- Phacelia capitata Rruckeberg is a member of the
Phacelia magellanica polyploid complex (species group
Magellanicae), a group of wide-ranging, polymorphic
perennials in western North America related to the South
American P. secunda (= P. magellanica). The purpose of
this project was to determine, to the extent possible, the
interrelationships of P. capitata within the Magellanicae,
using studies of its distribution, cytology, morphology,
reproductive biology, and population ecology.
Phacelia capitata is endemic to ultramafic (serpentine
and peridotite) outcrops, or similar metamorphic
substrates, in Coos, Douglas, and Jackson Counties,
southwestern Oregon. Field surveys revealed 22 extant
populations. The species occurs on open serpentine slopes,
outcrops, and roadbanks or similarly disturbed sites,
generally in areas with a south to southeast exposure. The
native plant community with which P. capitata is frequently
associated is a Pinus ieffreyi/grass savanna.
Soils from five P. capitata sites and two E. corvmbosa
sites were chemically analyzed. At six of these locations
the soils were derived from fully serpentinized parent
materials, and were found to possess low levels of
phosphorus and very low calcium/magnesium ratios when
compared to an eighth sample from a non-serpentine soil. A
soil sample from one P. capitata site, derived from a
metamorphic rock similar in appearance to serpentine, had
chemical characteristics intermediate between those of
serpentine and non-serpentine soils.
Chromosome counts from 20 P. capitata populations
revealed the existence of diploids (n=11) at 19 of them;
tetraploid (n=22) counts were obtained from two
populations, one of these populations also containing
Taximetric studies of living plants from 26 Phacelia
populations verify the morphological distinctiveness of P.
capitata when compared to P. corvmbosa, P. hastata, and P.
heterophylla ssp. virgata. Phacelia capitata is most
readily distinguished from the other species by its narrow,
mostly entire, silvery-pubescent rosette leaves, its often
capitate or sub-capitate cyme branches, and its sparse,
Significant seed set in P. capitata does not occur in
the absence of insect visitors. When natural pollinators
collect nectar from the congested flowers, it is likely that self-pollination between adjacent flowers of an
inflorescence is predominant (geitonogamy). However,
cross-pollination occurs to some extent in members of the
Magellanicae, as evidenced by hybridization and
introgression at the tetraploid level.
The origin of the polyploids of P. capitata is
problematic. It is possible that P. capitata may have
interbred in the past with P. Jeterophvlla ssp. virciata, or
with a more southerly member of the complex, P. egena.
There is also some morphological evidence that P. capitata
may have occasionally hybridized with P. hastata. However,
no plants have been found that suggest any hybrid contact
between E. capitata and Z. corymbosa.
At many locations there has been an increase in
numbers of P. capitata following habitat disturbance, and
this appears to have "released" the species from any
paucity that formerly characterized its populations. The
species evidently possesses poor competitive ability in the
more closed vegetation which exists on non-serpentine
soils, and also appears to be a poor competitor even in
some undisturbed serpentine communities.
Owing to its favorable response to disturbance, P.
capitata is not endangered with extinction. However, it
should be maintained on a monitoring list, because of its
very narrow geographic distribution and restriction to
serpentine or sub-serpentine soils.