Genecology and Phenotypic Selection in Whitebark Pine (Pinus albicaulis) and Ponderosa Pine (Pinus ponderosa) Under Warm-Dry Climate

Abstract

To study patterns of adaptation to the contemporary climate and phenotypic selection under future predicted climate in whitebark pine (Pinus albicaulis), seedlings from 49 populations representative of the interior northwestern USA were grown in two low-elevation, common-garden field tests over a 12-year period. The primary objectives were to: 1) assess genetic variation in growth and survival among populations (Chapter 1); 2) model this genetic variation in relation to climate at the seed source (Chapter 1); 3) describe phenotypic selection on growth rhythm (Chapter 2); and 4) model growth rhythm in relation to climate of seed source (Chapter 2). Results showed moderate genetic divergence in growth and survival among populations that corresponded with climatic clines. Populations originating from locations with earlier spring warming exhibited greater inherent growth under favorable conditions, while populations originating from locations with lower spring precipitation exhibited greater survival under drought conditions. Selection on measures of growth rhythm was detected and varied between experimental sites and among years. More complex selection on measures of growth rhythm was observed on the experimental site with more favorable growing conditions. Curved directional selection and stabilizing selection tended to occur in earlier years and was chiefly associated with mortality in the year of expression. Differences among seed sources in growth rhythm were mild and were explained to a moderate extent (r2 = 0.08 - 0.28) by variation in temperature or precipitation among seed origins. Individuals originating from milder climates tended to elongate slightly longer through the growing season and exhibit slightly higher shoot elongation rates. To study phenotypic selection of ponderosa pine (Pinus ponderosa Doug.) in response to predicted future climate, seedlings were grown from seed in the field using three drought-imposed treatments under a climate warmer and drier than the climates of seed origins. The seedlings originated from 36 maternal trees from Priest River Experimental Forest, ID and 3 provenances (8 maternal trees per provenance) representative of the interior northwestern USA. The primary objectives were to 1) evaluate the magnitude, form and temporal dynamics of selection on seed and seedling traits in response to variation in the timing of growing-season drought for a single provenance and (2) assess differences in selection among provenances representative of the interior northern USA (Chapter 3). Differences in selection among treatments and provenances on traits associated with drought adaptation were detected and described. For the Priest River provenance, mortality differed among families within provenances. Selection via differential mortality in the year of emergence was particularly strong, while selection in subsequent years was far less pronounced. The form of selection was influenced by timing of drought. Differences in the magnitude and form of selection were also detected among provenances. In addition, patterns of variation for selection among provenances corresponded with dryness of seed origin. These results suggested that during the seed-to-seedling stage, populations within the region will likely differ in their selection response to changes in the timing of growing-season drought.