Selection for fewer, water- and carbon-conservative needles in black spruce trees under warm, dry climates

Trees are increasingly at risk of maladaptation to their environment as climates change rapidly worldwide. Although adaptive evolution through natural selection is a key mechanism by which populations and species can persist in changing environments, we have limited information regarding the phenotypic traits under selection in warm and dry environments. We answer the following research questions: (1) What ecophysiological traits are under selection in warm, dry environments? (2) Does intrapopulation trait integration affect the response to selection in the warmer, drier site? (3) Is the plastic response of traits under selection adaptive?

Using Picea mariana (black spruce) as a case study, we studied 425 trees representing seven provenances across three 50-year-old common garden trials established along a spatial climate gradient across eastern Canada. We measured height growth rate as a performance metric, and ten traits that reflect water use, thermoregulation, structural support and photosynthetic rate.

All traits were under selection in at least one site, mostly in combination with other traits. For two trait combinations, the strength of selection gradients significantly increased from the colder, wetter site to the warmer, drier site: water use efficiency (WUE) with Huber value (HV), and carbon-to-nitrogen ratio (CN) with HV. In the warmer and drier site, trait-trait correlations among these three traits were largely absent, except for CN:HV in two provenances. Overall, reaction norms suggest that the plastic response was not aligned with selection for trait pairs in warm, dry climates.

Results suggest that adaptive evolution in response to climate change in P. mariana may favour phenotypes with fewer needles that are conservative for water and resource use. In the seven study provenances, intrapopulation trait integration should minimally impede adaptive evolution, but plastic responses to warmer and drier conditions may constrain the expression of optimally adapted phenotypes.