*1A.
LA-UR-11-12187
Park Williams, 2Dave Meko, 2Connie Woodhouse, 3Craig D. Allen, 2Tom Swetnam, 2Alison Macalady, 2Daniel Griffin, 1Sara Rauscher, 1Xiaoyan Jiang, 4Ed Cook, 5Henri Grissino-Mayer, 1Nate McDowell, 1Michael Cai
* williams@geog.ucsb.edu; 1Los Alamos National Laboratory, NM; 2University of Arizona, Tucson; 3USGS, Bandelier National Monument, NM; 4Lamont-Doherty Earth Observatory, Columbia University, NY; 5University of Tennessee, Knoxville
Abstract
We derive an annual forest-drought-stress index (FDSI) for 1000–2008 AD using 330 tree-ring records from North American Southwest (NASW) forests. Warm-season maximum daily temperature and cold-season precipitation correspond with
81% of FDSI variability during 1910–2008 and influence FDSI nearly equally, but in opposite directions. FDSI during the 2000s and 1950s was the most severe in 300 years. FDSI in 2002 was the most severe since 1000 AD, corresponding with extensive
wildfires and bark-beetle outbreaks. Among the 3 primary conifer species in the NASW, the number of dead trees approximately doubled during 2001–2006. Despite the 2000s and 1950s droughts, 1900–2008 FDSI was well above the 1000–1899 AD average
and the frequency of severe FDSI years during 1900–2008 was unexceptional. According to an ensemble of climate models, severe drought years will occur approximately 50 times per century by 2100, far beyond the range of variability of the driest centuries
in the past millennium. Strong correspondence between FDSI and probabilities of extensive wildfires and bark-beetle outbreaks suggests the frequency of NASW forest mortality events will increase beyond last millennium’s range of variability by 2100.
Past
Present
Tree-ring records from 330 piñon pine, ponderosa pine, and
Douglas fir sites
Future
Modeled future climate is used to forecast forest-drought
stress
2000s drought and forest mortality
Forest-Drought-Stress Index (FDSI)
No Drought
Drought
No Drought
Drought
Drought
Recent mortality
due to bark beetles and wildfire
Forest-Drought-Stress Index (FDSI) vs. Climate
FDSI " 0.44[zscore(ln(PPTcold #season ))] # 0.56[zscore(Tmax warm #season )]
Tree-ring record
1,100 year record of Forest-Drought-Stress Index
Model climate
record
cold-season: November – March
warm-season: May – July & previous August – October
Correlation: estimated vs. actual FDSI (r)
High confidence that warm-season Tmax contributed
more than cold-season PPT to FDSI during 1910–2008
1910–1958
1910–2008
95%
confidence
1959–2008
Climate-based FDSI
Forest-Drought-Stress Index (FDSI)
Tree-ring-based FDSI
Piñon pines killed by bark beetles & drought stress during the early-2000s drought at
Walnut Canyon National Monument, Arizona. Photo by Craig D. Allen, 4 June 2005.
Burned area classified as “moderately or severely” burned by the Monitoring Trends
in Burn Severity Project (mtbs.gov) during 1984–2008 or as burnt by the MODIS
burned-area product during 2009–2011 (modis-fire.umd.edu/BA_getdata.html).
Beetle areas were classified as areas where ≥ 10 trees/acre were killed by bark beetles
according to the Forest Health Technology Enterprise Team (fs.fed.us/foresthealth/
technology).
# of years per century when drought stress
exceeds that of the 1950s and 2000s droughts
Annual burned area corresponds with ForestDrought-Severity Index (FDSI).
4-year smooth
Burned area
Annual area where > 10 trees/acre are killed by
bark beetles corresponds with multi-year trends
in FDSI.
Forest-Drought-Stress
Index
FDSI vs Southwestern FireScar Data (1650–1899 AD)
FDSI vs Southwestern seedling
recruitment (1600–1980 AD)
39-yr running ForestDrought-Stress Index
95% & 99%
significance
No Drought
Bark-beetle area
Model CMIP3 climate data come from ftp-esg.ucllnl.org.
Conclusions
Satellite-derived measurements of regional
surface greenness (NDVI) correspond with
annual variability in FDSI. (glcf.umd.edu/data/
Vegetation
Greenness
gimms)
Drought
Drought doesn’t only impact when mortality occurs, it impacts where mortality occurs
Expected
Probability
Fire scars within cross-sections of trees tell us when & where wildfires occurred in the
past. Fire-scar data come from 281 sites within Tom Swetnam’s (& others) southwestern
fire-scar network. A ‘big fire year’ is defined as a top 10% fire year: the proportion of
sites recording fire scars during a big fire year exceeds that of 90% of years during
1650–1899 AD.
Tree-age data from 12 sites in the Southwest were used to derive a regional ponderosa
pine recruitment index, which can be used to see that seedling recruitment occurs
during wet and/or cool periods (high FDSI).
Swetnam TW and Brown PM (2011) Climatic inferences from dendroecological reconstructions. In:
Hughes MK, Swetnam TW, Diaz H (eds) Dendroclimatology: Progress and Prospects, vol 11.
Springer, New York, pp 263–295.
Fraction of forest area
where ≥ 10 trees per acre
were killed by bark beetles
during 1997–2010
Long-term mean FDSI
Fraction of forest area
Index of ponderosa pine
recruitment at 12 Southwestern sites
(Swetnam & Brown 2011)
Fraction of forest area
# of lag years
Forest-Drought-Stress Index (FDSI)
4-year smoothed Forest-Drought-Stress Index
# of dead trees approximately doubled during
the 2000s drought, according to annual Forest
Inventory & Analysis Data. (fia.fs.fed.us/tools-data)
% contribution of warm-season Tmax
Probability of a
big fire year
!
1,100 years of past, present, and future forest response to drought in
the North American Southwest
Fraction of forest area moderately – severely
burned by wildfire during 1984–2011
•  Warm-season maximum temperature (Tmax) and cold-season precipitation account for 81% of variability in a
regional record of tree-ring widths in the North American Southwest, making ring widths an excellent record
of forest-drought stress.
•  Warm-season Tmax is at least as important as cold-season precipitation in dictating forest-drought stress.
•  Southwestern forest-drought stress has been exceptionally variable during the past millennium, with many
droughts stronger and longer than those of the 1950s & 2000s.
•  The 2000s drought was associated with a doubling of the number of dead piñon pine, ponderosa pine, and
Douglas fir trees in the Southwest due to a combination of bark beetles, wildfire, and drought stress.
•  Considering climate-model projections of warm-season Tmax and cold-season precipitation, we predict forestdrought stress will match or exceed that of the 2000s drought during the majority of years by 2100 AD.
•  Such a substantial and sustained increase in forest-drought stress would lead to massive forest die-off during
the 21st century in the North American Southwest.
Acknowledgments This project was supported by grants from LANL-LDRD and DOE-BER. Observational climate
Long-term mean FDSI
data come from the PRISM Climate Group, Oregon State University (prism.oregonstate.edu). Most tree-ring data come from the International TreeRing Databank (ncdc.noaa.gov/paleo/treering.html). Some tree-ring data are unpublished. Thanks to all contributors of tree-ring data.