Empirical determination of N critical loads for alpine vegetation

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Empirical determination of N critical loads for
alpine vegetation
William D. Bowman, Julia L. Gartner, Keri
Holland, and Magdalena Wiedermann Department of
Ecology and Evolutionary Biology and Mountain
Research Station, University of Colorado, Boulder
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N Critical Loads Does one size fit all?
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(No Transcript)
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Indicators of Ecosystem Response to Elevated N
Inputs

• episodic acidification loss of acid
  neutralizing capacity and elevated NO3- in
  upper Green Lakes Valley (Nel Caine Mark
  Williams)
• changes in diatom composition (lake cores)
  (Jasmine Saros, Alex Wolfe and Jill Baron)
• needle and forest floor chemistry in old-growth
  subalpine forests (East-West slope comparison)
  (Heather Rueth and Jill Baron)
• changes in alpine plant species composition in
  long-term monitoring plots

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Paradox of simultaneous N limitation N excess
Experimental N additions in alpine result in
greater plant growth, yet growing season export
of NO3- is occurring (?)

• Adaptation to low soil nutrient supply- some
  species dont respond to increased N availability

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Paradox provides an opportunity changes in
species composition indicative of N
inputs Alternative view how much N input does it
take to produce a change in species composition?
( N critical load using biotic response)
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species composition response
treatment x year P lt 0.01
similar response for Trisetum spicatum
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Community response ordination score
treatment x year P lt 0.05
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• Establishing a critical load from response data
• assume a dose response i.e. magnitude of change
  is related to treatment level
• 2) assume no other forcing factor is altering
  response variable (e.g. climate change)
• 3) set 0 level to ambient deposition rate (8
  kg/ha/yr)

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Empirical estimation of N critical load for plant
species responses in alpine dry meadows
N Critical load 4-12 Kg N/ ha/ yr
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Estimates of N critical loads in the
alpine Amount source basis (kg ha-1
yr-1) 4-12 this study vegetation change 4
Williams Tonnessen surface water
chemistry (2000) 1.5 Baron (2006) hindcasting
analysis 3-4 Baron et al. (1994) CENTURY
model (N leaching) 10-15 Bobbink et al.
(2002) vegetation change wet only
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Indications of ongoing vegetation response to N
deposition on Niwot Ridge

• Recensus of long-term plots (Marr plots- Korb
  Ranker)
• Analysis of LTER monitoring plots (Suding
  Bowman)

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Ecosystem (soil) responses
inorganic N loss to resin bags (15 cm depth)
during the growing season
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Soil solution NO3-- N (early season-prior to
fertilization)
note apparent higher critical load for N leaching
relative to vegetation response
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N cycling rates net N mineralization and
nitrification
b
b
b
ab
ab
ab
a
a
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Exchangeable Aluminum
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Summary Take-Home Messages

• N Critical load estimation possible using
  community/ population level approach (most
  probable in chronically N limited vegetation
  alpine, arctic, grassland, herbaceous
  understory) coupled experimental monitoring
  approach
• Sampling intensity and disturbance lower using
  plant species monitoring
• Responses by vegetation may precede more serious
  soil changes that may lead to greater
  environmental degredation (acidification)
• Changes in plant species composition may have a
  positive feedback on inorganic N leaching

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Research needed to establish N critical loads in
sensitive sites e.g. governed as class 1 areas of
Clean Air Acts e.g. similar empirical approach
will be used to establish N critical loads for
alpine vegetation in Rocky Mountain and Glacier
National Parks
Chapin Pass
Appistoki Valley