Genetic responses to climate in Pinus contorta: Niche breadth, climate change, and reforestation

Fundamental plant-environment relationships were revealed by analyses of 20 -yr height and survival of 118 populations representing two subspecies of P inus contorta growing in common gardens at 60 environmentally disparate tes t sites in British Columbia. The approach involved (1) preparing models tha t described the general climate of British Columbia, (2) developing populat ion-specific response functions driven by predicted climate variables, (3) developing general transfer functions that predict performance from the cli matic distances over which populations were transferred, and (4) interpreti ng the results in terms of niche breadth, effects of climate change on adap tedness of populations, and reforestation in a changing environment. Polynomial regression models used physiographic descriptors to predict seve n climate variables from normalized records of 513 weather stations. Values of R-2 ranged over 0.80-0.97 for thermal variables and 0.54-0.61 for preci pitation variables. Validations with independent data from 45 stations were strong and suggested that the models were generally free of bias within th e limits of the original data. Response functions describing the height or survival of each population wer e developed from quadratic regressions using predicted climate variables fo r each test site. Mean annual temperature and mean temperature in the colde st month were the most effective variables for predicting population height , while the ratio of summer temperature to summer moisture was the best pre dictor of survival. Validation of the response functions with independent d ata from two additional test sites produced values of R-2 between actual an d predicted values that were as high as 0.93 for height and 0.73 for surviv al. The results demonstrated that natural populations have different climat ic optima but tend to occupy suboptimal environments. Nevertheless, the gen eral transfer functions showed that optimal growth and survival of the spec ies as a whole is associated with the null transfer distance. These seemingly anomalous results suggest that the same processes thought t o determine the distribution of species control the distribution of genotyp es within species: (1) environmental selection to produce a broad fundament al niche, and (2) density-dependent selection to produce a relatively narro w realized niche within which most populations are relegated to suboptimal environments. Consequently, the steep geographic dines typical of P. contor ta seem to be driven more by density-dependent selection than by environmen tal selection. Asymmetric gene flow from the center of distribution toward the periphery is viewed as a primary regulator that provides the fuel for b oth environmental and density dependent selection and thereby indirectly pe rpetuates suboptimality. The response functions predict that small changes in climate will greatly a ffect growth and survival of forest tree populations and, therefore, that m aintaining contemporary forest productivities during global warming will re quire a wholesale redistribution of genotypes across the landscape. The res ponse functions also provide the climatic bases to current reforestation gu idelines and quantify the adjustments necessary for maintaining adaptedness in planted trees during periods of small (similar to 1 degrees C) temporal temperature shifts.