GC23E – 0677
AGU 2014
Can nutrient availability explain low and declining white spruce growth
near Arctic treeline in the eastern Brooks Range, Alaska?
Sarah Ellison and Patrick Sullivan — Environment and Natural Resources Institute, University of Alaska Anchorage
Introduction
Conclusions
• 20th century warming has caused
increased growth of white
spruce in the western Brooks
Range, but declining growth in
the east (Wilmking et al. 2004)
• Drought-induced stomatal
closure cannot explain this trend
(Brownlee et al. in review, AGU
poster GC23E - 0674)
• We hypothesize that low soil
nutrient availability may explain
low and declining growth in the
eastern Brooks Range
The low and declining growth of
white spruce stands near Arctic
treeline in the Brooks Range,
Alaska may be partially due to low
nutrient availability, following
findings in similar habitats in the
western Brooks Range (Sullivan et
al. in press).
Sites are remote and accessible only by bush plane in the
summer months, with the exception of the Dietrich river site,
which is located on the Dalton highway. We visit each site four
times during the summer and camp on location. It takes seven
days to travel between all four sites.
Study sites. We located our sites in comparable white spruce stands
on riverside terraces to measure stands most likely to experience
drought-induced stomatal closure. In 2014, we also installed 3
auxiliary sites near each terrace representing a range of soil water
conditions.
Our easternmost site is located on the Wind River in the Arctic
National Wildlife Refuge (above). Microclimate stations record
hourly data at all sites. Except for the Kugururok river site, sites
are uniform white spruce with similar stand density and
understory.
Results
White spruce growth is lower at our the
eastern Brooks Range sites than the
western sites
Needle nitrogen concentration may be
related to growth
Low nitrogen concentration cannot entirely
explain low photosynthesis, especially at
the Wind River site
Methods
• We visited each site four to five
times during the growing
seasons of 2013 and 2014
• Microclimate data (above- and
below ground) is recorded
hourly at each site
• We measured branch extension
growth and needle nitrogen
concentration at the end of the
growing season (n=2-4 branches
from 10 trees).
• We measured needle gas
exchange using an LI-6400
portable infrared gas analyzer,
pictured below (n=2 branches
of 10 trees).
• Trees in the east have had the lowest branch extension for
the past four years (n=40 trees per site)
• Higher needle N concentration is associated with greater
branch extension across all four regions (n=16 sites)
• Ring widths are also significantly smaller in the east
(Brownlee et al. in review)
• Points in grey do not follow this trend, likely due to local
fungal and insect infestations
• The eastern sites have the lowest photosynthesis for both
2013 and 2014, but span the full range of needle N
concentration (n=80 measurements per point)
Low eastern Brooks Range soil temperatures have a large effect on photosynthesis through stomatal conductance
Soil Temperature
Air Temperature
Relative Humidity
Needle N Concentration
• To examine the controls on
photosynthesis, I performed a random
forest regression with soil and air
temperature, relative humidity, needle
N concentration and soil water content.
Soil H2O
• Low photosynthesis and conductance
at our eastern sites is related to low soil
temperatures in both 2013 and 2014.
Soil Temperature
• Soil temperature has the largest effect
on photosynthesis, particularly below
soil temperatures of 8 C. This
supports past findings (Goldstein and
Brubaker 1985, Sullivan and
Sveinbjornsson 2011).
Relative Humidity
However, nutrient availability may
only be able to explain the low
growth, but not the low
photosynthetic capacity at the
eastern sites. Effects of colder soils
in the eastern Brooks Range may
depress photosynthesis via stomatal
conductance and contribute to low
growth there.
Our future work includes analysis
of:
1. Needle nonstructural
carbohydrates
2. Needle phosphorous
3. Soil nutrient availability in
different pools of availability, as
determined through:
• Ion exchange membranes
• Soils extracted in KCl
• Soils extracted in water
• Microbial biomass
4. Soil pH
Air Temperature
Needle N Concentration
Soil H2O
• Low soil temperature affects stomatal
conductance via reduced membrane
permeability in roots (Running and Reid
1980)
Agashashok River
Acknowledgments
Contact
Funding was provided by the National Science
Foundation. We thank A. Krichels, E. McKnight,
A. Brownlee, S. Cahoon and C. Gamm for
excellent field support. CH2MHill Polar Field
Services provided logistical support.
• Sarah Ellison, M.S. student,
sbzellison@gmail.com
• Dr. Patrick Sullivan,
pfsullivan@alaska.edu
Literature cited
Brownlee, A.H., P.F. Sullivan, A.Z. Csank, and B. Sveinbjörnsson. In review. Tree-ring δ13C suggests drought-induced stomatal closure cannot explain white spruce growth
declines in the eastern Brooks Range, Alaska. Ecology.
Goldstein, G.H., L.B. Brubaker, and T.M. Hinckley. 1985. Water relations of white spruce (Picea glauca (Moench) Voss) at tree line in north central Alaska. Canadian Journal of
Forest Research 15: 1080-1087.
Running, S. W., and C.P. Reid. 1980. Soil temperature influences on root resistance of Pinus contorta seedlings. Plant Physiology 65: 635-640.
Sullivan, P.F., S.B.Z. Ellison, R.W. McNown, A.H. Brownlee, and B. Sveinbjornsson. In press. Soil nutrient availability constrains growth and allocation of treeline trees in
northwest Alaska. Ecology.
Sullivan, P.F., and B. Sveinbjornsson. 2011. Environmental Controls on Needle Gas Exchange and Growth of White Spruce (Picea glauca) on a Riverside Terrace near the Arctic
Treeline. Arctic, Antarctic, and Alpine Research 43: 279-288.
Wilmking, M., G.P. Juday, V.A. Barber, and H.S. Zald. 2004. Recent climate warming forces contrasting growth responses of white spruce at treeline in Alaska through
temperature thresholds. Global Change Biology 10: 1-13.