p-j is not just one vegetation type: key variation in structure ......p-j is not just one vegetation...

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


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


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


Five junipers … in different

climate zones

figure by Brian

Jacobs


climate zones

Juniperus occidentalis in the west Nevada, SE California, SW Idaho * DRY SUMMERS

figure by Brian

Jacobs


climate zones


Juniperus monosperma & deppeana in the southeast: SE New Mexico, SE Colorado, S Arizona

* WET SUMMERS

figure by Brian

Jacobs


climate zones


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


Young stands (post-1880 origin, expansion) most likely on concave or depositional sites, and in southeast


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



Has the 20th

century expansion

event ended?

Has the 20th

century expansion

event ended? Will

contraction be our

management challenge in

the 21st century?

p-j is not just one vegetation type: key variation in structure ......p-j is not just one vegetation...

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