Impact of climate change on cold hardiness of Douglas-fir (Pseudotsuga menziesii): environmental and genetic considerations

The success of conifers over much of the world's terrestrial surface is largely attributable to their tolerance to cold stress (i.e.,cold hardiness). Due to an increase in climate variability, climate change may reduce conifer cold hardiness, which in turn could impact ecosystem functioning and productivity in conifer-dominated forests. The expression of cold hardiness is a product of environmental cues (E), genetic differentiation (G), and their interaction (G×E), although few studies have considered all components together. To better understand and manage for the impacts of climate change on conifer cold hardiness, we conducted a common garden experiment replicated in three test environments (cool, moderate, warm) using 35 populations of coast Douglas-fir (Pseudotsuga menziesii var. menziesii) to test the hypotheses: 1)cool-temperature cues in fall are necessary to trigger cold hardening, 2)there is large genetic variation among populations in cold hardiness that can be predicted from seed-source climate variables, 3)observed differences among populations in cold hardiness in situ are dependent on effective environmental cues, 4)movement of seed-sources from warmer to cooler climates will increase risk to cold injury. During fall 2012, we visually assessed cold damage of bud, needle and stem tissues following artificial freeze tests. Cool-temperature cues (e.g., degree-hours below 2°C) at the test sites were associated with cold hardening, which were minimal at the moderate test site owing to mild fall temperatures. Populations differed 3-fold in cold hardiness, with winter minimum temperatures and fall frost dates as strong seed-source climate predictors of cold hardiness, and with summer temperatures and aridity as secondary predictors. Seed-source movement resulted in only modest increases in cold damage. Our findings indicate that increased fall temperatures delay cold hardening, warmer/drier summers confer a degree of cold hardiness, and seed-source movement from warmer to cooler climates may be a viable option for adapting coniferous forest to future climate.