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P-J is not just one vegetation type: key variation in structure and disturbance dynamics
Bill Romme, Colorado State University PJ symposium, Albuquerque, Oct. 12, 2016
(Photo by Dan Binkley)
Workshop in Boulder, CO August 2006
• Academic and agency researchers from throughout the West
– Experience from throughout the 100 million acres of PJ vegetation in the West
• Objectives:
– Summarize the science -- what we know with high / moderate / low confidence
– Assemble & weigh the evidence
– Interpret both pattern & process
Participants & regions of experience:
• New Mexico … Craig Allen, Brandon Bestelmeyer, Esteban Muldavin
• Arizona … John Bailey, Dave Huffman, Tom Swetnam
• Colorado … Bill Baker, Karen Eisenhart, Lisa Floyd-Hanna, Bill Romme
• Utah … Peter Brown
• Nevada … Robin Tausch, Peter Weisberg
• Oregon … Rick Miller
Major Findings
• Geographic variation … critical! – Dominant species
– Climate (temperature & timing of precipitation)
– Historical stand & landscape structures
– Historical fire regimes
– Patterns & mechanisms of 20th century change
• But some patterns & processes apply to all
Some sources of information
• Forest Restoration Institutes – New Mexico (Highlands U.) … Colorado (CSU)
… Arizona (NAU) – clearinghouses of information … workshops,
meetings & field trips – focused research to address the most pressing
questions
• “Historical and Modern Disturbance Regimes of Piñon-Juniper Vegetation in the Western U.S.” 2007. – A short version (published by TNC) is available
online as pdf (CFRI or TNC websites) – And a longer version, with full literature
review & citations, is available online as pdf, with paper copies available soon (published by CFRI)
https://sites.warnercnr.colostate.edu/coloradoforestrestorationinstitute/wpcontent/uploads/sites/32/2016/05/2007_PJDisturbanceRegimes_Synthesis.pdf
https://sites.warnercnr.colostate.edu/coloradoforestrestorationinstitute/publications/
Proportion of Total Annual Precipitation that Falls During the Growing Season
(Related to the Summer Monsoon) … and Distribution of Piñon-Juniper Woodlands (shown in black)
… figure prepared by Brian Jacobs, Bandelier NM & CSU
WET SUMMERS
DRY SUMMERS
Figure by
Brian Jacobs
Three pinyons … in three different climate zones
Figure by
Brian Jacobs
Pinus monophylla in the west Nevada, SE California, W Utah, S idaho * DRY SUMMERS
Three pinyons … in three different climate zones
Figure by
Brian Jacobs
Pinus edulis in the east: New Mexico, Colorado, SE Utah, NE Arizona * WET SUMMERS OR BIMODAL PRECIPITATION Pinus
monophylla in the west Nevada, SE California, W Utah, S idaho * DRY SUMMERS
Three pinyons … in three different climate zones
Figure by
Brian Jacobs
Pinus edulis in the east: New Mexico, Colorado, SE Utah, NE Arizona * WET SUMMERS OR BIMODAL
Pinus cembroides in the south: S Arizona, Texas * WET SUMMERS
Pinus monophylla in the west Nevada, SE California, W Utah, S idaho * DRY SUMMERS
Three pinyons … in three different climate zones
Five junipers … in different
climate zones
figure by Brian
Jacobs
Five junipers … in different
climate zones
Juniperus occidentalis in the west Nevada, SE California, SW Idaho * DRY SUMMERS
figure by Brian
Jacobs
Five junipers … in different
climate zones
Juniperus occidentalis in the west Nevada, SE California, SW Idaho * DRY SUMMERS
Juniperus monosperma & deppeana in the southeast: SE New Mexico, SE Colorado, S Arizona
* WET SUMMERS
figure by Brian
Jacobs
Five junipers … in different
climate zones
Juniperus occidentalis in the west Nevada, SE California, SW Idaho * DRY SUMMERS
Juniperus monosperma & deppeana southeast: SE New Mexico, SE Colorado, S Arizona
* WET SUMMERS
Juniperus osteosperma in the north N New Mexico W Colorado E Utah N Arizona
* BIMODAL PRECIPITATION
figure by Brian
Jacobs
Figure by
Brian Jacobs
And the fifth
juniper:
Juniperus scopulorum
at higher elevations
Rocky Mountains & Great Basin … New Mexico to
British Columbia
* COOLER TEMPS BUT VARIABLE
PRECIP
If you’re working in southern New Mexico, you’ll have: - Pinus edulis (Colorado pinyon) and/or Pinus cembroides (Mexican pinyon) - Juniperus monosperma (one-seed juniper) and/or Juniperus deppeana (alligator juniper) - Juniperus scopulorum (Rocky Mountain juniper … at higher elevations)
If you’re working in central New Mexico, you’ll have: - Pinus edulis (Colorado pinyon) - Juniperus monosperma (one-seed juniper) - Juniperus scopulorum (Rocky Mountain juniper … at higher elevations)
If you’re working in northern New Mexico, you’ll have: - Pinus edulis (Colorado pinyon) - Juniperus osteosperma (Utah juniper … especially in northwest) - Juniperus monosperma (one-seed juniper … especially in northeast) - Juniperus scopulorum (Rocky Mountain juniper … at higher elevations)
To summarize for New Mexico …
If you’re working in southern New Mexico, you’ll have: - Pinus edulis (Colorado pinyon) and/or Pinus cembroides (Mexican pinyon) - Juniperus monosperma (one-seed juniper) and/or Juniperus deppeana (alligator juniper) - Juniperus scopulorum (Rocky Mountain juniper … at higher elevations)
If you’re working in central New Mexico, you’ll have: - Pinus edulis (Colorado pinyon) - Juniperus monosperma (one-seed juniper) - Juniperus scopulorum (Rocky Mountain juniper … at higher elevations)
If you’re working in northern New Mexico, you’ll have: - Pinus edulis (Colorado pinyon) - Juniperus osteosperma (Utah juniper … especially in northwest) - Juniperus monosperma (one-seed juniper … especially in northeast) - Juniperus scopulorum (Rocky Mountain juniper … at higher elevations)
To summarize for New Mexico …
SO, NOT JUST ONE TYPE
OF PINYON JUNIPER
VEG IN
NEW MEXICO
…
Stand dynamics: lots of variability … four basic kinds
Expansion … establishment of pinyon and/or juniper trees in former grasslands or shrublands.
• A widespread phenomenon during the modern period (post-1880)
• “invasion” “encroachment”
• Associated with land use change & climate (more later on mechanisms)
• Especially prominent with one-seed juniper
Expansion … establishment of pinyon and/or juniper trees in former grasslands or shrublands.
• A widespread phenomenon during the modern period (post-1880)
• “invasion” “encroachment”
• Associated with land use change & climate (more later on mechanisms)
• Especially prominent with one-seed juniper
• But expansion is not occurring everywhere …
Brian Jacobs’ work
Brian Jacobs’ work
Young stands (post-1880 origin, expansion) most likely on concave or depositional sites, and in southeast
Brian Jacobs’ work
Young stands (post-1880 origin, expansion) most likely on concave or depositional sites, and in southeast
Old stands (pre-1880 origin) most likely on convex or erosive sites, and in northwest
Infill … the process of increasing tree density
within existing woodlands. • Widespread in the 20th
century … not the same as expansion … occurring in places that were already woodland prior to the modern period
• Associated largely with climate (land use change may play a role also)
• Has occurred in most kinds of pinyon-juniper … but not in all places
Contraction … reduction in tree density and cover due to drought, insects, or fire
• Widespread occurrence in past 20 years
• Drought, insect outbreaks, fire
• Probably occurred historically as well – old stands commonly
have lots of ancient dead wood
– small decrease in PJ extent in two Colorado landscapes 1910s-1980s
( photo by Diana Selby)
Recovery … tree re-establishment following severe disturbance, e.g., stand-replacing fire or chaining.
• easy to mistake areas of recovery for expansion
• key difference is that recovery occurs after disturbance of a previously existing woodland, whereas expansion involves establishment of a new woodland in a place that was formerly shrubland or grassland.
Three broad types of piñon-juniper vegetation
1. Persistent woodland
2. Wooded shrubland
3. Savanna
1.
2.
3.
1.
2.
1.
Three broad types of piñon-juniper vegetation
1. Persistent woodland
2. Wooded shrubland
3. Savanna
1.
2.
3.
Must emphasize: much additional variation … this is just a start at characterizing this diverse vegetation type!
1.
2.
1.
1. Persistent piñon-juniper woodland
• Found where site conditions have long been inherently favorable for piñon and juniper – Typically shallow rocky soils
(though not always) and moderate climate
– Infrequent fire (always)
• Stands may be dense or sparse, depending on site conditions & history
• Tree density fluctuates with climate & disturbance … but piñon and juniper always well represented … historically and currently
2. Wooded shrubland • Found where climate & soils
are suitable for piñon and/or juniper and for shrubs, and where fire occurs periodically
• Shrubs are the dominant & consistent life form on the site
• Tree densities wax & wane over decades & centuries
– increase during wet periods – decrease during dry periods
… drought, insects, fire
Photo by Brian Jacobs
3. Piñon-juniper savanna
• Found where climate & soils are suitable for piñon and/or juniper and for grasses
• Many savannas have only juniper, no piñon
• Grasses are the dominant & consistent life form on the site
• Tree densities wax & wane over decades & centuries – increase during wet periods – decrease during dry periods …
drought, insects, fire
Nogal Mesa –Circa 1900 Nogal Mesa - Today
Extensive infill & expansion in southwestern savannas during the past century
(Photo comparison by Hollis Fuchs, slide shared with me by Sid Goodloe)
Nogal Mesa –Circa 1900 Nogal Mesa - Today
Extensive infill & expansion in southwestern savannas during the past century
(Photo comparison by Hollis Fuchs, slide shared with me by Sid Goodloe)
This kind of change has been most prominent where one-seed juniper or alligator juniper is the major tree
What was/is the mechanism driving juniper expansion?
• Fire exclusion? – Logical … but little empirical data on pre-1880 fire history in these
ecosystems … few fire scars
• Livestock grazing? – Inconsistent evidence for direct grazing effect … often little/no
difference in tree densities with vs. without grazing
– but indirect effect of grazing, by reducing/eliminating fine fuels, could be a reason for reduced fire frequency
• Climate? – Documented pulses of recruitment during wet periods, and 20th
century climate was generally favorable for trees … but was that enough to explain the magnitude of change?
– Increasing CO2 concentration also may enhance juniper growth & survival
What was/is the mechanism driving juniper expansion?
• Fire exclusion? – Logical … but little empirical data on pre-1880 fire history in these
ecosystems … few fire scars
• Livestock grazing? – Inconsistent evidence for direct grazing effect … often little/no
difference in tree densities with vs. without grazing
– but indirect effect of grazing, by reducing/eliminating fine fuels, could be a reason for reduced fire frequency
• Climate? – Documented pulses of recruitment during wet periods, and 20th
century climate was generally favorable for trees … but was that enough to explain the magnitude of change?
– Increasing CO2 concentration also may enhance juniper growth & survival
* A key research priority identified by the 2006 PJ group *
Recent studies have provided empirical evidence that fire was important in maintaining low-
density savannas
• Rowe Mesa savanna, New Mexico (Ellis Margolis) – Pinyon & juniper with multiple fire
scars pre-1880 – Onset of grazing in ca. 1879 followed
by abrupt cessation of frequent & extensive fires, and by pulse of pinyon & juniper establishment
• Savanna areas in Davis Mtns & Big Bend NP in Texas (Poulos et al.) – Pinyon & juniper with multiple fire
scars pre-1880 – Multi-age pinyon cohorts
• Wupatki National Monument – Fires began to occur soon after
cessation of grazing Fire-scarred pinyon
(shared with me by Sid Goodloe)
Wupatki: • Heavy grazing ca.
1880-1930s … less intense grazing through 1989 … no fires of any size during that time
• Grazing terminated in 1989
• Six fires since 1989 … in 1995, 2000, 2002 (two fires), 2013, & 2016
• Total area burned 1995-2013 = 3,600 acres = 22% of grassland area in monument
2013 fire, photo 3 weeks later
(photo by Paul Whitefield)
2013 fire, photo 2 years later
2013 fire, photo 2 years later
2000 fire, photo 15 years later
Has the 20th
century expansion
event ended?
Has the 20th
century expansion
event ended? Will
contraction be our
management challenge in
the 21st century?