Transcript Slide 1

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
N Critical Loads:
Does one size fit all?
NADP-NTN
trends in inorganic N
concentration
Niwot Ridge, CO
25
 eq/L
20
P < 0.01
NO3
-
15
NH4
P < 0.05
10
5
0
1980
1985
1990
1995
2000
2005
Year
total annual deposition ca. 8 kg N/ha
+
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
responsiveness to N supply
200
150
100
50
Species
m
tu
Tr
ise
D
es
ch
am
ps
ia
ro
st
is
is
ru
pe
st
r
C.
Ca
la
m
ag
Ca
re
x
sc
op
u
lo
r
um
sia
0
Ko
br
e
Percent change in biomass
(high to low N)
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 don’t respond to increased N availability
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)
Addressed experimentally in alpine
(species rich dry meadow), using
additions of 2, 4, 6 g N/m2/yr
response variables:
species composition
soil solution chemistry
N leaching (resin bags)
biomass production
soil N transformation rates
soil cation chemistry
species composition response:
Carex rupestris
projected cover (%)
50
treatment x year P < 0.01
Carex rupestris
40
N added:
0
2
4
6
30
20
10
06
20
04
20
01
20
99
19
19
96
0
similar response for Trisetum spicatum
Community response: ordination score
120
treatment x year P < 0.05
N added:
0
2
4
6
100
DCA 1
80
60
40
20
0
1996
1998
2000
Year
2002
2004
Establishing a critical load from response data:
1) 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)
Change in DCA1 scorechange in Carex cover
(value /year)
(% projected cover/yr)
Empirical estimation of N critical load for plant
species
responses in alpine dry meadows
4
3
2
1
0.0
0
-1
0
1
2
3
4
N input (g N m
-2
5
6
-1
yr )
7
N Critical load:
4-12 Kg N/ ha/ yr
-2.5
-5.0
-7.5
-1
0
1
2
3
4
-2
-1
5
N input (g N m yr )
6
7
Estimates of N critical loads in the alpine:
Amount:
source:
basis:
4-12
this study
vegetation change
4*
Williams & Tonnessen
(2000)
surface water chemistry
1.5
Baron (2006)
hindcasting analysis
3-4
Baron et al. (1994)
CENTURY model (N leaching)
10-15
Bobbink et al. (2002)
vegetation change
(kg ha-1 yr-1)
*wet only
Indications of ongoing vegetation response to N
deposition on Niwot Ridge
ln cover change of species in
long term monitoring plots
Recensus of long-term plots (Marr plots- Korb & Ranker)
Analysis of LTER monitoring plots (Suding & Bowman):
1.5
1.0
Comparison of species in Saddle permanent
plots with fertilization plots
species responses to fertilization:
**
decreasers
non-responders
increasers
**
0.5
0.0
-0.5
dry meadow
all communities
Ecosystem (soil) responses:
inorganic N loss to resin bags (15 cm depth) during the
growing season
resin bags (2001)
mg N bag
-1
3
-
NO3 -N (P<0.05)
+
NH4 -N (n.s.)
2
1
0
0
+2
+4
+6
2
Treatment (N addition- g/m /yr)
Soil solution NO3-- N
(early season-prior to fertilization)
soil solution lysimeters
2003 (7th year)
(mg N/ L)
-
soil solution NO 3 -N
10
1999 (3rd year of experiment)
1
note apparent higher
critical load for N
leaching relative to
vegetation response
0.1
0.01
0
2
4
6
-2
-1
N added (g m yr )
8
N cycling rates:
net N mineralization and
nitrification
(from Aber et al. 1998)
2003
b
1.0
-1
a
0.5
0.0
0
2
4
6
-2
-1
N addition (g N m yr )
-1
b
2.0
ab
1.5
-1
ab
2003
2005
2.5
(mg N Kg soil day )
b
N nitrification rate
2005
-1
(mg N Kg soil day )
N mineralization rate
1.5
1.0
ab
a
0.5
0.0
0
2
4
6
-2
-1
N addition (g N m yr )
Exchangeable Aluminum
Aluminum- Niwot Ridge
P = 0.08
60
[Al 3+] mg/g
50
40
30
20
10
0
0
2
4
N addition
6
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
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