Spruce hybrids show superior lifespan growth but intermediate response to climate stress compared to their ecologically divergent parental species

Climate change brings new constraints to which trees will have to adapt, including more frequent severe weather events due to climate anomalies. Black spruce and red spruce are phylogenetically close but adapted to different ecological conditions, and they form a natural hybrid zone where their natural 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 eventually 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 F₁ hybrids. A dendroecological analytical approach relying on wood cores was used and revealed similar wood responses to climatic variations between species, both through lifespan 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. F₁ hybrids exhibited hybrid vigor for lifespan cumulative growth under the test site conditions but showed intermediate values for traits related to climate response. These results may assist the management of forest genetic resources and contribute to a better understanding of the dynamics of adaptation to climate in hybrid zones.