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Examining Changes in Forest Ecosystem Carbon
Exchange with Stand Age in the Upper Midwest US
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Not So Old
Ankur Desai, Ken Davis, Weiguo Wang Department
of Meteorology Pennsylvania State
University Paul Bolstad, Eileen Carey, Bruce
Cook Jon Martin, Leslie Kreller, Deborah
Hudleston Department of Forest
Resources University of Minnesota
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Acknowledgements

• Department of Energy
• Office of Biological and Environmental Research,
  Terrestrial Carbon Processes
• National Institute for Global Environmental
  Change (NIGEC) Midwestern Region
• Ameriflux / Fluxnet
• U.S. Forest Service
• Chequamegon-Nicolet National Forest, Park Falls,
  WI
• Ottawa National Forest, Watersmeet, MI
• North Central Research Station, Rhinelander, WI

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Motivation

• While interannual variability in forest carbon
  exchange appears to be primarily controlled by
  climate, long-term forest carbon exchange is
  strongly constrained by stand age since
  initiation
• Scaling carbon fluxes across space and time (past
  or future) requires understanding of the
  relationship between forest carbon exchange and
  site age
• Eddy flux measurements in old-growth forests are
  few but increasing. None are in the upper
  Midwest
• Old growth forests are expanding in the upper
  Midwest
• Old growth forests are not carbon neutral as was
  typically theorized, but are probably small
  carbon sinks

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ChEAS Region

• Chequamegon-Ecosystem Atmosphere Study allows
  for intensive study of carbon exchange
• Most forests in region arerecovering from
  logging in early 20th century
• Pre-European settlement vegetation is found in
  the 8,500 Ha Sylvania Wilderness and Recreation
  area (Upper Peninsula, Michigan)
• Eddy flux towers have been installed in an old
  growth stand adjacent to Sylvania and a nearby
  mature hardwoods forest (Willow Creek)

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Hypotheses

• Ecosystem respiration is greater at an old-growth
  forest compared stand to a mature forest stand
• Primarily due to increased coarse woody debris
• Gross ecosystem production is slightly lower at
  the old-growth site compared to the mature site
• Possibly due to decreased light use efficiency,
  stomatal conductance, water use efficiency and/or
  hydraulic conductivity
• The old-growth site is more sensitive to drought
  stress

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Site Descriptions

• Sylvania old-growth
• Established late 2001
• Hemlock, sugar maple, basswood, yellow birch,
  LAI 3.6
• 0-350 year old trees, very low disturbance
  frequency (5.4 / decade), long-term
  compositional equilibrium
• Lots of coarse woody debris (42 Mg/Ha)
• 3-30 Ha patchy regions of s. maple or hemlock
  dominance
• Willow Creek mature hardwoods
• Established mid-1998
• Sugar maple, basswood, green ash, LAI 5.3
• Approx. 70 yrs old (initiated after logging)
• Very little coarse woody debris

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Site Descriptions

• Willow Creek pre-European settlement vegetation
  is very similar to Sylvania

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Site Descriptions

• Willow Creek Sylvania

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Climate

• Average (1971-2000, Watersmeet, MI)
• Annual 3.9o C, -12.7o C in Jan, 18.5o C in Jul
• 771 mm of precipitation (57 during May-Sep)
• Willow Creek is slightly colder and drier (esp.
  in winter)
• 2002
• Warmer 5.0o C, -7.1o C in Jan, 20.7o C in Jul
• Wetter 866 mm, but very dry in Nov and Dec
• 2003
• Colder up through Oct -14.3o C in Jan, 18.0o C
  in Jul
• Very dry winter and spring (50 of average
  precip.)
• August-October also dry (59 of average precip.)
• Soil moisture lower than 2002 in Jan-Mar and
  Jul-Sep

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Climate
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Methods

• Eddy covariance measurements and high-precision
  CO2 mixing ratio profiles measured from 30 m (W
  Creek) and 37 m (Sylvania) towers, 10 hz, similar
  instrumentation (closed-path IRGAs) and flux
  calculation methods
• Standard micrometeorological and soil
  measurements and profiles (temperature, water,
  radiation, etc)
• Ground-based biometric and physiological
  measurements in vicinity of towers and across
  ChEAS region, including soil efflux

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Methods

• Nighttime u cutoff filter
• 0.3 m s-1 at Sylvania
• 0.175 m s-1 at Willow Creek
• Directional heterogeneity NEE data screening
• non-SW winds on summer nights at Sylvania
  (dilution by lakes?)
• SE-winds (typically with temperature inversions)
  at Willow Creek (flux pooling in low-lying
  wetland?)

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Methods

• Ecosystem respiration (ER) modeled with nighttime
  NEE and 5-cm soil temperature
• Using an Arrhenius style equation
• 28-day moving window fit
• GEP computed as ER NEE
• Light response curves modeled with daytime GEP
  and above canopy incoming total PAR
• Rectangular hyperbola curve
• Similar moving window fit as for ER
• Data gaps filled using ER and GEP models
• 2003 results are preliminary (computed last week!)

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Average Daily NEE (Jun-Aug)

• Mean nighttime summer NEE at Willow Creek was
  smaller than Sylvania nighttime NEE decreased at
  Sylvania in 2003 vs. 2002, but not at Willow
  Creek
• Mean daytime summer NEE at Willow Creek was
  larger than Sylvania daytime NEE was similar in
  02 and 03

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Net Ecosystem Exchange

• Total summer (Jun-Aug) NEE at Willow Creek was
  larger than Sylvania 244 larger in 2002, 181
  in 2003
• Summer Sylvania NEE increased 60 in 2003 vs.
  2002, while Willow Creek NEE increased 30

gC m-2 day-1
gC m-2 day-1
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Cumulative NEE

• Annual total NEE in 2002 at Willow Creek (-447 gC
  m-2 yr-1) was much larger than Sylvania (-72 gC
  m-2 yr-1)
• 2003 is also on target for a large difference up
  through September, NEE Jan-Oct 2003 at Willow
  Creek (-701 gC m-2 yr-1) was 1.75x larger than
  Sylvania (-254 gC m-2 yr-1). Total uptake
  appears to be larger in 2003 vs. 2002

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Respiration Response

• Sylvania had a greater temperature dependency
  (dR/dT) to respiration than Willow Creek
• Respiration for a given temperature decreased
  slightly for Sylvania, but increased for Willow
  Creek in 2003
• Summer soil temperature was 1.2 C cooler in 2003

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Total Ecosystem Respiration

• Summer respiration at Willow Creek was smaller
  than Sylvania 41 smaller in 2002, 24 in 2003
• Summer Sylvania respiration decreased 26 in 2003
  vs. 2002, whereas Willow Creek declined only 3

Total Sylvania (965 gC m-2 yr-1) Willow Creek
(667 gC m-2 yr-1)
gC m-2 day-1
gC m-2 day-1
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Soil Chamber Efflux

• Sylvania
• Total 2002 soil respiration was 908 gC m-2 yr-1
  94 of total ecosystem respiration (965 gC m-2
  yr-1)
• Mean summer soil respiration was 3.9 mmol m-2 s-1
  57 of mean summer eddy flux respiration (6.8
  mmol m-2 s-1)
• Willow Creek
• Total 2002 soil respiration was 1147 gC m-2 yr-1
  172 of total ecosystem respiration (667 gC m-2
  yr-1)
• Mean summer soil respiration was 7.0 mmol m-2 s-1
  175 of mean summer eddy flux respiration (4.0
  mmol m-2 s-1)

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Daytime PAR Response

• GEP to PAR response was slightly smaller at
  Sylvania than Willow Creek in 2002 bigger
  difference in 2003
• Willow Creek response curves were relatively
  similar in 2002 and 2003 Sylvania was smaller in
  2003 vs. 2002

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Total Ecosystem GEP

• Summer GEP at Willow Creek was slightly larger
  than Sylvania 15 larger in 2002 46 in 2003
• Summer Sylvania GEP declined 7 in 2003 vs 2002,
  whereas Willow Creek GEP increased 17

Total Sylvania (1036 gC m-2 yr-1) Willow Creek
(1136 gC m-2 yr-1)
gC m-2 day-1
gC m-2 day-1
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GEPER

• Mean daily GEPER ratio for summer (Jun-Aug)

2002 2003
Willow Creek 2.5 /- 0.5 2.9 /- 0.5
Sylvania 1.3 /- 0.3 1.8 /- 0.9
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WUE

• Ratio of mean monthly GEP (gC m-2 day-1) LE (kg
  H2O m-2 day-1)
• Significant change from 2002 to 2003 at Willow
  Creek
• LE decreased significantly in summer at Willow
  Creek, but not at Sylvania from 2002 to 2003

June July August
Willow Creek
2002 4.1 3.2 3.5
2003 6.1 6.6 6.0
Sylvania
2002 3.4 3.6 3.5
2003 2.7 3.6 3.9
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Summary

• Ecosystem respiration at Sylvania appears to be
  more sensitive to climate than Willow Creek.
  Both Willow Creek and Sylvania had significant
  GEP interannual variability, though more at
  Willow Creek
• The two sites had opposite responses in summer
  GEP when comparing 2002 to 2003
• Total annual NEE at both sites was larger in a
  cool, dry year (2003) compared to a warm, wet
  year (2002)
• This NEE increase was driven at Sylvania mainly
  by ecosystem respiration decline, but at Willow
  Creek, the increase was due to an increase in GEP

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Summary

• Chamber soil efflux suggests opposite respiration
  differences between the two sites than eddy flux
• CWD respiration calculations underway
• GEPER increased slightly at both sites in 2003
  compared to 2002
• WUE increased at Willow Creek in 2003 vs. 2002
• Sapflux data can be used to separate evaporation
  and transpiration to find source of WUE change
• Had Willow Creek never been logged or managed, it
  probably would have much larger ecosystem
  respiration (most likely due to greater CWD), and
  slightly or moderately smaller gross ecosystem
  production

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Future Work

• Analysis of biometric and physiological
  measurements to calculate NEP and individual
  carbon pool fluxes
• BIOME-BGC modeling of carbon exchange
• Integration of remote sensing / FIA data to
  extrapolate fluxes over the region
• More information at http//cheas.psu.edu