Patterns of Drought Induced Tree Mortality in PinyonJuniper Woodlands

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Patterns of Drought Induced Tree Mortality in
Pinyon-Juniper Woodlands

• Michael J. Clifford, Neil S. Cobb,
• and Paulette Ford

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Key Issues

• Expansion of pinyon-juniper (Pinus edulis -
  Juniperus spp.) woodlands since EuroAmerican
  settlement (York et al. 1994, Brockway et al.
  2002)
• Severe drought of 2002 caused major tree die-off
  in pinyon-juniper woodlands (Breshears et al.
  2005, Shaw et al. 2005)
• Role of climate in restructuring or resetting
  woodlands

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Questions Addressed

• How has drought altered stand structure in
  pinyon-juniper woodlands?
• What are the differences in pinyon and juniper
  mortality?
• Has recent mortality returned pinyon-juniper
  woodlands to historical levels?

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• Drought in the Southwest
• Persistent drought since 1996
• Severe/extreme drought in 1996 and 2002
• Major bark beetle outbreak in 2002 and 2003

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Photo by N.S. Cobb (Newman et al. 2006)
Photo by Craig Allen
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Stand Structure Impacts

• How has drought altered stand structure in
  pinyon-juniper woodlands?
• Examine drought mortality on pinyon and juniper
  populations
• Differences between tree size
• Use basal trunk diameter as a proxy for age

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• Northern Arizona Study Area
• 53 transects
• Sites established from 1998-2001
• Study area 528 km2

San Francisco Peaks
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Mortality of pinyon and juniper age classes

• More mortality among smaller sized junipers
• Average juniper mortality 10
• Average size of dead junipers was
• 11.5 cm basal diameter

• Trend of larger pinyons
• Average pinyon mortality 48
• Average size of dead pinyons was
• 7.9 cm basal diameter

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Temporal Dynamics of Pinyon Juniper Mortality
Year
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Spatial Patterns of Tree Mortality
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Biotic Abiotic Factors of Mortality Show
Similar Patterns as Abiotic Factors
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Key Points

• Older pinyons and younger junipers were impacted
  most
• Similar response of mortality to drought from
  pinyons and junipers, but at different levels.
• Biotic mechanism (i.e., bark beetles) magnify
  drought mortality in pinyons

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What Does Tree Die-off Mean to Historic P-J
Expansion?

• Determine cover from 1883 government land office
  (GLO) map
• Classified canopy cover on aerial photos and
  satellite images from 1936, 1997, and 2004

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Pinyon-Juniper Woodland Expansion Through Time
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1883 Government Land Office Map

• Convert cords/acre to canopy cover
• Variation based on original survey data
• Average canopy cover 6.66 1.81

5-10 cords/acre (15 of area)
10-20 cords/acre (85 of study area)
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• From 1883-1997 (114 years) increase of 7.4
  canopy cover
• From 1997-2004 (7 years) loss of 7.8 canopy
  cover
• From 1883-2004 (121 years) loss of 0.26 canopy
  cover

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Temporal Canopy Cover Dynamics
Error bar is not standard error, but range of
potential canopy covers
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Conclusions

• Reduced populations of pinyons and junipers
• Trend of older pinyons and younger junipers
• Biotic factors (i.e., bark beetles) of mortality
  magnify climatic stressors
• Climatic events can quickly restructure woodlands
  to pre-EuroAmerican levels
• Global climate change-type drought can suddenly
  alter vegetation patterns

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Acknowledgements

• Jessica Vespi, Danielson Barbone, Jacob Higgins,
  and Robert Delph
• All field workers that surveyed the transects
  since 1998

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Interpolations of Early Tree Mortality
Pinyon mortality prior 2002
Keep???
Juniper mortality prior 2002
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Average Mortality of Size Classes for Pinyons
and Junipers
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Temporal Canopy Cover Dynamics
P-J Expansion
Drought Mortality
Percent canopy cover
1936
1997
2004
Year
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Distribution of Pinyon and Juniper Mortality