Canopy gas exchange and growth of white spruce near the Arctic treeline: confronting measurements with models along natural and experimental resource gradients

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The position of the arctic treeline has important implications for surface energy budgets and carbon cycling in a changing climate. Modeling efforts suggest these effects are relevant on regional and global scales. Our understanding of the controls on tree growth at the arctic treeline has been developed using tree ring studies, which are necessarily correlative and not mechanistic in nature. These tree ring studies have identified both positive and negative radial growth responses to warming in the later half of the 20th century. Investigators have speculated that negative growth trends reflect an increasing importance of temperature-induced drought stress and that treeline advance may be expected in mesic and wet areas, but not in dry areas, with future climate warming. Recent work has revealed several important complexities that clearly show we have oversimplified the relationships between climate and tree growth at the arctic treeline. Detailed measurements of seasonal changes in tree physiology and growth in response to changes in resource availability are now required to take our understanding to the next level. This work will coordinate continuous measurements of white spruce canopy gas exchange with weekly measurements of branch gas exchange and leader, branch, radial and fine root growth in trees receiving factorial nutrient and water supplements along a gradient of parent material depth. Results of the study will, for the first time: resolve the seasonality of C uptake and water loss in treeline white spruce; compare the seasonality and magnitudes of growth in all major organs; articulate the consequences of changes in resource availability for white spruce gas exchange physiology and growth along a gradient where resource availability varies naturally; and identify a process-based model that accurately describes the relationships between climate and tree growth at the arctic treeline.