The Niwot Ridge Mountain Biosphere Reserve:

Tipping points in high-elevation ecosystems

in response to changes in climate and

atmospheric deposition

Mark Williams, University of Colorado

High-elevation areas are important

bellwethers of global change:

we need long-term research

Glaciated

valley SADDLE

External Drivers: Temperature

Increasing air temperature since early 1980’s

Summer air temps warming fastest

Earlier lake ice-out dates

5ºC increase in 25 years

External Drivers: Precipitation

Greater precipitation with increasing elevation

Increases in the winter months (more snow)

Summer drought starting in 2000

External Drivers: N deposition

• Increased rates of N deposition (wetfall)

• N loading increases, despite drought

ROCK

GLACIER

ARIKAREE

GLACIER

D1 CLIMATE

GREEN

LAKE 4

3,600 m

Earlier Snowmelt 2-6 days decade-1

Precipitation Effects on Streamflow

GL4

MART

225-ha

8-ha

Wet Year vs. Dry Year

-Increasing subsurface

pathways.

-- Snowmelt peak is not

snow – old, reacted water

(talus + baseflow) is

pushed out by infiltrating

snow.

-- Source of baseflow

changes and volumes are

not significantly different.

Arikaree glacier is dying

Drought

Tipping point

Arikaree Glacier: Mass balance (Bn), cm water equivalent.

N dep + warming T = N saturation

Annual VWM concentrations of nitrate increase at all stream sites

N Critical load: Aquatic 4 Kg N/ ha/ yr

Stoichiometric controls on N-

cycling

Scatterplot of NO3- vs. DOC: NO3- ratio

for eight sites in Green Lakes Valley.

Saddle site

how much N input does it take to produce a change in

species composition? (= N critical load using biotic response)

Addressed experimentally in species rich dry meadow, using additions of 2, 4, 6 g N/m2/yr

19961998

20002002

20042006

2008

0

10

20

30

40

50

0

2

4

6

N added:

Car

ex r

upe

stri

s

pro

ject

ed c

ov

er (

%)

species composition response:

Carex rupestris

treatment x year P < 0.01

Treatment specific rate of change in cover

similar response for Trisetum spicatum

-1 0 1 2 3 4 5 6 70

1

2

3

4

N input (g N m-2

yr-1

)

chan

ge in

Car

ex c

over

(% p

roje

cted

cov

er/

yr)

N Critical load: 10-40 Kg N/ ha/ yr

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

-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

)

Ch

ange

in D

CA

1

sco

re (

valu

e /

year

)

Whole community response

Woody Willow Encroachment

Willow Encroachment Experiment

Factorial manipulation of N (fertilizer), snow (snowfence), summer temperature (open-topped chambers), in all possible combinations

+ Addition of Salix glauca seedlings

Shrub growth benefits fromincreased N, snow, and temperature

Salix

gro

wth

in h

eig

ht

after

2 y

ears

(cm

)

Feedbacks to ecosystem processes

Ne

t N

min

era

liza

tio

n(m

g N

/g s

oil

/30

da

ys)

Absent Present

Willows

Willow encroachmentincreases litter, N availability,and snow depth.

These effects may accelerateencroachment

Summary

• Disappearing glaciers

• Snowmelt occurring earlier

• Drought years little discharge because of groundwater deficit

• Increasing N deposition increases nitrate in streamflow

• Critical loads for terrestrial veg 10 kg/ha/yr

• Shrub invasion of alpine

QUESTIONS?