Abstract
Northern forests at the leading edge of their distributions may not show increased primary productivity under climate warming, being limited by climatic extremes such as drought. Looking beyond tree growth to underlying physiological mechanisms is fundamental for accurate predictions of forest responses to climate warming and drought stress. Within a 32-year genetic field trial, we analyze relative contributions of xylem plasticity and inferred stomatal response to drought tolerance in regional populations of a widespread conifer. Genetic adaptation leads to varying responses under drought. Trailing-edge tree populations produce fewer tracheids with thicker cell walls, characteristic of drought-tolerance. Stomatal response explains the moderate drought tolerance of tree populations in central areas of the species range. Growth loss of the northern population is linked to low stomatal responsiveness combined with the production of tracheids with thinner cell walls. Forests of the western boreal may therefore lack physiological adaptations necessary to tolerate drier conditions.
Highlights
Northern forests at the leading edge of their distributions may not show increased primary productivity under climate warming, being limited by climatic extremes such as drought
Diverse physiological mechanisms underlie the differences observed in drought tolerances of divergent lodgepole pine populations
Wood anatomical responses and isotopic signatures revealed different relative contributions of xylem and stomatal control, suggesting varied strategies to cope with drought
Summary
Northern forests at the leading edge of their distributions may not show increased primary productivity under climate warming, being limited by climatic extremes such as drought. Prolonged droughts may increase forest mortality through slow depletion of tree internal carbon reserves, known as carbon starvation[14] Both wood hydraulic traits and stomatal regulation of water loss affect vascular damage and photosynthesis under moisture limitation[15,16,17,18,19]. Trees may produce more cavitation-resistant xylem through the formation of tracheids with thicker cell walls and smaller lumen diameters This drought-tolerant behaviour could enable stomata to remain open under prolonged drought, which may help avoid depletion of carbon reserves. While trailing-edge populations are expected to accumulate extinction debts[32], decreased forest health and productivity can be expected throughout a species' range due to locally adapted populations being genetically maladapted to new climate conditions[24]
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