Growth rhythm that is well synchronized with seasonal changes in local climatic conditions is understood to enhance fitness; however, rapid ongoing climate change threatens to disrupt this synchrony. To evaluate phenotypic selection on growth rhythm under expected warmer and drier future climate, seedlings from 49 populations of whitebark pine (Pinus albicaulis Engelm.) were grown and measured over more than 10 years in two common garden field experiments on sites that approximate the projected future climate of the seed origins. Selection on growth rhythm was assessed by relating individual plant fitness to timing and rate of shoot elongation. Differential survival clearly evidenced selection on growth rhythm. We detected directional and stabilizing selection that varied in magnitude between experimental sites and among years. The observed phenotypic selection supports the interpretation of clinal variation among populations within tree species as reflecting adaptive variation in response to past natural selection mediated by climate. To the extent that growth rhythm is heritable, results of the present study suggest evolution of whitebark pine toward a more distinct timing of shoot elongation and generally more rapid elongation in the immediate next generation under ongoing climate change in environments similar to the study sites.
Bibliographical noteFunding Information:
The authors thank the USDA Rocky Mountain Research Station for funding. Many people provided assistance for this project including Donna Dekker-Robertson, Paul Leigh, Mose Harris IV, Amanda Link, Debra Eastman, Drs. Charles Geyer, Nicholas Crookston, Gerald Rehfeldt, and Ned Klopfenstein. Thank you to Drs. Ned Klopfen-stein, MeeSook Kim, Andrew David, Peter Tiffin, Jean-nine Cavender-Bares, Sylvie Oddou-Muratorio and several 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.
- Pinus albicaulis
- adaptive variation
- aster models
- natural selection
- unconditional expected height