Apparent climatically-induced increase of
mortality rates in temperate forests of the
Sierra Nevada
Phil van Mantgem
Nate Stephenson
USGS Sequoia and Kings Canyon Field Station
U.S. Department of the Interior
U.S. Geological Survey
The Western Mountain Initiative
The Upshot
Background mortality rates are increasing
in old growth forests of the Sierra Nevada.
Increasing mortality rates appear to be
related to increasing water stress.
A growing body of evidence suggests that environment
(particularly climate) affects forest demographic rates
Large-scale die-off
Credit: USFS
Background mortality
Credit: N. Stephenson
A growing body of evidence suggests that environment
(particularly climate) affects forest demographic rates
Large-scale die-off
Background mortality
(1) subtle, slow, neglected
(2) ... but important!
Credit: USFS
Tropical forests are changing
RAINFOR plot network
• Increasing
recruitment and mortality rates (Phillips et al. 2004)
• Increasing
stand biomass (Körner et al. 2004)
• Changes
in species composition (increasing liana densities)
(Phillips et al. 2002, Laurence et al. 2004)
Credit: O. Phillips
Credit: Y. Malhi
Could similar changes be occurring in temperate forests?
To find out, we used 21 long-term
old-growth forest plots:
-- Established 1982-1994
-- 0.9 to 2.5 ha (26 ha total)
-- 1900 m elevational gradient
-- Excludes “disturbed” plots
-- The fates of > 20,000 trees
(≥1.4 m tall) tracked annually
-- Causes of death recorded
Yosemite
transect
Sequoia
transect
Credit: USGS
No change in ingrowth rates in the Sierra Nevada
Annual ingrowth rate (%)
Thick black line, mean
3
2
1
0
1984
1988
1992
1996
Year
2000
2004
No change in ingrowth rates in the Sierra Nevada
3
y = 2 .7 6 - 0 .0 0 0 6 6 x
-1
Forest turnover (% yr )
Forest vs. seedling demographics
r 2 = 0 .4 9 , P < 0 .0 0 1
2
1
0
1500
2000
2500
3000
-1
Seedling turnover (% yr )
E le v a tio n ( m )
20
3500
Stephenson and
van Mantgem (2005)
y = 0 .1 5 9 - 0 .0 0 0 0 4 x
R 2 = 0 .2 1 , P = 0 .0 4 6
15
10
5
0
1500
2000
2500
3000
E le v a tio n (m )
3500
van Mantgem and
Stephenson (2006)
Mortality rates are increasing in the Sierra Nevada
Annual mortality rate (%)
Thick black line, mean
3
2
1
0
1984
1988
1992
1996
Year
2000
2004
Trends in stand biomass
Increased mortality from self-thinning?
• Biomass from allometric equations (Means et al. 1994, BIOPAK)
• No detectable biomass trend (P = 0.99)
-1
Live stem biomass (Mg ha )
• Increased mortality primarily in small trees (<20 cm DBH)
1500
1000
500
0
1984 1988 1992 1996 2000 2004
Year
Mortality rates are increasing in the Sierra Nevada
Annual mortality rate (%)
Abies increase = 3% yr-1, P = 0.0001
Pinus increase = 9% yr-1, P < 0.0001
Abies
Pinus
5
4
3
2
1
0
1984
1988
1992
Year
1996
2000
2004
Mortality rates are increasing in the Sierra Nevada
Pinus species
Credit: NPS
Forest structure and composition change profoundly
over elevation, controlled by the climatic water balance.
Credit: Tony Caprio
Mortality rates are increasing in the Sierra Nevada
Annual mortality rate (%)
Low (1500 - 2000 m) increase = 2% yr-1, P = 0.52, n = 4 plots
Mid (2000 - 2400 m) increase = 3% yr-1, P = 0.001, n = 8 plots
High (2400 - 2600 m) increase = 9% yr-1, P < 0.0001, n = 5 plots
Veryhigh (2600 – 3500 m) increase = -2% yr-1, P = 0.56, n = 4 plots
low
mid
high
veryhigh
3
2
1
0
1984
1988
1992
Year
1996
2000
2004
Mortality rates are increasing in the Sierra Nevada
but not uniformly!
Subalpine plots
a) No trend
b) Low sample size, n = 4
3
y = 2 .7 6 - 0 .0 0 0 6 6 x
-1
Forest turnover (% yr )
c) Trend hard to detect
2
r = 0 .4 9 , P < 0 .0 0 1
2
1
0
1500
2000
2500
3000
3500
E le v a tio n ( m )
Stephenson and
van Mantgem (2005)
The Upshot
Background mortality rates are increasing
in old growth forests of the Sierra Nevada.
Increasing mortality rates appear to be
related to increasing water stress.
Mortality rates are increasing in the Sierra Nevada
Annual mortality rate (%)
Thick black line, mean
Drought
3
2
1
0
1984
1988
1992
1996
Year
2000
2004
Changes for the Sierra Nevada
Temperature increases
March – April Temps
1950-1997
Cayan et al (2001)
Changes for the
Sierra Nevada
Maximum snow water content
Summers are getting
longer and drier.
Snowpack has been
decreasing over most of
the West in recent
decades …
Mote et al., 2005
… and spring
streamflow has been
arriving earlier.
Spring-pulse dates
Stewart et al., 2004
Photo credit: A. Caprio
Modeled climatic trends
12
8
Min. temperature = increasing
(P < 0.0001)
4
0
-4
16
Max. temperature = increasing
(P < 0.0001)
12
8
2000
1500
Precipitation = no trend
(P = 0.49)
1000
Water deficit (mm)
Precipitation
(mm)
Maximum
temperature (oC)
Minimum
o
temperature ( C)
Thick black line, mean
500
400
1984
1988
1992
1996
2000
2004
Annual water deficit = increasing
(P < 0.0001)
Year
300
200
100
0
1984
1988
1992
Year
1996
2000
2004
Water balance: integrating temperature and precipitation
mm Water
150
100
S
D
50
AET
0
1
J
F2
3
M
4
A
5
M
J6
7J
8
A
9
S
10
O 11
N 12
D
Month
Compared with traditional measures of climate, AET and D better represent
aspects of climate of direct physiological importance to plants (Stephenson 1990)
AET:
D:
Climatic potential for primary production
Drought stress, (PET – AET = D)
2.0
Average annual mortality rate
Water deficit (3 yr. running average)
280
260
240
1.5
220
1.0
200
180
0.5
160
0.0
1985
1990
1995
2000
Year
βyear = 0.02, s.e. = 0.01, P = 0.04
βD = 0.002, s.e. = 0.001, P = 0.03
2005
Water Deficit (mm)
Annual mortalityrate (%)
Water deficit partially explains mortality rate trend
Developing a mechanistic understanding
Stress
mortality
(standing dead:
insects, fungi,
or no symptoms)
Mechanical
mortality
(breaking,
uprooting, or
crushing)
Stress = 3356 deaths
Mechanical = 542 deaths
Stress
86%
Mechanical
14%
Related to water deficit
Related to storm intensity
βyear = 0.01, s.e. = 0.01, P = 0.21
βD = 0.004, s.e. = 0.001, P = 0.002
Average annual stress mortality rate
Water deficit (3 yr. running average)
2.0
280
260
240
1.5
220
1.0
200
180
0.5
Water deficit (mm)
Stress
mortality
Annual mortality rate (%)
Developing a mechanistic understanding
160
0.0
1984
1988
1992
1996
2000
2004
βyear = 0.03, s.e. = 0.02, P = 0.13
βmax.3.precip = 0.001
s.e. = 0.0002, P = 0.0004
Average annual mechanical mortality rate
Sum max 3 mo. precip.
1200
Precipitation (mm)
Mechanical
mortality
Annual mortality rate (%)
Year
1000
1.0
800
0.5
600
400
0.0
1984
1988
1992
Year
1996
2000
2004
The decline spiral model of tree death
recovery
healthy tree
drought
suppression
death
pitch
defense
dominance
recovery
bark
beetles
competition
Franklin et al. 1987
Mountain pine beetle increasing range in British Columbia
Carroll et al. 2003
The Upshot
Background mortality rates are increasing
in old growth forests of the Sierra Nevada.
Increasing mortality rates appear to be
related to increasing water stress.
Hemingway on long-term research
There are some things which cannot be
learned quickly, and time...must be paid
heavily for their acquiring.
-- Ernest Hemingway
Hemingway was a jerk.
-- Harold Robbins
Die-back event of piñon pine in the Southwest
October 2002
May 2004
Photo credit: C. Allen
Die-back event of piñon pine in the Southwest
Annual temperature (oC)
Annual precipitation (mm)
Die-back occurred in the context of increasing temperatures coupled with a
reduction in precipitation -leading to stronger drought response than in the past!
Breshears et al. 2005
Changes are occurring in conjunction with other stressors
Credit: Eric Knapp
Credit: USFS
Air pollution
Forest fragmentation
Credit: USFS
Altered disturbance regimes
Credit: Eric Knapp
Invasive species