Genetic variation in leaf traits and gas exchange response to vapor pressure deficit in two contrasting conifer species

1. Mechanistically predicting the evolutionary response of tree species to
climate change requires an understanding of genetic variation in relevant
traits. Here we compared the phenotypic and genetic variation in the Leaf
Economics Spectrum (LES) traits and the response of gas exchange to vapor
pressure deficit (VPD) in lodgepole pine (Pico) and white spruce (Pigl),
an early and a late successional species dominating the boreal forests of
western Canada. 2. We measured gas exchange, foliar nitrogen, and lamina
mass to area ratio in 697 app. 30-year-old trees in two field progeny
trials. We analyzed the response of gas exchange rates to VPD using a
novel quantitative genetic model, the function-valued trait approach. 3.
Pico showed greater phenotypic variation in the LES traits and greater
genetic variation in photosynthetic rate than Pigl, but the species showed
no significant difference in their phenotypic correlations between the LES
traits. Pico showed a less sensitive stomatal response to VPD than Pigl
and no significant genetic variation in stomatal sensitivity. In contrast,
Pigl showed a positive correlation between the genetic values of stomatal
sensitivity to VPD and stomatal conductance under low VPD. 4. Our study
region is projected to see an increase in VPD with climate change; the
less sensitive and genetically diverse stomatal response to VPD in Pico
could make this species more vulnerable to climate change-induced
droughts.