Genetic variation and mating system of ponderosa pine in the Willamette Valley of Oregon

Thesis

Transfer Guideline: Recommendation

The population genetic structure of ponderosa pine (Pinus ponderosa Dougi.) in the Willamette Valley of Oregon was investigated. Cones were collected from native stands of ponderosa pine from the Willamette Valley, eastern Oregon, southwest Oregon and the Puget Sound Basin of Washington. Seeds were subjected to isozyme analysis of 12 enzyme systems controlled by 20 different loci. Population genetic parameters for progeny, parent trees and pollen were estimated from isozyme data using BIOSYS-2, a population genetic statistical software package. For progeny data, total gene diversity (HT) was 0.249; average genic diversity within stands (Hs) was 0.231; average genic diversity among stands (DST) was 0.017; the proportion of total genic diversity due to differences among stands (GST) was 0.067. Based on gene diversity statistics and genetic distances, stands of Willamette Valley ponderosa pine do not differ substantially from one another or from other populations of ponderosa pine in Washington and Oregon. These results suggest that protection of the Willamette Valley population can be accomplished by conserving a large number of individuals in a few large stands or many small stands. Multilocus and single-locus estimates of outcrossing rate were determined for six stands within the Willamette Valley. Outcrossing rates were estimated for seven polymorphic loci using MLTR, a mating system analysis software package. Estimates of multilocus outcrossing for these stands (mean 0.642; range 0.365 to 0.879) were substantially lower than estimates previously reported for other populations of ponderosa pine, other pine species, and other species in the family Pinaceae. No significant difference was detected among stands in outcrossing rate. The high level of inbreeding observed in this population could be attributable to population fragmentation due to human activities, or to reduced levels of pollen production in this population compared to other populations of ponderosa pine. Genetic conservation strategies for this population should include the protection of the large native stands that remain and the use of local, adapted sources for afforestation. Management of pollen in a seed orchard should improve the genetic quality of seed over wild stand seed by reducing inbreeding levels.