Mechanisms Controlling the Effects of Weather
and Climate on California's Ecosystems
•Dramatic ~2003 tree mortality in SW US
•Followed extraordinarily dry period –
triggered by drought and bark beetles
•Possibly amplified by warming
•Possibly an early climate change impact
Sierra Nevada CZO: A. Kelly, G. Winston,
M. Meadows, R. Bales
Transect
Surveys
Eddy
Covariance
San Jacinto
Transect:
A. Fellows
Deep Canyon
Transect:
A. Kelly
Drier
Warmer
Wetter
Colder
Sierra Nevada CZO:
Winter 2009-10
Midmontane forest
Summer
Winter
Net CO2 on warm days
(µmol m-2 s-1)
Net CO2 on warm days
(µmol m-2 s-1)
Winter
Subalpine forest
Summer
Winter
cold
limited
Year-round growing season
Oak/Pine Savanna
Net CO2 on warm days
(µmol m-2 s-1)
Winter
Summer
Summer
drought
limited
•GPP at upper elevations
limited by winter cold
•GPP at mid-elevations
limited by neither drought
nor cold in a typical year
•GPP at lower elevations
limited by drought
San Jacinto Transect:
• Surveyed live and dead trees on
the west (wet) side of San
Jacinto/Santa Rosa Mountains in
2007-08
• Inferred 2002 composition
Mt. San Jacinto 3,302m
Highest transect
3,002 m
Lowest transect 1,295 m
Expansion
Mortality
Deep Canyon
Transect
Normalized montane tree cover
0.8
Live 2008
0.6
0.5
Mortality
0.4
0.3
0.2
0.1
0
-650
San Jacinto Transect:
• Surveyed live and dead trees
on west (wet) side
• Mortality greatest down low
• Approach not sensitive to
expansion
• No evidence of high seedling
establishment/ range
expansion
0.7
Live 2002
-450
San Jacinto
Transect
-250
-50
150
Elevation difference from center (m)
350
Deep Canyon Transect:
• Surveyed live shrubs and
trees on east (dry) side of
San Jacinto/Santa Rosa
Mountains; Compared to
1970s observations by Jan
Zabriskie
• Mortality greatest down low
• Expansion greatest up high
(suspect mostly established
plants getting bigger)
550
Average cover weighted elevation
Deep Canyon Transect
Desert
shrubs
Midmontane
conifers
1977
Elevation
2006-07
Elevation
Difference
(m)
965 m
1034 m
68 m
2216 m
2273 m
57 m
San Jacinto Transect
2002
Elevation
Midmontane
conifers
Subalpine
conifers
2007-08
Elevation
Difference
(m)
1999 m
2032 m
34 m
2872 m
2870 m
-2 m
Both Transects:
• Very rapid net upslope
movement of species
distribution – as much as
60 m for midmontane
conifers
• Roughly half (30-m) of
upslope movement caused
by mortality in lower parts
of ranges (both
approaches sensitive to
this mechanism)
• Other half (30-m) caused
by expansion in upper
parts of ranges (only Deep
Cyn approach sensitive to
this mechanism)
• No change in distribution
of subalpine conifers
Fraction of trees that died
0.6
0.5
0.4
0.3
0.2
Midmontane
conifer (mid
elevation, long
growing season,
possibly drought
limited in dry
years)
Subalpine conifer
(high elevation,
winter cold
limited)
0.1
0
-650
-150
350
Elevation difference from center (m)
San Jacinto Transect:
• Progressively greater
mortality at lower
elevation in
midmontane
• Little or no mortality in
subalpine
• Consistent with flux
tower observations
• Lower montane
strongly limited by
drought in dry years
• No obvious effect of
drought on lower
subalpine, even in dry
period
Historical context
•Los Angeles Times archives
•September 30, 1903
- LA Times Jul 27, 1925
- LA Times Sept 30, 1903
“Beetles Lay Waste to Forest;
Bugs' attack on droughtweakened pines may strip Lake
Arrowhead, Idyllwild of alpine
look”
- LA Times Feb 5, 2003
- LA Times Oct 31, 1960
Historical context
•Cyclical patterns
of LA precipitation
(mm/yr); especially
obvious in 5-year
running mean
•Consistent reports
of forest mortality
during dry phases
•Cautious
interpretation is we
are seeing
something that has
happened many
times before
Conclusions
• Primary Production at lower elevations limited by (and
susceptible to) drought; upper elevations limited by winter cold.
Should expect differential effects of climate change.
• Species distributions change very rapidly with mortality and
expansion in extreme parts of ranges – establishment in new
locations is slow. Climate change impacts mediated by further
amplifying a natural and powerful mechanism. This mechanism
carries implications for loss of local diversity and vulnerability to
more extreme climate change.
• The early 2000s mortality may be (1) a natural cycle of species’
range expansion and contraction associated with climate
variability (LA Times), or it may be (2) an early impact of global
climate change – it’s too early to tell.
• The early impacts of climate change and the natural cycles of
species’ range expansion and contraction may look exactly the
same – an interesting and tricky attribution issue.