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: N Critical Loads: Does one size fit all?Does one size fit all?

NADP-NTNtrends in inorganic N

concentrationNiwot Ridge, CO

1980 1985 1990 1995 2000 20050

5

10

15

20

25

NO3-

NH4+

P < 0.01

P < 0.05

total annual deposition ca. 8 kg N/ haYear

eq/

L

episodic acidification= loss of acid neutralizing episodic acidification= loss of acid neutralizing capacity and elevated [NOcapacity and elevated [NO33

--] in upper Green Lakes ] in upper Green Lakes Valley (Nel Caine & Mark Williams)Valley (Nel Caine & Mark Williams)

changes in diatom composition (lake cores) changes in diatom composition (lake cores) (Jasmine Saros, Alex Wolfe and Jill Baron)(Jasmine Saros, Alex Wolfe and Jill Baron)

needle and forest floor chemistry in old-growth needle and forest floor chemistry in old-growth subalpine forests (East-West slope comparison) subalpine forests (East-West slope comparison) (Heather Rueth and Jill Baron)(Heather Rueth and Jill Baron)

changes in alpine plant species composition in long-changes in alpine plant species composition in long-term monitoring plotsterm monitoring plots

Indicators of Ecosystem Response to Elevated N InputsIndicators of Ecosystem Response to Elevated N Inputs:

Paradox of simultaneous N limitation & N excessParadox of simultaneous N limitation & N excessExperimental N additions in alpine result in greater plant growth, yet growing season export of NO3

- is occurring (?)

Adaptation to low soil nutrient supply- some Adaptation to low soil nutrient supply- some species don’t respond to increased N species don’t respond to increased N availabilityavailability responsiveness to N supply

0

50

100

150

200

Species

Per

cent c

han

ge in

bio

mas

s(h

igh to

low

N)

Paradox provides an opportunity: changes in Paradox provides an opportunity: changes in species composition indicative of N inputsspecies composition indicative of N inputs

Alternative view: how much N input does it take to Alternative view: how much N input does it take to produce a change in species composition? (= N produce a change in species composition? (= N

critical load using biotic response)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

0

10

20

30

40

50

0246

N added:

Car

ex r

upes

tris

pro

ject

ed c

over

(%

)species composition response:species composition response:

Carex rupestrisCarex rupestris

similar response for similar response for Trisetum spicatumTrisetum spicatum

treatment x year P < 0.01treatment x year P < 0.01

Community response: ordination scoreCommunity response: ordination score

treatment x year P < 0.05treatment x year P < 0.05

1996 1998 2000 2002 20040

20

40

60

80

100

120N added:

6420

Year

DC

A 1

Establishing a critical load from response data:Establishing a critical load from response data:

1) assume a dose response i.e. magnitude of assume a dose response i.e. magnitude of change is related to treatment levelchange is related to treatment level

2) assume no other forcing factor is altering assume no other forcing factor is altering response variable (e.g. climate change)response variable (e.g. climate change)

3) set “0” level to ambient deposition rate (8 set “0” level to ambient deposition rate (8 kg/ha/yr)kg/ha/yr)

-1 0 1 2 3 4 5 6 7-7.5

-5.0

-2.5

0.0

N input (g N m-2 yr-1)

Chan

ge

in D

CA

1 sc

ore

(val

ue

/yea

r)

N CriticalN Critical load: load: 4-12 Kg N/ ha/ 4-12 Kg N/ ha/ yryr

Empirical estimation of N critical load for plant Empirical estimation of N critical load for plant species responses in alpine dry meadowsspecies responses in alpine dry meadows

Estimates of N critical loads in the alpineEstimates of N critical loads in the alpine::Amount:Amount: source:source: basis:basis:(kg ha(kg ha-1-1 yr yr-1-1))

4-12 4-12 this studythis study vegetation changevegetation change

4 *4 * Williams & TonnessenWilliams & Tonnessen surface water chemistrysurface water chemistry(2000)(2000)

1.51.5 Baron (2006)Baron (2006) hindcasting analysishindcasting analysis

3-4 3-4 Baron et al. (1994)Baron et al. (1994) CENTURY model (N CENTURY model (N leaching)leaching)

10-15 10-15 BobbinkBobbink et al. (2002)et al. (2002) vegetation changevegetation change

*wet only*wet only

Indications of ongoing vegetation response to Indications of ongoing vegetation response to N deposition on Niwot RidgeN deposition on Niwot Ridge

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

dry meadow all communities

-0.5

0.0

0.5

1.0

1.5

decreasersnon-respondersincreasers**

**

species responses to fertilization:

ln c

over

cha

nge

of s

peci

es in

long

term

mon

itori

ng p

lots

Comparison of species in Saddle permanent plots with fertilization plots

resin bags (2001)

0 +2 +4 +60

1

2

3

Treatment (N addition- g/ m2/ yr)

NO3--N (P<0.05)

NH4+-N (n.s.)

mg

N b

ag-1

inorganic N loss to resin bags (15 cm depth) inorganic N loss to resin bags (15 cm depth) during the growing seasonduring the growing season

Ecosystem (soil) responses:Ecosystem (soil) responses:

soil solution lysimeters

0 2 4 6 80.01

0.1

1

101999 (3rd year of experiment)

2003 (7th year)

N added (g m-2 yr-1)

soil s

olution

NO

3- -N

(mg

N/ L

)

Soil solution NOSoil solution NO33--- N - N

(early season-prior to fertilization(early season-prior to fertilization))

note apparent note apparent higher critical load higher critical load for N leaching for N leaching relative to relative to vegetation responsevegetation response

0 2 4 60.0

0.5

1.0

1.52003

2005

N addition (g N m-2 yr-1)

N m

iner

aliz

atio

n ra

te

(mg

N K

g-1

soi

l day

-1)

0 2 4 60.0

0.5

1.0

1.5

2.0

2.52005

2003

N addition (g N m-2 yr-1)

N n

itrifi

catio

n ra

te

(mg

N K

g-1

soi

l day

-1)

(from Aber et al. 1998)

N cycling rates:N cycling rates:net N mineralization and net N mineralization and

nitrificationnitrification

a

b

ab

bb

a

ab ab

Exchangeable AluminumExchangeable Aluminum

Summary: Take-Home MessagesSummary: Take-Home Messages

N N Critical load estimation possible using community/ Critical load estimation possible using community/ population level approach (most probable in chronically N population level approach (most probable in chronically N limited vegetation: alpine, arctic, grassland, herbaceous limited vegetation: alpine, arctic, grassland, herbaceous understory); coupled experimental – monitoring approachunderstory); coupled experimental – monitoring approach

Sampling intensity and disturbance lower using plant Sampling intensity and disturbance lower using plant species monitoringspecies monitoring

Responses by vegetation may precede more serious soil Responses by vegetation may precede more serious soil changes that may lead to greater environmental changes that may lead to greater environmental degredation (acidification)degredation (acidification)

Changes in plant species composition may have a positive Changes in plant species composition may have a positive feedback on inorganic N leachingfeedback on inorganic N leaching

Research needed to establish N critical loads in Research needed to establish N critical loads in sensitive sites e.g. governed as class 1 areas of Clean sensitive sites e.g. governed as class 1 areas of Clean

Air ActsAir Acts

e.g. similar empirical approach will be used to establish e.g. similar empirical approach will be used to establish N critical loads for alpine vegetation in Rocky Mountain N critical loads for alpine vegetation in Rocky Mountain

and Glacier National Parksand Glacier National Parks

Chapin PassChapin Pass Appistoki ValleyAppistoki Valley