Temperature as a Potent Driver of Regional
Forest Drought Stress and Tree Mortality
Park Williams, Craig Allen, Alison Macalady, Daniel Griffin, Connie Woodhouse, Dave
Meko, Tom Swetnam, Sara Rauscher, Richard Seager, Henri Grissino-Mayer, Jeffrey Dean,
Edward Cook, Chandana Gangodagamage, Michael Cai, Nate McDowell
Published this week in Nature Climate Change
Southwest United States
NDVI
Average summer Normalized Difference
Vegetation Index (2000–2011; MODIS)
Goal: Figure out how much temperature
(relative to precipitation) influences
regional forest productivity and
mortality in the Southwestern USA
Last 117 Years of Climate in the
Southwest: PRISM data
5-Year Running Average
1895 – July 2012
Vapor Pressure Deficit (kPa)
Warming Drives Increasing Atmospheric
Moisture Demand
Tmax (°C)
Goal: Figure out how much temperature
vapor-pressure deficit (relative to
precipitation) influences regional forest
productivity and mortality in the
Southwestern USA
Goal: Figure out how much temperature
vapor-pressure deficit (relative to
precipitation) influences regional forest
productivity and mortality in the
Southwestern USA
335 Tree-Ring Sites
Regional Ring-Width Record
More Growth
Less Growth
Regional Ring-Width Record
3 Primary Tree Species
More Growth
Less Growth
Ring-Widths Vs. Climate
When do precipitation and vapor-pressure deficit dictate
regional tree growth?
Previous year
May-July
Aug.-Oct.
Growth year
Nov.-Jan.
Feb.-April
May-July
Aug.-Oct.
Cool season precipitation and warmseason vapor-pressure deficit dictate
regional tree growth.
Previous year
VPD
May-July
Aug.-Oct.
R2 = 0.82
Precip.
Nov.-Jan.
Growth year
VPD
Feb.-April
May-July
Aug.-Oct.
Forest Drought-Stress Index (FDSI)
FDSI = 0.44[zscore(cold-season Precip.)] –
0.56[zscore(warm-season VPD)]
Tree-ring record
Climate record
Forest Drought-Stress Index (FDSI)
FDSI = 0.44[zscore(cold-season Precip.)] –
0.56[zscore(warm-season VPD)]
Tree-ring record
Climate record
What else does FDSI
correspond to?
Summer NDVI
1981–2012
Summer NDVI
1981–2012
FIA % Dead
1997–2009
Summer NDVI
1981–2012
FIA % Dead
1997–2009
Area killed by
Bark Beetles
1997–2011
Summer NDVI
1981–2012
FIA % Dead
1997–2009
Area killed by
Bark Beetles
1997–2011
Area killed by
Wildfires
1984–2011
What are the implications
for the future?
What are the implications
for the future?
Extreme drought is a trademark of the
Southwest!
Extreme drought is a trademark of the
Southwest!
Late-1500s
Megadrought
1500s Megadrought is a “Drought-Stress Threshold,”
beyond which there is probably widespread forest
decline
Late-1500s
Megadrought
Most severe 50% of years during
1500s Megadrought
Climate Model Projections through 2100
Cold-season precipitation
5-15% decrease in coldseason precipitation by
2100
40% increase in warmseason VPD
Warm-season vaporpressure deficit
standard deviation anomaly
Climate Model Projections of Forest DroughtStress Index (FDSI) through 2100
Climate models project persistent “megadrought”
conditions by the 2050s
Forest Drought-Stress Index
FDSI
standard deviation anomaly
Most severe 50% of years during
1500s Megadrought
Frequency of Megadrought-Type Years
Extreme Events!
50-year running frequency of years when FDSI is
more severe than the worst years in the 1500s
megadrought
Tree-ring record
18%
Extreme Events!
50-Year Running Frequency of Megadrought-Type Years
Business-AsUsual A2
Scenario
80%
Tree-ring record
18%
Frequency of Megadrought-Type Years
Extreme Events!
50-Year Running Frequency of Megadrought-Type Years
A2
Scenario
80%
78%
A1B
Scenario
53%
B1
Scenario
Tree-ring record
18%
Conclusions
April – July Dew Point in New Mexico
& West Texas: 1895 – 2011
Williams et al. (to be submitted very soon)