Indirect effects of climate change:
Importance of plant species loss and increased N availability
Molly Smith Cross
John Harte
University of California, Berkeley
Climate
Productivity
Carbon Cycling
Nitrogen Cycling
Direct Effects
Climate
Indirect Effects
Productivity
Carbon Cycling
Nitrogen Cycling
Plant Species
Composition
Available
Nitrogen
Site Description
Rocky Mountain Biological Laboratory (RMBL), CO
Warming Experiment
Climate
Productivity
Carbon Cycling
Nitrogen Cycling
Plant Species
Composition
Available
Nitrogen
Warming Experiment
• Began in 1990
• Microclimate Responses to heating:
Increase in soil T by ~2˚C
Decrease in soil M by ~14%
Earlier snowmelt by ~2 weeks
Harte et al. (1995)
Experimental Heating Has Resulted In…
Forb Aboveground Biomass (AGB)
Shallow-Rooted Forb success
Tap-Rooted Forb AGB (No ∆, or )
Harte and Shaw (1995)
De Valpine and Harte (2001)
Saavedra et al. (2003)
Tap-Rooted Forb
Shallow-Rooted Forb
Helianthella quinquenervis
Linum lewisii
Experimental Heating Has Resulted In…
Forb Aboveground Biomass (AGB)
Shallow-Rooted Forb success
Tap-Rooted Forb AGB (No ∆, or )
Net Nitrogen Mineralization
Harte and Shaw (1995)
De Valpine and Harte (2001)
Saavedra et al. (2003)
Shaw and Harte (2001)
Plant Species Removal + N Addition Experiment
Climate
Productivity
Carbon Cycling
Nitrogen Cycling
Plant Species
Composition
Available
Nitrogen
Control
Shallow-Rooted Forb Removal
Removal
Treatments
Random Forb Removal
Nitrogen Addition Treatments
1) Ambient N
2) Addition of 6 gN/m2/yr
Ecosystem Response Variables
• Aboveground Plant Biomass
• By plant type -- tap-rooted and shallow-rooted forbs, grasses
• Soil N Cycling
• Net nitrogen mineralization and nitrification rates
• NO3- at 30 cm depth (using ion exchange resin bags)
Aboveground Plant Biomass (AGB)
450
After 3 years of removals…
Grasses
Shallow-Rooted Forbs
Tap-Rooted Forbs
375
g/m2
300
225
150
75
0
Control
Cross + Harte (in press) Ecology
Random Removal
Shallow Removal
Error bars = 1 s.e.
Aboveground Plant Biomass (AGB)
After 3 years of removals…
450
Control
Random Removal
Shallow Removal
400
* (p = 0.08)
g/m2
350
300
250
200
ambient N
+N
Error bars = 1 s.e.
2003 Shallow Soil Moisture (0-10cm)
40
Control
Random Removal
Shallow Removal
35
30
25
20
15
10
5
5/11/03
5/21/03
5/31/03
6/10/03
6/20/03
6/30/03
7/10/03
Date
7/20/03
7/30/03
8/9/03
8/19/03
8/29/03
Net N Mineralization and Nitrification
2002
2003
ugN/g soil/day
1.6
1.2
a
0.8
0.4
0
b
ab
control random shallow
removal removal
1.6
ugN/g soil/day
Net
Mineralization
(N-MIN)
control random shallow
removal removal
1.2
0.8
b
ab
Net
Nitrification
(N-NITR)
a
0.4
0
control random shallow
removal removal
control random shallow
removal removal
Error bars = 1 s.e.
Nitrate (NO3-) at 30 cm
2003
3
Control
Random Removal
Shallow Removal
ln ((mgN/resin bag)+1)
2.5
2
1.5
1
0.5
0
ambient N
+N
Error bars = 1 s.e.
Summary and Conclusions
Loss of shallow-rooted forbs =
•
Increase in tap-rooted forb and grass growth;
•
No change in N-MIN, N-NITR, and NO3- at 30 cm beyond
what we saw with the removal of random biomass.
→ Tap-rooted forbs and grasses fully compensated for the loss of
shallow-rooted forbs for these ecosystem processes.
Summary and Conclusions
Addition of nitrogen =
•
No main effect of N addition on total aboveground biomass.
•
No change in N-MIN, N-NITR; but an increase in NO3- at
30 cm.
→ Maybe water limitation more important than N limitation
during experiment?
Summary and Conclusions
Loss of shallow-rooted forbs plus N addition =
•
Enhanced biomass response to nitrogen addition relative to
more species-rich Control and Random Removal.
→ The loss of shallow-rooted forbs seems to have made the
system more sensitive to a perturbation in N availability.
→ Maybe due to changes in water limitation and movement of
nitrogen down through soil profile?
Summary and Conclusions
•
Interactions between multiple environmental changes (i.e.,
species loss and increased N availability), and multiple limiting
resources (i.e., soil water and N), may be complex.
•
Information about indirect effects of warming should be
incorporated into ecosystem models of climate change impacts.
Many thanks to:
NASA, DOE, NSF
the Harte Lab
RMBL Staff
D. Bushey
A. Chau
H. Cooley
E. Conlisk
L. Cushing
C. DiVittorio
Z. German
L. Goldstein
M. Grant
L. Jin
N. Kraft
K. McClure
C. Pederson
S. Carey
T. Perfors
J. Preist
A. Smith
C. Stumpf
K. Trinh
N. Tsui
M. Wang