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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 vapour 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.We measured gas exchange, foliar nitrogen and lamina mass to area ratio in 697 c. 30‐year‐old trees in two field progeny trials. We analysed the response of gas exchange rates to VPD using a novel quantitative genetic model, the function‐valued trait approach.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.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. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

The prevailing hypothesis states that treeline positions are defined by the direct effects of cold temperatures on cell division and tree growth. Meanwhile, photosynthesis is thought to be relatively unrestricted in treeline trees. Support for this hypothesis comes from the global correlation between temperature and treeline position, the observation that many treelines have advanced in response to recent warming and the frequent use of increment cores from treeline trees to reconstruct past climates. However, studies of the physiology of trees at the Arctic treeline are rare, and this hypothesis remains largely untested., To improve our understanding of the relationships between temperature and performance of white spruce near the Arctic treeline, we made measurements of needle gas exchange, needle nutrition and soil nutrient availability over 2 years in three contrasting habitats: riverside terrace, hillslope forest and treeline. The sites had similar above-ground microclimates, but very different soil conditions. Soils were warm and dry on the terrace, cool and moist in the forest and cold and seasonally wet the treeline., Photosynthesis, needle nitrogen (N) concentration and soil N availability declined from the terrace to the forest to the treeline. Low N availability at the treeline was likely a consequence of limited microbial activity in the cold and seasonally wet soils. Soils at the treeline were colder than the terrace during the growing season and colder than the forest in winter, when the treeline maintains a shallow snowpack., Our results highlight the potential for an indirect effect of temperature on the growth of trees at the Arctic treeline and suggest that treeline responses to changes in climate may be more complex than previously thought. [ABSTRACT FROM AUTHOR]

1. The abundance and performance of the Eastern Spruce Gall Adelgid, Adelges abietis, was evaluated on young, open-grown trees of White Spruce, Picea glauca, whose growth rates had been increased through fertilization or decreased through root-pruning. 2. In general, gall densities were highest on control trees and on mid-crown branches. Reduced galling success on fertilized trees was largely due to higher overwintering mortality of first-generation nymphs. Foliar magnesium, total tannin and total phenol contents were positively, and nitrogen and total monoterpene contents negatively, related to gall density and/or galling success. 3. Although short shoots were more abundant, shoot length was parabolically related to gall density. There was a non-significant parabolic trend between shoot size and galling success and volume. The number and average size of A. abietis females emerging from galls were positively related to gall volume, and realized fecundity was positively correlated to female size. Consequently, fitness was approximately twice as high for nymphs colonizing intermediate-sized than small or large shoots. 4. These results do not support the plant vigour or plant stress hypotheses. The results do, however, agree with predictions of the modified plant stress hypothesis for sucking insects. It is speculated that Adelges abietis lacks the necessary resources for successful gall formation on small shoots and is unable to produce a stimulus large enough to induce gall formation on large shoots. [ABSTRACT FROM AUTHOR]