Spruce hybrids show superior cumulative growth and intermediate response to climate anomalies, as compared to their ecologically divergent parental species

Climate change brings new constraints to which trees will have to adapt, including more frequent weather extremes. Black spruce and red spruce are phylogenetically close but adapted to different ecological conditions, and they form a natural hybrid zone where their distributions come into contact. Thus, they represent an interesting model to study the effect of introgressive hybridization in the context of climate change, given that interspecific gene flow could affect their capacity to adapt where their natural distributions overlap. Using a common-garden field test gathering 20-year-old progeny trees resulting from rigorous controlled crosses including previously verified genetic identity of the parents, growth patterns and wood density differences could be observed between species and between them and their F ₁ hybrids. A dendroecological analytical approach relying on wood cores was used and revealed similar wood responses to climatic variations between species, both through lifetime climate sensitivity and through episodic stress response indexes. They were however differentially expressed in early- and latewood between black spruce and red spruce, differences likely driven by diverging cambial phenology adaptations to different growing season lengths. F ₁ hybrids exhibited hybrid vigor for cumulative growth under the test site conditions, but showed intermediate values for traits related to climate response. These results offer new perspectives for understanding the dynamics of adaptation in hybrid zones in the context of climate change, as well as for guiding conservation and genetic improvement efforts.