Evidence of soil nutrient availability as the proximate constraint on growth of treeline trees in northwest Alaska

• DOI: 10.1890/14-0626.1
• Publication Date: 2014-09-02T17:33:47Z
• Authors: Patrick F. Sullivan; Sarah B. Z. Ellison; Robert W. McNown; Annalis H. Brownlee; Bjartmar Sveinbjö...
• Keywords: Soil nutrients; Proximate

The position of the Arctic treeline, which is a key regulator surface energy exchange and carbon cycling, widely thought to be controlled by temperature. Here, we present evidence that soil nutrient availability, rather than temperature, may proximate control on growth treeline trees at our study site in northwest Alaska. We examined constraints allocation white spruce three contrasting habitats. habitats had similar aboveground climates, but temperature declined from riverside terrace forest treeline. identified six lines conflict with hypothesis direct and/or point importance availability. First, magnitude along gradients diminishing nitrogen (N) availability needle N concentration. Second, peak rates branch extension, main stem radial fine‐root were generally not coincident seasonal air maxima. At particular, well before temperatures reached their peaks. Third, contrast temperature‐limited growth, growing season average net photosynthesis was positively related sum normalized across sites. Fourth, nonstructural carbohydrate concentration significantly higher terrace, where greatest. Fifth, annual extension snow depth, consistent deeper promotes microbial activity greater Finally, tree ring record revealed large increase during late 20th‐century climate warming relatively high. Meanwhile, showed progressively smaller increases. Our results suggest effects sites mediated making responses change more complex ability interpret challenging previously thought.