Low photosynthesis of treeline white spruce is associated with limited soil nitrogen availability in the Western Brooks Range, Alaska

1. 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. 2. 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 two years in three contrasting habitats: riverside terrace, hillslope forest and treeline. The sites had similar aboveground microclimates, ...