Local adaptation in eastern white pine and implications under a changing climate
Eastern white pine (Pinus strobus) is a valuable commercial species and a major component of several forest cover types providing habitat and food to a multitude of wildlife species. Its large geographic range encompasses substantial climatic variability, with a temperature range of 18 – 23° C during the month of July and an annual precipitation range of 510 – 2030 mm. Forest tree species inhabiting such variable environments tend to differentiate genetically into populations adapted to the local environment; white pine populations have been shown to differentiate in growth, phenology, and survival, among other traits. In the context of projected climate change, identifying the links between climate and populations’ growth is crucial to understand potential changes in a species’ range and to project forest growth. The objective of this study is to identify linkages between climate and local adaptation of white pine populations, and to quantify the effects of a changed climate on the populations’ growth. Provenance tests data from 7 published articles provided a total of 147 populations and 23 test sites for the present study. Mixed-effects models as well as classification and regression trees were used to relate population growth with the climate the populations are supposedly adapted to and the climate at the test sites. Climate transfer distance, calculated as the difference between test site climate and climate at population origin, was also included in the models to elucidate potential responses under a warming climate.
Preliminary results show that populations differentiate along climatic clines providing evidence of adaptation to the local environment. Of the variables tested, the sum of degree days below 0°C characterizing the populations’ provenance presented the strongest negative association with populations height growth; the trade-off between growth and adaptation to cold has been well documented for several tree species. Populations showed different growth responses when moved to warmer climates. Populations originating in relatively cold and mild climates increased growth, whereas populations originating in warmer climates drastically decreased their growth when tested at warmer sites. The latter provides further evidence of a higher risk of extirpation for the populations at the trailing edge of forest tree species distribution under a warming climate.