom soon after the first rainfall of the year.

However, dormant buds have at least two drawbacks. First, such buds are exposed to predation and environmental hazards such as extreme cold. Second, a single severe drought year can eliminate two years of blooms. In the pathetic rainfall year of 2001-2002, these species aborted their buds made in 2001, and never formed buds in 2002, and hence did not bloom in both 2001 and 2002.

Developing such dormant buds is apparently a strategy that has evolved independently at the species level or a bit higher, since these genera are unrelated in general, and not every species in a given genera develops dormant buds. For example, many Ceanothus species do not produce dormant buds.

Jepson (1938) was the first to notice that essentially all Arctostaphylos species produce such buds, and found that these were quite useful for discriminating many of the species. As a result, these buds have been given a special name, nascent inflorescences, in this genus.

Keeley (1997) made the perceptive observation that Arctostaphylos pringlei ssp. drupacea, henceforth referred to as A. pringlei for brevity, is an exception within Arctostaphylos and apparently does not produce a nascent inflorescence. He followed up his observation with detailed field observations of a set of plants from June through December 1992, finding no nascent inflorescences on any of those plants during that interval. In April 1993, he observed shrubs beginning to flower, with all stages of the development of the inflorescence present at that time, from buds just breaking dormancy to flowers. He then concluded that the buds were produced just prior to flowering, without any dormant period.

Keeley also pointed out that A. pringlei seems so unique in other respects that Wells classified it in its own section within the subgenus Arctostaphylos.

During discussions with Mike Vasey and Jon Keeley about my discovery of the F1 hybrid A. patula X A. pringlei at San Jacinto Mountain, both Mike and Jon expressed a desire for further documentation of the bud development in A. pringlei. As a result, I have begun such a program for plants at San Jacinto Mountain.

Note added 9 January 2009: I followed these plants all the way to bloom time in 2007, and all of them only developed their inflorescences immediately prior to bloom. Keeley was exactly correct. Many inflorescences were still developing as others came into bloom. I obtained pix of the same plants throughout the entire cycle, but I haven't yet had time to put the rest of the pix online.

Observations On 26 October 2006

I surveyed the many specimens of A. pringlei on the South Ridge Trail from an elevation of 6500 to 7500 feet. Essentially all of them were the same in terms of bud development except for specimens at the highest elevation. The vast majority had single undeveloped buds in the axils of the top ~5-10 leaves on nearly all of the flowering twigs that bloomed this year, as shown below:

See also a picture twice as large for details on the buds. All of these pictures are named with the GPS point for each specimen; the coordinates are given in the table at the end of this section. Many more specimens were photographed than are shown here.

The above picture shows essentially the entire twig that developed this year, and terminated with flowers that bloomed in ~April. Buds have developed recently in the leaf axils, and for this specimen, the uppermost buds are somewhat larger than the lowest buds and look like they possibly have begun growth.

The following picture shows all the twigs that developed this year on an older branch:

Notice the black ~1.5-year-old dead inflorescence in the middle of the picture, partially hidden by a single leaf, that terminates the ~1.5-year-old branch, and the three ~0.5-year-old twigs, each with ~10 leaves and an inflorescence, that developed and bloomed this year. This year's flowers may have fruited as well; many fruits have fallen by the time I took this picture.

On this ~1.5-year-old branch, the uppermost three buds developed into twigs earlier this year, whereas the lower buds all aborted. In general, I observed two to three twigs that developed on the almost-two-year-old branches. Presumably, the lower buds are mainly back-ups in case something happens to the uppermost buds (but not always; see below). An example of a two-twig branch, along with its ~1.5-year-old dead inflorescence, is shown in the next picture:

The above pictures nicely show the growth habit of manzanitas in general, and why the twigs and even larger stems have a zig-zag growth habit. The stem lengthens from the uppermost axillary bud, which curves upward and soon makes the former terminal inflorescence appear as an axillary inflorescence.

Specimens at the highest elevation showed some bud development, but it wasn't the development I would have predicted from the above pictures; the lowest buds are the ones that have already begun to produce leaves! The following picture shows this bud development:

Presumably, the specimens at lower elevation will soon show this same bud development, but only future observations will tell. Such earlier development at high elevations is expected. Everyone knows that spring comes later to higher elevations; not as many people know that fall and winter come earlier.

For comparison, I also analyzed specimens of A. patula. All specimens along this trail developed their nascent inflorescences at least a month or so ago. They also show buds very similar to the ones seen in A. pringlei (of course, buds of A. patula do not have the incredibly-glandular hairs of A. pringlei):

The difference seems to be solely in when the buds develop. The following is a scenario suggested by the above observations, as well as Keeley's observations. Specific development times will be verified over the next year.

One possible speculative explanation for this difference might be that the leaves and inflorescence of A. pringlei are less cold-tolerant than those of A. patula. Perhaps A. pringlei devotes its resources to making its vegetative buds cold-tolerant, whereas A. patula can cold-harden its leaves and nascent inflorescence. Laboratory experiments can readily verify this speculation.

Observations On 8 November 2006

I surveyed the specimens of A. pringlei on the Devils Slide Trail, and ones just below Humber Park, from an elevation of 6400 to 7400 feet. Essentially all of them were the same in terms of axillary bud development, and similar to specimens observed at the highest elevation on the South Ridge Trail on 26 October 2006, with the buds beginning to grow.

However, it also appeared that some twiglets had begun growing at their tip. I didn't notice such growth on the South Ridge Trail in the field, but looking back at my pictures, it is possible that such growth had just begun there. The tips of the branches there on 26 October 2006 either contained buds or very small leaves.

However, it is not possible to definitively state when the small leaves at the tips of the branches were produced without followup observations on the same plants. The tips of the branches on all plants may have stopped growing during the summer, freezing the small leaves at that point. My next observations in about a month should be able to distinguish whether these branches are actively growing at the tip.

The following pictures show a sampling of the twigs on 8 November 2006:

No nascent inflorescences were observed on any plant, but the tips of some branches look like nascent inflorescences might be imminent, as in the last picture above. The first and last picture above were from the same plant, which is at higher elevation than the plant in the middle picture above.

Stay tuned for future results!

GPS Points for the Photographed Specimens on 26 October 2006

All positions are in NAD27 decimal degrees.

SpeciesGPS #Latitude (° N)Longitude (° E)Elevation (feet)
A. pringlei333.73534-116.694976505
A. pringlei6833.74502-116.684187521
A. pringlei7033.74474-116.684887527
A. patula6533.75347-116.680987953


Jepson, W.L. 1938, Erythea 8:97-99.

Keeley, J. 1997, Madrono 44:109-111.

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Copyright © 2006-2009 by Tom Chester
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Last update: 30 October 2006 (link to Keeley 1997, and brief note on final findings, added on 9 January 2009)