Biosystematic studies of Phacelia capitata (Hydrophyllaceae), a species endemic to serpentine soils in southwestern Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/hh63sz72k

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  • 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 diploids. 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, obscure glandulosity. 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.
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