Is local the best? Phenotypic plasticity vs local adaptation in a reciprocal transplant experiment with white spruce in Alaska

Boreal forests situated in high latitudes face heightened susceptibility to climate extremes and global warming. Understanding the relative influence of adaptation mechanisms like phenotypic plasticity or local adaptation on key traits is crucial to better understand and project species distribution, forest growth and vitality. To address this, we conducted a reciprocal transplant experiment featuring two white spruce (Picea glauca [Moench] Voss) provenances in Alaska, representing cold and dry treelines. Trees from each provenance were reciprocally transplanted across a gradient spanning from dry bluff sites, dry treelines via old growth forests to cold limited treelines and beyond. From 2015 - 2022, we monitored survival, vitality, growth, and various needle morphology traits. Results showed that the dry provenance had a superior performance in its home environment. Whereas both provenances performed similar at the cold site. Survival and vitality rates indicated that elevated temperatures favoured tree growth. Seedling survival and growth are possible beyond the current cold treeline. Further, needle morphology traits were more influenced by the current environment than by origin, thus showing a high phenotypic plasticity. Nevertheless, significant differences in needle morphology among provenances hinted at a genetic base of these traits. Results suggested that local is best can still be applied.