Soil nutrient availability constrains growth and allocation of treeline trees in northwest Alaska

Background/Question/Methods

The position of the Arctic treeline, which is a key regulator of surface energy exchange and carbon cycling, is widely thought to be controlled by temperature.  Here, we present evidence that soil nutrient availability, rather than temperature, is the proximate control on growth of treeline trees at our study site in northwest Alaska.  We examined constraints on growth and allocation of white spruce in three contrasting habitats.  The habitats had similar aboveground climates, but soil temperature declined from the riverside terrace to the forest to the treeline, making for a natural experiment.  

Results/Conclusions

We identified five lines of evidence which conflict with the hypothesis of direct temperature control and/or point to the importance of soil nutrient availability.  First, the magnitude of and proportional allocation to aboveground growth declined from the terrace to the forest to the treeline, along gradients of diminishing soil nitrogen availability and needle nitrogen concentration.  Second, peak rates of branch extension, main stem radial and fine root growth generally occurred well before seasonal air and soil temperature maxima.  Third, in contrast with expectations of temperature-limited growth, growing season average net photosynthesis was positively related to annual growth.  Fourth, annual branch extension growth was closely related to snow depth, consistent with the hypothesis that deeper snow promotes microbial activity and greater soil nutrient availability.  Finally, the tree ring record revealed a strong positive growth response to recent climate warming on the terrace, where soil nitrogen availability is relatively high.  Meanwhile, trees in the forest and at the treeline showed progressively smaller positive growth responses.  Our results suggest temperature effects on tree growth at the Arctic treeline may be mediated by soil nutrient availability, making responses to climate change more complex and our ability to interpret the tree ring record more challenging.