A range-wide common garden experiment of white spruce indicates population differentiation in drought tolerance traits

Abstract

In western regions of the Canadian boreal forest, mean annual temperature has increased by 1.5 to 2.5 °C, significantly exceeding the mean global temperature increase. Climate conditions have also become drier and together these climate trends have induced widespread growth reductions, dieback and mortality of tree populations due to drought stress, with white spruce having been identified as particularly vulnerable. Here I evaluate genetic population differentiation in a 40-year old provenance test and simultaneously conduct seedling trials with the original seed that has been stored for 40 years to investigate drought traits in controlled greenhouse environments. Patterns of height growth differed between seedlings and adult trees. Seedling growth potential was highest in eastern provenances. In contrast, local sources matched or exceeded eastern trees in adult field trials in north central Alberta. This variation in growth indicates that eastern trees may be adapted to high competition at the seedling stage but are maladapted to the cooler and drier climates of western Canada. Fluorescence measurements used to investigate impairment of photosynthesis during drought indicated the far eastern Maritime provenances and far north western Yukon provenances were the first to seize photosynthesis under drought. Carbon isotopes showed strong variation between provenances and indicated greater stomatal response in Ontario trees, indicating no trade-off between growth and stomatal response in these trees. Maritime and Yukon provenances adapted to less drought-prone conditions showed little stomatal response. Surprisingly, Alberta provenances, which were expected to be drought tolerant, also showed relatively low stomatal response to drought. We further observed associations of growth and drought traits with the origin climate of the seed sources. Populations vulnerable to drought were found at opposite ends of the spectrum: iii warm and wet provenances from the maritime and very cold and dry provenances from northern Alberta and the Yukon, indicating that neither of these populations is selected for by drought (due to cold in the northwest and due to ample precipitation in the eastern maritime). The study highlights genetic variation in white spruce populations. This variation between populations indicates particular vulnerability of northern populations where climate shifts are likely to be most significant. These results indicate assisted migration should be possible to replace vulnerable Yukon populations with more drought tolerant populations from Ontario and Southern Alberta.