Ecology and Diversity of Piiion-Juniper
Woodland in New Mexico
William A. ~ i c k - ~ e d d i e '
Until recently there has been little research conducted upon
woodland vegetation This has been largely because woodland
vegetation was of less economic importance than either forest
or grassland vegetation.
OCCURRENCE OF PINON-JUNIPER
VEGETATION
One of the most important factors dictating where a given
type of vegetation is found is "available moisture". Available
moisture is a complicated factor. It is a function of precipitation
(amount, form, season, frequency, and intensity); infiltration rate
(surface material and slope); percolation (soil structure and
texture); and rate of evapo-transpimtion from plants and soil.
Vegetation types have different avdable moisture requirements.
It is not surprising then that in New Mexico, forest, woodland,
grassland, and scrubland are established on a gradient of
decreasing available moisture. Woodland has a higher available
moisture requirement than grassland but lower than that required
by forest.
CHARACTERISTICS AND DIVERSITY OF
PINON-JUNIPER WOODLAND AND
JUNIPER SAVANNA
Woodlands are characterized by tree species whose canopies
do not overlap and whose sizes are smaller than top-canopy
forest tree species. Tree density in woodlands varies from 280
treeslacre down to 130 treeslacre. Scattered stands with densities
of less than 130 treeslacre are considered to be savanna starads.
This symposium is concerned with one type of woodland Piiion-Juniper Woodland. Even though there are no piiion pines
in New Mexico savannas, the juniper savannas are considered
part of the Piiion-Juniper Ecosystem at this symposium. You
must remember that Piiion-Juniper Woodland in New Mexico
includes a number of different species in different combinations,
in Merent parts of the state. The composition and structure of
' Professor Emeritus, Department of Biology, New Mexico State
Univeru'ty, Las Cruces, New Mexico.
the Piiion-Juniper types vary on an available moisture gradient.
Moir and Carleton (1987) proposed three subzones of woodlands
for New Mexico as follows:
1. Mesic (cool, wet) closed woodlands - Tree
cover: 50-80%, Height of tallest trees:
7-13m
2. Typical or model open woodlands - Tree
cover: 30-50%, Height of tallest trees: 4-8m
3. The aridic (warm, dry) juniper savannas Tree cover: 5-30%, Height of tallest trees:
5m
There are two species of piiion pine found in New Mexico.
They are the Colorado Piiion (Pinus edulis), and Border Piiion
(I! discolor). There are six or seven species of juniper in the
state but the most widespread are Alligator Juniper (Juniperus
deppeana), One-seed Juniper (J. monosperma), Rocky Mountain
Juniper (J. scopulorum), and Utah Juniper (J. osteospema). I
have classified the vegetation of Piiion-Juniper Woodland and
Juniper Savanna found in New Mexico (Dick-Peddie, 1993).
Series as used by the Forest Service was used as the category
above the basic category of Association (Community type or
Habitat type). The classification recognized the following Series
(the number in parentheses is the number of distinct Associationfound in the Series):
Coniferous Woodland
Colorado PifionUne-seed Juniper Series (10)
Colorado Piiion-Alligator Juniper Series (4)
Colorado Piiion-Utah Juniper Series (1)
Colorado Piiion-Rocky Mountain Juniper Series (1)
Colorado Piiion-Mixed Juniper Series (3)
Colorado Piiion-Mixed Juniper Series (1)
Mixed Woodland
Colorado Piiion-Oak-Juniper Series (2)
Savanna
One-seed Juniper Series (8)
One-seed Juniper-Rocky Mountain Juniper Series (1)
Utah Juniper Series (1)
You can see that the greatest number of Associations (10) are
found in the Colorado Piiion-One-seed Juniper Series. This
series is the most widespread in the state. It is interesting that
Woodin and Lindsey (1954) noted that although One-seed
Juniper was the most important juniper in New Mexico
woodlands, it is replaced in the north (Colorado) by Rocky
Mountain Juniper and in the south (Mexico) by Alligator
Juniper. Alligator Juniper is usually found in New Mexico at
the upper limits of the P-J Woodland and some even suggest
that Alligator Juniper is a subdominant member of the Ponderosa
Pine Forest in much of the state. Large old Alligator Junipers
are found in the Sacramento Mountains in areas which were
cleared of Ponderosa forest in the past.
I have outlined for you the type and degree of diversity found
in the Piiion-Juniper Woodland and Juniper Savanna vegetation
of New Mexico. It is obvious to all of us that Piiion-Juniper
Woodland and Juniper Savanna are generalized, generic terms
and that single or uniform management schemes cannot be
successfully applied to these diverse ecosystems. Each Series or
possibly even each Association should be approached
independently as to manipulation and management.
the establishment (ecesis) of a species on a new site denotes
that there is not only an opening in the community but that the
microhabitat meets the needs of the new arrival. Knowledge of
this ecology avoids the misleading concept implied by the terms
"invasion" and "invader". These terms imply a dynamics of
the advance of one species into another species' habitat and
outcompeting it, thereby pushing it out. A great deal of money
has been spent over the years, based upon this "invader"
premise, to remove the "invader" species as the culprit
responsible for the decline in vigor and density of the initial
species. Rather it should be assumed that the establishment of
the "invader" species serves as an indicator or symptom that
the initial habitat has been modified and may no longer be
optimum for the initial species.
My view is that the rapid increase in the amount of juniper
savanna in New Mexico over the past 100 years is an example
of ecesis following microhabitat modification A more striking
example can be seen in northcentral to northwestern New
Mexico where the microhabitat has been so modified that
grassland vegetation has disappeared and there now is a
transition from Piiion-Juniper Woodland to Great Basin Desert
Scrubland. Changing patterns where Colorado Piiion is
expanding appears to actually be its re-establishment on
previously Piiion-Juniper sites (Sallach, 1986).
I would suggest that brush removal on grasslard might well
hasten gr;lss recovery provided that post-removal management
rectifies the situation which allowed the brush to become
established in the fmt place. If such management is not
economically feasible, periodic removal will be necessary.
CHANGED AND CHANGING PATTERNS
OF PINON-JUNIPER AND JUNIPER
SAVANNA VEGETATION
LITERATURE CITED
Lastly, I want to address changes in the patterns of
Piiion-Juniper and Juniper Savanna vegetation during the last
100 years.
Synecologists (community ecologists) have been wrestling
with the concept of "competition" for many years. It now
appears that in plants competition is at best subtle and may only
be operative at the time of germination. Actually, as we instruct
our young school children, plants tend to "share" the
environment. In fact we should expect selective pressures to
stimulate a reduction of competition through the incorporation
of adaptations which permit a species to use different features
of the environment and/or at Merent times than other species
in the habitat. I have gone in to this so we will understand that
Dick-Peddie, W.A. 1993. New Mexico vegetation: past, presea
and future. University of New Mexico Press, Albuquerque,
NM 244.
Moir, W. H.; Carleton, J.O. 1978. Classification of
pinyon-juniper (P-J) sites on National Forest in the Southwest.
In: Proceedmgs Pinyon-Juniper Conference. USDA Forest
Service General Tech Rpt. INT-2 15S 81
Sallach, B.K. 1986. Vegetation changes in New Mexico
documented by repeat photography. Thesis. New Mexico
State University, Las Cruces, NM. 68.
Woodm, H.E.; Lindsey, A.A. 1954. Juniper-pinyon east of the
Continental Divide as analyzed by line-strip method. Ecology.
35: 473-489.