A Pleistocene Clone of Palmer's Oak in Southern California
1
1
2
2,3
2
4
Michael R. May , Mitchell Provance , Norman C. Ellstrand , Andrew Sanders , and Jeffrey Ross-Ibarra
2
Department of Evolution and Ecology, University of California Davis, E-mail: [email protected]; Department of Botany and Plant Sciences, University of California Riverside;
3
4
Biotechnology Impact Center, Institute for Integrative Genome Biology, University of California Riverside; Department of Plant Sciences, University of California Davis
Introduction
Materials and Methods
Palmer’s oak (Quercus palmeri Engelm, synonymous with Quercus dunnii
Kellogg) has a scattered distribution throughout its natural range,
usually existing at elevations between 900 and 1500 meters. Populations
in southern California are separated by many kilometers, and usually
consist of only a few individuals.
Trees from the Jurupa population were permanently tagged and
numbered, and voucher collections of multiple individuals were made
and deposited in the herbarium at University of California Riverside.
Plants were examined for maturing fruits and new growth eight times
over a six year period. Comparative specimens were collected from
larger, actively interbreeding populations of Q. palmeri in Garner Valley,
in the San Jacinto Mountains about 75km southeast of the Jurupa
Mountains, and Aguanga, about 80km southeast.
Conclusions
Figure 3. Crosssectioned stems were
sanded and polished,
then digitally imaged
for ring counts. Dots
indicate annual
growth rings.
Because growth
rings were rarely
visible throughout
the entirety of a
radius, correlation
between
discontinuous
segments of growth
rings was necessary.
To establish the clonal nature of the population, leaf tissue was collected
from 32 individuals in the population and tested for allozyme activity at
9 different loci. Activity was compared against the same loci in 15
samples of the Garner Valley population.
To determine the age of the population, 11 dead stems were collected and
their annual growth rings counted (Fig. 3). Our observation suggests that
this population propagates only by clonally re-sprouting from crowns
following a fire (Fig. 4), the size of the population and average rate of
growth (as determined by annual growth ring count) were used to
estimate the potential age of the population. Ring counts were also taken
from the Garner Valley and Aguanga populations for comparison.
We conclude that this population of Q. palmeri is a relict of an ancient
population that existed in the Jurupa Mountains at a time when
environmental conditions were more conducive to its success. Warming
since the last glacial period has pushed the ideal elevation of Q. palmeri
ever higher, leaving behind the small, disjunct populations that we find
scattered around southern California today. Our findings at Jurupa are
suggestive that cloning may be a significant contributor to the
persistence of these disjunct populations, which may themselves be
ancient clones.
Step 1. Original parent
tree
We explored the possibility that this population represents a single clone,
and the implications of this clonality on the natural history of the
population.
Step 2. Fire burns parent tree
down to the crown.
Step 3. Clones sprout from the
burned crown.
Step 4. Fire burns clones to
their crowns, which then
sprout their own clones.
Step 5+. Repeated burns
gradually increase the size of
the population.
Figure 4. Simplification of the vegetative expansion of a clonal population of Q. palmeri. The line represents the length of the clonal axis, which grows as the population is burned and re-sprouted. Note that resprouting will take place in all directions, not just away from the center, so natural populations will not actually be rings of live trees surrounding crowns. Because the growth of the stand proceeds at a rate equal to
growth of an individual, the age of the clonal population can be estimated from the length of the clonal axis and the average rate of growth of individual trees.
Literature Cited
Results
Growth form and leaf morphology is homogenous throughout the
Jurupa population. The population was in flower when first discovered
and acorns were clearly being produced. However, despite active
flowering, only 4 fully mature acorns were collected throughout the six
years that the population was observed. Populations in other locations
produced large amounts of mature, viable acorns.
At 7 of the 9 loci studied for allozyme activity, the Jurupa population was
monomorphic for a single homozygous genotype. By contrast, the
individuals sampled from the Garner Valley population varied at several
of the loci that were monomorphic in the Jurupa population. Of the loci
that varied within the Garner Valley population, the most frequent
alleles were the same alleles that were fixed in the Jurupa population. At
the remaining 2 loci, each individual in the Jurupa population
invariantly exhibited a two-band pattern, which we interpret as
representing a fixed heterozygous genotype.
Figure 2. Jurupa Mountains population of Q. palmeri as seen aerially from the northeast. Red
outline indicates the extent of the population. The population is sheltered from strong winds
by granite outcrops on either side.
We were able to estimate the minimum age of the population to be 3,300
years, based on maximal annual growth for the species in optimal
conditions. A more realistic estimate, based on the average annual ring
size of stems from Jurupa, suggests that the clone may be up to 18,600
years old.
This age places the origination of clonal reproduction in the Late
Pleistocene, possibly soon after the last glacial period began to break
about 20,000 years ago. Pollen records from packrat middens show that
Q. palmeri occurred in the Mojave Desert at an elevation of 850m starting
9,500 years ago, replacing the previously dominant Pinus monophylla. It is
presumed that Q. palmeri existed at lower elevations before this time.
Climatic warming is invoked to explain the upward migration of Q.
palmeri in to cooler, higher elevation territory (Leskinen 1975).
Figure 1. Distribution of Q. palmeri populations throughout California and their elevations.
The black dot represents the Jurupa Mountains population.
A small population of Palmer’s oak was recently discovered at an
elevation of 400 meters in the Jurupa Mountains of the San Bernardino
Valley on the northwestern edge of Riverside County (Provance et al.
2000). This population consists of roughly 70 stem clusters over an area
of 25 meters by 8 meters. Morphological homogeneity, high rates of
acorn abortion, and abundant evidence of post-fire re-sprouting suggest
that this population may in fact be a single clone.
The analysis of allozymes within the Jurupa Mountains population of Q.
palmeri is clear evidence that the population is clonal in nature. The
population demonstrates no genetic variation at all 9 loci, with 2 of those
loci being fixed for heterozygous genotypes. The population is
morphologically homogenous and exhibits a clustered multi-stem
growth form, both characteristics previously shown to be associated with
clonality within Q. palmeri populations in northern California (Tucker et
al. 1982). Further, high rate of acorn abortion is consistent with the loss of
productivity expected for a predominantly out-crossing plant when
clone density is high (Handel 1995). All lines of evidence support the
hypothesis that the Jurupa population is indeed a clone.
The average growth rate of stems in the Jurupa population, as
determined by counts of annual growth rings, was 0.8 ± 0.02mm per
year. The average annual growth rates in the Garner Valley and Aguanga
populations were 1.2mm and 0.9mm per year, respectively. The average
growth rate for all three populations combined was 0.96mm per year.
Additionally, the largest single ring from either of the non-clonal
populations was found at Aguanga and measured 3.8mm; we take this to
represent the maximal annual growth rate for Q. palmeri in ideal
conditions. The largest single ring observed in the Jurupa population
was 2.4mm. We believe that this lower value reflects poorer growing
conditions in the Jurupa population’s current environment.
Handel, S.N. 1985. American Naturalist 125: 367-384.
We conservatively assumed that all clones arose from a single parent at
the center of the population. Because the longest axis of the population is
25m, it would be necessary in this case for the population to expand by
12.5m in a single direction. We calculated the time required for the
Jurupa population to achieve the observed size according to each
possible growth rate (Fig. 5).
Growth Rate
(mm per year)
Estimated By
3.8
Single largest ring in any stem
from any population
3,300
0.96
Average ring count from all
stems collected
13,000
0.8 ± 0.02
Average ring count of stems
collected from Jurupa
13,500 to 18,600
Leskinen, Paul H. 1975. Madroño 23: 234-235.
Provance, M.C., et al. 2000. Madroño 47: 139–141.
Tucker, J.M., et al. 1982. Aliso 10: 321-328.
Projected Age
(years)
Figure 5. Estimates of the age of the Jurupa population based on different calculations of
growth rate. A rate of 3.8mm per year is assumed to be near the physiological maximum for
the species. More conservative growth rates are based on actual observed average size of
annual growth rings counted in the collected stems. A rate of 0.96mm per year is the average
growth rate of all observed stems from all populations, while the rate of 0.8 ± 0.02mm per
year is based on stems collected from the Jurupa population alone.
Acknowledgements
We would like to thank Leslie Blancas and Janet Lee for help with
allozyme analysis, and Craig Provance and Jim May for their
woodworking expertise.
Additional Information
For more pictures of stem cross-sections and examples of ring counts,
visit www.rilab.org/treerings/treerings.html