Plants of Southern California: Arctostaphylos glandulosa Subspecies Number of Stone Segments

Tom Chester, Craig Denson, Don Rideout, and Jim Roberts

After checking how many stone segments were produced by four plants of Arctostaphylos parryana ssp. desertica, and from one plant of A. glandulosa ssp. adamsii on 3 November 2023, we were curious to get more data on A. glandulosa, and see if the average number of stone segments depended on subspecies. On 12 November 2023, the authors spent the afternoon splitting 128 fruit from ten different plants from a 1.8 mile section of the Pacific Crest Trail east of Cuyamaca Lake in San Diego County. This webpage gives the results.

We sampled plants at six roughly-equally spaced locations; see a map of the locations of the measured plants, which also links to photos of nine of the ten plants we studied.

We began in the northern part of this stretch, and sampled four plants at the beginning, two plants that corresponded to ssp. adamsii and two plants of ssp. glandulosa. The only other criteria we used to pick the plants was that they had to have enough fruit to measure, and to have nascent inflorescences to check for glands.

After that, we hiked ~one quarter to one half mile before looking for the next plant to study. We tried to sample equal numbers of ssp. adamsii and ssp. glandulosa, which wasn't difficult since both species were completely intermixed here. Near the end of the day, we were running out of time so stopped sampling plants after measuring four plants of ssp. glandulosa; three plants of ssp. adamsii, and one plant that appeared to be ssp. adamsii at first, but we then discovered it had some nascent infloresences on another stem that were glandular. At the south end of the PCT section, we had time to sample two additional plants right next to each other that made ssp. glandulosa a bit over-represented in the ten plants that we measured. In fact, Tom intended to stop with the ninth plant, but the other authors were having so much fun opening fruit to gather more data that they had started on the tenth plant before Tom had time to put his pack back on!

For each plant, we broke open at least ten fruit with our fingernails, and recorded how many segments each stone naturally split into as the fruit was opened. Tom Parker said this was the procedure used by manzanita experts to see whether a stone is separable or not. Except for a small number of cases, we did not try to further split the stones with our fingernails. We did try to do that every so often just to see if we could further split the stones, but in every case the largest stone segment proved impossible to split further. We photographed at least one nascent inflorescence and twig for every plant except the last one, and often some of the stones, and posted the pix in separate observations at iNat, each labeled with the plant number in the order we encountered them. Direct links to each plant number are given in Appendix 2.

For plant #1, see a photo of an intact stone that did not split, although you can see deep crevices where it was thinking about splitting, and a photo of a stone that split into three segments, a single seed and two segments that remained with some fused seeds.

Several of us checked different nascent inflorescences, and the twigs just below the nascent inflorescence, for their trichomes to make sure we agreed on the subspecies. Plants that had only white hairs, without any glandular hairs, on both the nascent inflorescence and the twigs, were called ssp. adamsii. Plants that had glandular hairs on both the nascent inflorescence and the twigs, were called ssp. glandulosa. One plant had some stems with only white hairs on ts nascent inflorescences and its twigs, but it had other stems coming out of the same burl that had some glandular hairs on its nascent inflorescences. We will refer to that plant as mixed. It is possible that these stems were genetically distinct, "resulting from grafting of seedlings that emerged from a fused seed propagule or a common animal horde" (like a kangaroo rat seed cache).

Fig. 1 shows the average number of stone segments for each plant we studied, with each subspecies identified.

Fig. 1. The average number of stone segments for each plant we studied, with the plants numbered in the order that we measured them, with each subspecies labeled. The "mixed" plant is a good ssp. adamsii on some branches, but has some glandular hairs in its nascent inflorescence on another branch.

The average number of segments per fruit is 3.2 for all ten plants. The average was 2.9 for the six plants of ssp. glandulosa; and 3.7 for the three plants of ssp. adamsii. We would have called the "mixed" plant adamsii if we hadn't checked another one of its stems. Including that "mixed" plant with the three other adamsii gives an average of 3.5.

Given the huge variation in the six plants of ssp. glandulosa, from an average of 1.7 to 3.8 segments per fruit, we believe there is no clear difference in our data between ssp. adamsii and ssp. glandulosa, and the best number to use from our data is the overall average of 3.2 segments per fruit. This is essentially the same as the average of 3.1 +- 0.2 given by Keeley for ssp. crassifolia.

Combining all ten plants, the histogram of the # of fruit with a given number of segments is shown in Fig. 2, along with the histogram for two individual plants.

Fig. 2. Histogram of the # of fruit with a given number of segments for the sum of all plants, and separately for plant #8 and plant #10.

The histogram combining all ten plants shows a quite smooth variation, with a peak at 3 and 4 segments per fruit. But individual plants can have a histogram that is wildly different. The histogram for plant #10 peaks at 1 segment per fruit; the histogram for plant #8 peaks at 5 segments per fruit.

Every plant had at least one fruit with at least three segments. All plants except two had at least one fruit that remained solid, not splitting into any segments. The complete data for each plant are given in Appendix 1.

Other observations from our study:

Appendix 1. The data for individual plants

# of Fruit with a given # of stone segments

# of segmentsPlant #All plants
1 3 2 1 2 3 1 1 518
2 3 2 1 6 1 5 2 1 324
3 7 5 3 1 1 2 4 3 2 230
4 2 5 4 3 5 3 3 3 6 34
5 1 3 2 1 9 3 19
6 1 1 1 3
# fruit 16 14 12 11 11 14 12 16 12 10 128
# segments 43 41 48 28 37 37 40 68 46 17 405
Average # segments / fruit 2.7 2.9 4.0 2.5 3.4 2.6 3.3 4.3 3.8 1.7 3.2
subspecies glandulosa adamsii adamsii glandulosa glandulosa mixed glandulosa adamsii glandulosa glandulosa

Appendix 2. Links to iNat observations of individual plants

Each plant was photographed by both Don and Tom.

We did not photograph the tenth plant, since it was harder to access for close-up photos.

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Copyright © 2023 by Tom Chester, Craig Denson, Don Rideout, and Jim Roberts. (Authors are listed alphabetically, since all contributed equally to the field work.)
Permission is freely granted to reproduce any or all of this page as long as credit is given to us at this source:
Comments and feedback: Tom Chester
Last update: 16 November 2023