DOI: 10.5061/dryad.s5c8k
title: Data from: Low photosynthesis of treeline white spruce is associated with limited soil nitrogen availability in the Western Brooks Range, Alaska

description: 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, but
 very different soil conditions. Soils were warm and dry on the terrace,
 cool and moist in the forest and cold and seasonally wet the treeline. 3.
 Photosynthesis, needle nitrogen (N) concentration, and soil N availability
 declined from the terrace to the forest to the treeline. Low N
 availability at the treeline was likely a consequence of limited microbial
 activity in the cold and seasonally wet soils. Soils at the treeline were
 colder than the terrace during the growing season and colder than the
 forest in winter, when the treeline maintains a shallow snowpack. 4. Our
 results highlight the potential for an indirect effect of temperature on
 the growth of trees at the Arctic treeline and suggest that treeline
 responses to changes in climate may be more complex than previously
 thought.
description: Ion Exchange
 MembranesEstimates of nutrient
 availability from ion exchange membranes installed on June 28, 2011 and
 retrieved on August 9, 2011 in eight plots at each site. Two cation and
 two anion membranes were installed in each plot and extracted
 together.Growing Season
 MicroclimatesHourly measurements of
 air temperature, humidity, photosynthetically active radiation, rain, soil
 temperature and soil water content at each of the sites between early June
 and late September of the 2010 and 2011 growing
 seasons.Needle Gas
 ExchangeMeasurements of
 light-saturated net photosynthesis, dark respiration and stomatal
 conductance made on approximately one-week intervals during the 2010 and
 2011 growing seasons in three habitat types.Needle N, 15N and
 PhotosynthesisMeasurements of needle
 nitrogen concentration, needle 15N and needle net photosynthesis on the
 final sampling date in each of the habitats during late August of 2010 and
 2011.Seasonal Needle N and
 13CMeasurements of needle N
 concentration and needle 13C made on approximately weekly intervals on 8
 supplementary trees in each habitat during the 2011 growing
 season.Fertilizer Branch
 Extension GrowthMeasurements of annual
 branch extension growth made in 2007 and 2008 on 15 control trees and 15
 trees that were fertilized with N, P and K adjacent to the treeline site
 in September of 2006, 2007 and 2008.