Drought-related selection during seedling emergence and early development may play a strong role in adaptation. Yet this process is poorly understood and particularly so in relation to ongoing climate change. To evaluate drought-induced differences in selection during early life stages, a total of 50 maternal families sampled from three climatically disparate ponderosa pine (Pinus ponderosa Doug.) populations were grown from seed in two common garden field experiments at a location that was warmer and drier than seed origins. Three drought treatments were imposed experimentally. Phenotypic selection was assessed by relating plant fitness measured as survival or unconditional expected height at age 3 to seed density (mass per unit volume), date of emergence, and timing of shoot elongation. In the year of emergence from seed, differential mortality was particularly strong and clearly indicated selection. In contrast, selection in subsequent years was far less pronounced. Phenotypes with high seed density, an intermediate but relatively early emergence date, and high 2nd-year early-season shoot elongation exhibited the greatest estimated fitness under drought. The form of selection varied among seed sources in relation to drought treatment. Selection was generally more acute in the cases of greatest difference between drought treatment and climatic patterns of precipitation at the site of seed origin. These results suggest that populations of ponderosa pine are differentially adapted to drought patterns associated with the climate of their origin. To the extent that the phenotypic traits examined are heritable or correlated with heritable traits, our results provide insight into how tree populations may evolve in response to drought.
Bibliographical noteFunding Information:
The authors thank USDA, Forest Service, Rocky Mountain Research Station, for funding. We also thank the Washington State University, Lind Dryland Experimental Station for access to land, facilities, and resources. Many people provided assistance to this project including John Hanna, Keith Hillock, Brian Fode, Bruce Sauer, Steve Schofstoll, Drs. Bill Schillinger, Charles Geyer, Nicholas Crookston, Gerald Rehfeldt, Bryce Richardson, and Ned Klopfenstein. Thank you to Drs. Ned Klopfenstein, MeeSook Kim, Andrew David, Peter Tiffin, Jeannine Cavender-Bares, and two anonymous reviewers for their helpful comments on earlier drafts of this manuscript. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.
© 2018 The Authors. Evolutionary Applications published by John Wiley & Sons Ltd
- Pinus ponderosa
- adaptive variation
- aster models
- date of emergence
- drought selection
- growth rhythm
- natural selection
- unconditional expected height