Plant Species of the Borrego Desert:
Fouquieria splendens, ocotillo, recruitment
Fouquieria splendens, ocotillo, recruitment
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| Fig. 1. Left: a first year seedling about 2 cm tall. Note the thick stem that even first year seedlings have.
Right: a plant that appears to have had two years of growth, about 7 cm tall. The latest year's growth is obvious by being a different color. Both plants photographed by Don Rideout on 24 December 2022 near Second Crossing. Click on the pictures to go to the iNat observations. | ||
Botanists use the word recruitment to describe how new plants are added to an existing population of a species. A species whose population numbers are steady with time must recruit one individual that survives to reproduction age for every individual that dies. If recruitment consistently falls below that one to one ratio, a population will eventually go extinct.
We became interested in studying young ocotillos after reading reports like this:
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Yet we fairly regularly observe young ocotillos!
We of course agree that climate change exists, and it may well have already had a large negative impact on desert vegetation. We certainly see huge numbers of dead ocotillo plants in many areas. But given the disparity between some of the comments above, and what we have seen in the Anza Borrego Desert, we have begun a project to study young ocotillos to see if we can determine what the recruitment rate actually is. Our study is not attempting to understand other important effects on the population, such as whether there have been increased death rates in the existing population.
Recruitment in desert shrubs often depends on special conditions. Some species only produce seedlings from monsoonal (summer) rain, and the seedling survival depends on having good winter rain within the ~six months after germination, followed immediately by another summer with decent monsoonal rain. Conditions like this can be quite rare in both time and space, occurring perhaps only once every ten or 20 years, and then only in areas that received the spotty coverage of monsoonal rains in two successive summers. As a result, short-term monitoring of a small number of plots may not give an accurate impression of a century long pattern.
About four years ago, Don began photographing young ocotillos as he encountered them, and Tom joined him in doing that in the last year. All of our photographs are posted at iNat, and we have attempted to mark other iNat posts of young plants, with tags such as "seedling" or "young plant". This page reports our initial findings from that set of young plants.
For each plant observed, we estimated its age by counting the segments visible on the stem. The latest year's growth is always quite visible since that stem segment is a different color than the rest of the stem, with an obvious joint at its base. Previous year's growth are usually easily visible except near the very base of the stem. At that point, it is unclear whether the short stem or segment at its base developed over a single year, or multiple years. We make our best guess as to how to count it, but usually just assign it to having developed in a single year. See an example of a plant estimated to be seven years old.
There are at least two caveats about our estimate of the plants age. First, if there are years in which the plant does not produce any stem length growth, it will not be counted in the plant's estimated age. If there have been years without any stem growth in these young plants, our "estimated age" should be interpreted as "number of stem growing years".
Second, plants might have two growth spurts per year, one from summer rain and one from winter rain. We should eventually be able to find out whether this is the case. One source implies that the stem of our species only grows in the summer, since Mark Dimmitt comments that the boojum differs from ocotillo "in being a winter grower".
The rest of the page shows some very preliminary results on sizes and ages of the plants we've found.
Fig. 2 shows the longest stem length vs. the estimated age of the plant.
Fig. 2. Stem height, measured for the longest stem, vs. the estimated age in years of the plant. Plants are counted as one year old when observed in their first year in fall / winter, after their summer growth.
It is well-known that older plants produce longer growth each year than do younger plants. Fig. 3 shows the average growth rate increases with age, with the lowest growth rate in plants of ages 1, 2 and 4 years, and largest in plants of ages 3 to 15 years. As expected, there is considerable variability in the growth rate, since it depends on location in many ways, especially rainfall and site characteristics, and also on the number of stems on the plant. For example, on this plant estimated to be seven years old, the new growth varies from 14 to 45 cm on its seven branches. The median new stem growth was 22 cm. The average stem growth over seven years of its longest stem was 21.4 cm, the largest value in Fig. 3.
Fig. 3. The average stem growth rate (longest stem divided by age of plant) vs. the estimated age in years of the plant.
On 3 December 2023, the authors measured the length of all the stem segments of the longest stem on five plants that were growing in the same wash over a distance of 0.2 miles (0.3 km). Plant #1 was the southernmost plant; plant #2 was the next one to the north; plants #3 and #4 were growing close together farther north; and plant #5 was the northernmost one. Fig. 4 shows the stem segment length for each plant, with the youngest segment given as segment #1.
Fig. 4. Stem segment length vs. segment number. The youngest stem segment is given as segment #1, with subsequent segments as #2, #3, etc. The last stem segment is the one at the base of the plant.
The data vary a lot! There is little synchrony among the five plants as to which years produce longer segments. Plant #2 produced the longest stem segment by far in its youngest segment (segment #1), whereas plant #3, a plant that had the same number of stem segments, produced its longest stem segment in its second youngest segment (segment #2).
However, the two plants growing in very close proximity, #3 and #4, had identical patterns in their variation from year to year. Stem segment #2 was the longest stem segment for both plants; both exhibited smooth changes in stem segment length for older stem segments; and both had major declines in the stem segment length for their youngest segment.
These data are consistent with stem growth produced by summer monsoonal rain, which is spotty in areal coverage at scales of 0.1 mile, and inconsistent with stem growth produced by winter rains which are usually correlated over large areas, at scales of 1 mile or more.
Data for each plant are given in Table 1, along with a link to iNat posts showing photographs of these five plants.
Table 1. Information about the five plants in Fig. 4
| iNat Link | # Segments | Growth Rate (cm / segment) | Longest stem length | |
|---|---|---|---|---|
| (cm) | (feet) | |||
| Plant #1 | 5 | 7.4 | 37 | 1.2 |
| Plant #2 | 8 | 19.6 | 157 | 5.2 |
| Plant #3 | 8 | 6.4 | 52 | 1.7 |
| Plant #4 | 6 | 12.3 | 74 | 2.4 |
| Plant #5 | 6 | 14.2 | 85 | 2.8 |
Additional data from other places will be needed to confirm this pattern.
On 18 December 2022, we, along with Jim Roberts, discovered eight first year plants near Ironwood Wash that probably germinated from the remnants of a tropical storm that dropped lots of rain in the desert in September 2022. We followed up those plants on 3 December 2023, and every single one of them had vanished without a trace. We surmised this was due to predation, since if they had died from the summer heat and lack of rain, there probably would have been remnants left. This is consistent with the findings of Bowers, Turner and Burgess 2002 that predation is a more frequent mortality source than drought.
Ted Caragozian did find one plant on 3 December 2023 that was well separated from the seedlings that disappeared, that may have been a first year plant a year ago, and may have escaped the predation the other first year plants suffered. However, it is also possible that this was an older plant in 2022 whose upper stem was predated, but whose lowermost stem was untouched.