BIOLOGY AND ECOLOGY OF
ASHE (BLUEBERRY) JUNIPER
F. E. Smeins and S. D. Fuhlendorf
Ashe juniper (Juniperus ashei Buchholz)(1) is
found from southern Missouri, through
Arkansas into Oklahoma, across central Texas
and into Mexico and Guatemala. It occurs
primarily on calcareous, shallow, rocky soils
derived from limestone and dolomitic parent
materials, although it can also grow on sandy,
granitically derived soils (Vines 1960). In
Texas, it is found in greatest abundance on the
eastern and southern portions of the Edwards
Plateau, but also extends into the Central
Mineral Region, Grand Prairie, Cross Timbers
and North Central Prairies Ecoregions (Texas
State Soil and Water Conservation Board 1991).
The exact distribution and abundance of Ashe
juniper on the Edwards Plateau at the time of
European settlement may never be known with
any degree of accuracy. Early observers,
recorded very dense and nearly closed-canopy
"cedar brakes" in some areas, particularly on the
canyon slopes and on rocky, sloping land of the
Balcones canyonlands over Glen Rose
limestone substrates. On the other hand, Ashe
juniper was apparently of minimal density in
many areas, particularly on the divide portions
of the central and northwestern Plateau (Smeins
1980, Weniger 1984, Smeins et al. this
volume).
In spite of lack of resolution about its original
distribution and abundance, since European
settlement this species has generally increased
in abundance and habitat range with fluctuations
based on periodic naturally and man-caused
fires or efforts to reduce it or harvest it for
various uses and commercial products (Bray
1904, Foster 1917, Smeins et al., this volume,
Garriga this volume). Since European
settlement large areas of cedar brakes have been
harvested for fence posts and other purposes
(Cartwright, 1966), but following harvest Ashe
juniper tends to rapidly reestablish itself (Huss
1954, Weniger 1984). Thus, reestablishment in
previously occupied areas, as well as spread into
new sites is an ongoing process particularily in
the absence of periodic fires.
The exact cause of increase in Ashe juniper is
difficult to determine. Overgrazing by
livestock, which reduces herbaceous plant
competition for establishing juniper seedlings is
frequently considered a factor. Purposeful or
inadvertent prevention of naturally occurring
fires, along with reduced fuel loads due to
grazing, have also had an influence. Since this
species does not sprout from the base when it is
topkilled, it is readily controlled by fire if
sufficient fuel is available and if the plants are
burned at less than mature size. Wink and
Wright (1973) found that 99% of Ashe juniper
plants less than 1.8 m tall(2) were killed with a
fine fuel (grass) load of 1500 kg/ha or more. As
the trees become larger, the denuded area
beneath their canopy expands and it becomes
increasingly difficult to ignite the plants or to
move the fire to the fire-sensitive central stem
(Dalrymple 1969). Periodic droughts,
particularly when associated with overgrazing,
may also play a role in their spread. The
weakened, open herbaceous cover could be less
resistant to juniper invasion following droughts
when favorable rainfall conditions return. On
the other hand, long-term droughts, such as the
one of the 1950's, can cause extensive mortality
of large, mature, Ashe juniper, although, smaller
size classes apparently survive droughts better
than associated species such as live oak
(Quercus virginiana Mill.) and shin oak (Q.
pungens Liebm. var. vaseyana (Buckl.) C.H.
Muller) (Merrill and Young 1959). Historical
soil erosion due to overgrazing, droughts and
other factors may increase the heterogeneity of
sites making them more susceptible to woody
plant invasion. Documented increase in
atmospheric carbon dioxide since the 1850's,
which may favor growth conditions for juniper
and other woody species, compared to the warm
season grasses, may also be a factor (Emanuel
et al. 1985). As with most ecological
phenomena, the increase of Ashe juniper is most
probably an interaction of all these factors
(Smeins et al., this volume).
Ashe juniper significantly alters the
composition and structure of rangeland plant
communities. Increasing density of Ashe
juniper reduces grazeable areas for livestock
and many species of wildlife because of its low
forage value. It significantly reduces the
production and diversity of associated plant
species (Blomquist 1990, Fuhlendorf 1992,
Yager 1993, Fuhlendorf and Smeins 1997).
Additionally, growth of Ashe juniper in the
immediate vicinity of more desirable woody
species such as live oak, not only denies
browsing animals access to these species but
may in time cause mortality of the desired
species (Rykiel and Cook 1988). Ashe juniper
does, however, serve as cover and food for
many wildlife species. Deer and domestic
livestock may consume large quantities of
berries, particularly when other forages are of
limited availability or quality. Bark from
certain size classes of Ashe juniper is apparently
essential as nesting material for the endangered
golden-cheeked warbler (Dendroica
chrysoparia) (Kroll 1980). Ashe juniper may
influence the hydrology of an area when it
grows in abundance. Alternatively, it may limit
soil erosion on many sites including steep
slopes of canyons where other vegetation cover
cannot grow or is sparse (Wright et al. 1975,
Marsh and Marsh 1993).
To better understand the variation in
abundance of this species, its influence on
ecosystem structure and function and to more
effectively develop strategies for its
management, a detailed understanding of its
biology and ecology is essential. This chapter
will summarize our knowledge of Ashe juniper
in terms of its biogeography, taxonomy, life
history characteristics, and relationships with
other plants and animals in the ecosystem.
Growth Habit and Taxonomy
Ashe juniper(3) is an evergreen, dioecious
(separate male and female plants) shrub or small
tree that is usually less than 6 m in height at
maturity. It is non-sprouting, typically
multistemmed, and has a broadly globular
growth form that usually branches near the base.
Its stems are often fluted and twisted and, as
they age, they develop a gray to reddish brown
bark that shreds into shaggy longitudinal strips.
Its wood is dense with a pale-brown heartwood
and a broad zone of nearly white sapwood.
Leaves on mature branches are scalelike and
occur in pairs or sometimes threes which are
closely appressed, triangular to ovate in shape,
and average 2 mm long and 1 mm wide.
Leaves on young shoots and seedlings are spiny
and awl-like and may be up to 12 mm long.
Small, inconspicuous flowers are located
terminally on branches. Staminate flowers are 4
mm long with 12 to 18 stamens; they are brown
in color. The green pistillate flowers are up to 3
mm long and are formed by cohesion of
enlarged fleshy scales to form a berrylike cone.
As the one-seeded (sometimes two) berry
matures, it takes on a deep blue color and varies
in size from 6.5 to 8mm in length and 5.5 to
7mm in width (Chavez-Ramirez and Slack
1994). Seeds are 2 mm long with a thick
woody seed coat (Correll and Johnston 1970).
The origin of Ashe juniper is not known
although most believe it to be part of the
southwestern Madrean Evergreen Woodland
that occurs in Mexico and extends into New
Mexico and the Southwestern U.S. (Brown
1982). It overlaps in geographic distribution
with Eastern Red Cedar (Juniperus virginiana
L.) to the east and north and with Redberry
Juniper (Juniperus pinchotii Sudw.) to the west
and northwest. Based upon anatomical and
morphological features it has been suggested
that Ashe juniper may form hybrids with both
of these species where they come into contact
with one another (Hall et al. 1961, Gehlbach
1988). Some authorities feel that Ashe juniper
is closely related to one-seeded juniper
(Juniperus monosperma (Engelm.) Sarg.),
which occurs from western Texas and
Oklahoma through New Mexico to Utah,
Colorado and Wyoming, and some would
consider one-seeded juniper to be only a variety
of Ashe juniper (Vines 1960). Chemical
analysis of the foliage of Ashe and other
junipers suggests that it is chemically distinct
from other species, and no chemical evidence
exists of hybridization between Ashe juniper
and other species that overlap its geographic
range (Rudloff 1968, Adams and Turner 1970).
The final judgment of its genetic,
biogeographic, and taxonomic status has yet to
be confirmed.
LIFE HISTORY
Introduction
Life history refers to the set of adaptations of
an organism that more or less directly influence
its survival and reproductive success. It includes
reproductive rate and output, age at first
reproduction, seed germination, longevity and
dispersal, growth rate and pattern as well as
many other characteristics of the organism. For
Ashe juniper information is available for some
of these parameters while for others they remain
unknown or are currently being analyzed.
Flowering and Seed Production
Flowers are typically produced between
December and February with pollen release and
pollenation occuring during this time. Cone
(fruit, berry) development and maturation
occurs from then until November, and fruit
ripening and dispersal between November and
April. Observations indicate that Ashe juniper is
capable of producing abundant berry and seed
crops. Trees begin to produce fruits at about 1
to 1.5 m in height (approximately 10 to 20
years old). Fruit and seed production varies
greatly from tree to tree within a year and for
the same tree across years (Table 1). A given
tree may produce abundant fruit crops in
successive years, if environmental conditions
are favorable, which suggests that
environmental rather than genetic limitation
controls berry (seed) crops. Large trees may
produce between 100,000 to 250,000 berries
per tree. Smaller trees produce proportionately
fewer berries (Table 1).
Weather (precipitation) appears to be a controlling factor in berry production. For example, several large trees on the Sonora Research Station in 1989-90 (wet year) produced 50,000 to 200,000 berries per tree, while in 1990-91 (dry year) they produced fewer than 1,500 berries and in 1991-92 (wet year) they all produced in excess of 100,000 berries (Table 1) (Chavez-Ramirez 1992). A plot with 18 fruit bearing trees in 1989-90 (wet) had only 4 bearing fruit in 1990-91 (dry). Reinecke (1996) also found mature trees that had prolific seed production in 1993 (wet) had limited production in 1994 (dry).
Table 1. Fruit production of four large Ashe juniper trees over a three year period. (From Chavez-Ramirez 1992)
| Fruit Numbers Per Tree | ||||
| Tree | Canopy
Area(m2) |
1989-90
(wet) |
1990-91
(dry) |
1991-92
(wet) |
| 1 | 16 | 57,500 | 1,500 | no data |
| 2 | 28 | 91,000 | 0 | 105,000 |
| 3 | 33 | 196,000 | 100 | 226,900 |
| 4 | 76 | 154,600 | 90 | 198,600 |
Seed Rain and Dispersal
Seeds of Ashe juniper are relatively heavy
and hence those that fall from the parent tree
tend to remain within or very near the canopy
area. If not removed by predators, most seeds
fall from the tree within days after ripening.
Near Uvalde, Texas, 40 trees were marked that
had abundant seed production. Even though
there were abundant birds and mammals (seed
foragers) present, few if any of the berries were
consumed. Most fell directly under or near the
parent tree (Owens and Schliesing 1995).
Perhaps in some years berry production exceeds
a threshold and is so high that even a great
number of seed foragers may miss individual or
entire stands of trees with abundant berry
production. Holthuijzen and Sharik (1984)
found most seeds of the similar Eastern Red
Cedar to fall beneath the canopy and few seeds
were found beyond 5 m of the parent plant. On
relatively level sites on the Sonora Research
Station no Ashe juniper seeds were found in soil
samples over 5 m from parent plant canopies
(Kinucan and Smeins 1992). If the tree occurs
on a slope or an area that has periodic overland
flow of water during rainfall events, many seeds
can be carried by gravity or water and deposited
some distance away (Huss 1954). When
deposited, they are often in sites with a
collection of well-watered debris and litter that
may favor germination and seedling
establishment.
Berry (seed) foragers may remove nearly all
berries from a given tree within a few hours or
days of ripening. Primary agents of long
distance dispersal are birds and mammals.
Berries are readily consumed by nearly all
mammals, including all classes of livestock,
many bird species and some insects. Raccoons
(Procyon lotor)(4) and ringtail cats (Bassariscus
astutus) as well as gray foxes (Urocyon
cinereoargenteus), coyotes (Canis latrans) and
jackrabbits (Lepus californicus) consume the
berries. American robins (Turdus migratorus)
and cedar waxwings (Bombycilla cedorum) are
important bird foragers primarily because of
their very high winter populations in the Ashe
juniper region (Chavez-Ramirez and Slack
1993, 1994).
Not only do all these animal species consume
the seeds but large numbers of ingested seed
avoid destruction by mastication or digestive
processes and pass intact through the animal and
exit with the feces. As a result they may be
deposited some distance from the parent plant
(Chavez-Ramirez 1992, Chavez-Ramirez and
Slack 1993). Robins are perhaps more effective
dispersal agents than waxwings, due to their
different post-foraging behavior (Chavez-Ramirez and Slack 1994). Robins have a much
greater range of movement away from parent
trees following ingestion of juniper berries and
they tend to individually visit a wider variety of
perch sites, particularly within grassland areas.
Berries are harvested by animal foragers and
disperers when the berries are ripe (fully
hydrated). It was speculated that seeds would
be at their peak of germination potential at this
time. Samples of berries were collected from a
series of mature Ashe juniper trees on the
Sonora Reasearch Station periodically from
mid-October 1993 to mid-February 1994 and
again from October to December 1994. Time
of peak germination and percent germination of
cold-treated seeds varied dramatically between
trees (see seed germination section). Some trees
never produced seeds that germinated while
other trees had a high of 55% germination on a
given date. Each tree had a unique germination
pattern. Some were highest in the fall, others in
mid-winter and other in late-winter. Highest
germination, for any tree in many cases,
corresponded to when the berries were ripe
(Reinecke 1996). Thus, it appears that
synchronous maturation of seeds occurs within
a given tree which coincides with berry
maturation and palatability. Seeds at other trees
mature and berries ripen at other times over as
much as a 4 to 5 month period. This
combination of events i.e. synchronous
maturation within a tree and asynchronous
maturation between trees, effectively favors
seed consumption and dispersal by animals and
retains dispersal agents, such as robins and
waxwings in the area for a long period of time.
It also provides an important forage component
for livestock and white-tailed deer during a time
of low forage availability.
Seed Bank
The number of seeds in the leaf litter and top
8 cm of mineral soil beneath 18 large female
Ashe juniper trees was determined on the Texas
A&M Research Station at Sonora. The number
of intact seeds was greatest in the litter and
varied from 11,000 to 23,000 seeds/m2. The
mineral soil varied from 250 to 650 seeds/m2.
Overall average was 16,600 seeds/m2 in the
litter and top 8 cm of the soil (Blomquist 1990).
These seeds were the result of seed rain from
several previous seasons. Tests were conducted
to test potential viability. Although viability was
only <0.3%, the large number of seeds in the
seed bank still would result in nearly 50 viable
seeds/m2. Most of the viable seeds were
considered to be seeds produced in the most
recent seed crop. Owens and Schliesing (1995)
found between 1197 and 1436 seeds/m2 in the
leaf litter and 275 to 366 seed m-2 in the soil on
a site nor Uvalde. High variation across studies
would be expected because of variable seed
production from year to year.
Seed Germination
Berries were collected from several trees on
the Texas A&M Research Station during the fall
of 1987 and subjected to treatments that may
naturally occur. Seeds were germinated after
receiving the following treatments: (1) no
treatment; (2) berries fed to goats, raccoons and
robins and seeds collected from the feces; (3)
berries mechanically removed from the seeds.
Subsamples of all of the above were cold
stratified at 2o C for 2 months and their
germination compared to non-cold treated
seeds.
Digestive scarification did not significantly
influence seed germination over mechanical
removal of berries. Mean germination across all
treatments was 29%. Cold stratification
significantly increased average germination to
45%. With the berry removed, germination
equalled 32%; while with the berry intact it was
only 0.02%. Seeds from individual trees varied
from 30 to 50% germination with the fruit
removed. The fruit appears to act as a physical
or chemical inhibitor to germination. The
advantage of seeds passing through the
digestive tract of an animal may be more
influenced by removal of the berry rather than
any physical or chemical scarification of the
seeds due to the digestive process.
It appears that favorable precipation during
the late spring-early summer period is related to
the degree of germination of seeds produced in
that year. Total annual rainfall or that of other
months or seasons was not strongly correlated
with level of germination (Reinecke 1996).
Significant tree to tree variation occurred in
amount (from 0 to up to 55% peak germination)
and timing of germination (some peaked as
early as mid-October and others as late as early
January). Cold-stratification of seeds with the
fruit removed increased overall germination
from 0 (non-stratified) to 15% (cold-stratified)
across all trees and sample dates. Presence of
the berry resulted in no germination of stratified
or non-stratified seeds. Laboratory storage of
seeds for 6 and 24 months did not significantly
alter germination responses.
Seed Longevity
Seeds were harvested from trees on the
Sonora Research Station during the fall of 1987.
They had an average cold-treated germination
percentage of 45% immediately after collection.
They were placed in nylon bags and either
positioned on the soil surface or buried 1 to 2
cm below the soil surface. Samples were
collected every 6 months for two years and
germination tests conducted. By the end of 18
months, germination had dropped to less than
1% regardless of being on the soil surface or
buried in the seed bank. This suggests that seed
persistence is rather limited under field
conditions, although 1% of the hundreds of
thousands of seeds produced in a stand of
mature trees would still be a significant number
for potential recruitment into the population.
On a site near Uvalde germination of seeds
from the seed bank was 0%, while newly
collected seeds of the year was 5% (Owens and
Schliesing 1995). These results are similar to
those found by Holthuijzen and Sharik (1984)
for Eastern Red Cedar where <5% of seeds
remained viable after 400 days. Reinecke
(1996) found, however, that seeds kept in the
laboratory for up to 24 months retained the
same levels of germination as immediate post-collection germination trials. The reason for the
contrasting results is uncertain, however, it is
believed, and inspection of field placed seeds
agrees, that in the field dessication, bacterial
and other degrading factors seem to destroy the
seeds after 18 months.
Seedling Distribution and Survival
Samples were taken from under juniper trees
or oak mottes and out in open grassland from
pastures with different grazing histories across
the Sonora Research Station to determine the
numbers and distribution of seedlings
(Blomquist 1990). The greatest density of
seedings (2.5/m2) occurs under female juniper
trees, whereas density under oaks or in open
areas is substantially less (Figure 1). However,
there could be great turnover of these seedlings under
juniper canopies, that is, they may establish
only to die during the following year and be
replaced by seedlings from the next seed crop.
Observation would suggest that some of this
phenomenon may occur but many seedlings
under parent trees appear to have survived for
several years. They have thick basal stems with
several annual rings, their leaves are of the
mature form, and some are up to 0.5 m in
height. Thus as juniper trees become larger they
serve as a large reservoir of seedlings either due
to protection from herbivores or as favorable
establishment sites.
Death of the large overstory tree can release
many seedlings that may grow rapidly due to
increased light, moisture and nutrient
availability. Where one large tree occurred a
cluster of small plants can develop. Thus,
removal of large trees by mechanical methods
not followed by burning or other practices may
result in rapid replacement of the removed
plants.
Another study on the Sonora Station
evaluated numbers of Ashe juniper seedlings
and saplings under juniper and oak canopies and
adjacent grass patches in four longterm grazing
treatments (Reinecke 1996). Greatest densities
were under female junipers followed by male
trees and oak mottes. Grass patches had very
low densities. Female trees in pastures with a
previous history, but not current heavy browsing
by goats, had the highest seedling densities.
This is perhaps a result of the still existing
browse line on these previously heavily
browsed plants which now provides a better
light environment for seedling survival. This
treatment also had the lowest juniper density
and cover.
It would appear that seedling establishment is
possible across a great variety of soil/vegetation
conditions. They may more readily establish in
heterogeneous, rocky, relatively, open sites,
however, they can and do establish on relatively
deep soils with a continuous cover of vigorous
herbaceous plants. In the latter case, increase in
size and numbers may be slowed by herbaceous
competition but invasion of juniper is ultimately
possible. Ability to establish under the dense
canopy of parent juniper and other woody
species suggests that high light intensity is not
essential for their germination or establishment
but may favor their survival.
Van Auken (1993) evaluated closed juniper
woodlands and adjacent grassland opening on
eastern portions of the Edwards Plateau in
Bandera County. He found high densities of
Ashe juniper seedlings in heavily grazed
grasslands and concluded that the low grass
biomass due to grazing reduced interference for
establishment of juniper seedlings and also
reduced fire potential. He inidcated low
densities under mature canopies of the closed
woodland perhaps due to low light intensities.
Comparison of this area, on quite different soils
and climatic conditions, to the more westerly
location of the Sonora Research Station, must
be made with caution since the different
environments, could produce contrasting results.
Stands at Sonora never reach the crown closure
of mature stands on the eastern Plateau. Hence,
his observation that mature stands exhibit
declining seedling densities could be a general
but yet undocumented phenomenon. Most
stands are regrowth from former thinning and
clearing and most have, perhaps, not reached a
mature stage of growth and corresponding
decline in seedling survival.
A recent investigation near Comfort, Texas
suggests that grazing and the presence of grass
vegetation, may influence seedling survival
different from the conclusions of Van Auken
(1993). Four treatments, ungrazed-high grass
cover, ungrazed-low grass cover, grazed-high
grass cover and grazed low-grass cover were
evaluated (Reinecke 1996). Of 1696 new
seedlings marked during the winter of 1995,
there was on overall 45% survival across all
treatments after nearly 500 days the
survivorship in each treatment was: ungrazed/
high grass cover-60%; ungrazed/low grass
cover-49%; grazed/high grass cover-40%;
grazed/low grass cover-31% (Figure 2).
These results suggest that moderately stocked
cattle, white-tailed deer, rabbits, armadillos, etc.
do reduce the number of seedlings through
consumption and trampling. Herbivore use and
selection of juniper seedlings is discussed in
more detail by Taylor et al. (this volume). Also,
high grass cover may enhance seedling
survivorship by providing protection from
herbivores and a favorable microenvironment
for growth and development. Variable
conclusions from seedling establishment studies
suggest that germination and survival of juniper
may be episodic where some years low grass
cover may enhance establishment while other
years, high cover may be more critical.
Growth of Individual Plants
Once established, rate of growth and
development determines the influence of the
juniper trees on the rest of the community. In
1949, nearly all junipers were hand-removed
from the Texas A&M Research Station. Aerial
photographs from 1955, 1966, 1973 and 1985
were obtained and 14 trees that could be
identified on photographs from 1955 were
followed through each photograph to assess
their individual rate of canopy increase
(Blomquist 1990). The average canopy showed
a steadily increasing growth rate over the 30
year period: 0.58 m2/yr from 1955 to 1966,
1.40 m2/yr from 1966 to 1973, and 1.94 m2/yr
from 1973 to 1985 (Figure 3). Great variation
existed in individual growth patterns. Some
plants exhibited little, if any, growth during the
first 10 to 20 years and then grew rapidly. For example, one tree
expressed little growth from 1955 to 1973 but
then increased in size by 42 m2 from 1973 and
1985, a rate of 3.50 m2/yr. Others grew rapidly
at first and then greatly reduced their growth
rate. Differences appear to be related to
different site conditions. Although first
production of seeds occurs at approximately 1
to 1.5 m in height or 10 to 20 years, as their age
and size increase their quantity of seed
production increases.
COMMUNITY RELATIONSHIPS
General Composition-Edwards Plateau
Ashe juniper has been a component of the
plant communities of the Edwards Plateau for
thousands of years (Bryant and Shafer 1977,
Smeins et al. this volume). The general
geographic range of the species has probably
not changed greatly since the time of settlement.
It has, however, increased in density in
communities where it has always occurred, and
in some instances spread into habitats where it
previously was absent or of limited abundance
(Smeins 1980, Riskind and Diamond 1988, Van
Auken 1988). Riskind and Diamond (1988)
provide an excellent general overview of the
plant communities and environments of the
Edwards Plateau and indicate, along with others
such as Gehlbach (1988) and Van Auken
(1988), that Ashe juniper can successfully grow
on nearly all geology/soil sites. Hence, given
sufficient time without fire or heavy use by
browsers and adequate dispersal, Ashe juniper
can probably become a dominant plant on
nearly all sites on the Edwards Plateau, as well
as many adjacent ecoregions.
Composition - Texas A&M University Research Station
Canopy cover and density of woody plants
was determined for pastures on the Texas A&M
Research Station that have had 3 different
grazing histories since 1948 (Tables 2 and 3).
Although, most juniper had been hand cut from
the Station in 1948, canopy cover was
significantly different across the three grazing
histories in 1984. The historically (1949-1969)
heavily browsed (goat) pastures had 15% total
canopy cover which was less than one-half the
cover of the ungrazed pastures (exclosure),
while light browsed (Merrill deferred rotation)
pastures had intermediate values. Percent
woody cover of live oak varied from 3.3 to
7.5%. Shin oak had only 2.1% cover in heavily
browsed pastures, but 14.1% in ungrazed
pastures. Ashe juniper varied from 4.4 to
11.1% with highest values in ungrazed pastures
(Table 2). Since no fires have occurred in these
pastures since 1948, it appears that long-term
grazing does not contribute to the increase in
woody plants and may actually inhibit it.
Eighteen woody species were encountered in
ungrazed pastures while light and heavy browse
pastures had 12 and 10 species, respectively.
Thirty-five percent of total cover was
contributed by other woody species in the heavy
browse pasture and the majority of that was
made up of Texas persimmon (Diosporous
texana Scheele) and algerita (Mahonia trifoliata
Moric. Fedda). Elbowbush (Forestiera
pubescens Nutt.), hackberry (Celtis laevigata
Willd.) and skunkbush sumac (Rhus aromatic
Ait), which are preferred browse species, were
absent from light and heavy browse pastures,
while in ungrazed pastures the latter two species
made up the majority of the composition of
other woody species.
Ashe juniper size and density was
significantly affected by grazing history (Table
3). Total density of heavily browsed pastures
was 276 plants/ha which was approximately 2.5
times less than light and no browsed pastures.
For all height classes heavily browsed pastures
had fewer plants than other grazing histories
except for the smallest size classes where no
significant differences were recorded (Table 3).
Heavy browsing by goats from 1948 to 1969
has continued to dramatically limit the invasion
and growth of juniper until recent years. There
are fewer large berry producing trees in these
pastures which further reduces the opportunity
for new recruitment.
Table 2. Percent cover of woody species for different grazing histories. Different letters indicate significant differences ( 0.05) between grazing histories (from Station records).
| Percent Cover | |||
| No
Graze |
Light
Browse |
Heavy
Browse | |
| Live Oak | 7.4 a | 7.5 a | 3.3 a |
| Shin Oak | 14.1 b | 8.6 b | 2.1 a |
| Ashe juniper | 11.1 b | 6.3 ab | 4.4 a |
| Other species | 4.4 a | 3.6 a | 5.2 a |
| Total | 37.0 c | 26.0 b | 15.0 a |
Table 3. Stem densities of Ashe juniper by canopy diameter size class for three grazing histories, Different letters indicate significant differences (< 0.05) between grazing histories (from station records).
| Grazing History | |||
| Canopy
Dia. (m) |
No
Graze |
Light
Browse |
Heavy
Browse |
| 0.0-0.25 | 170 a | 143 a | 145 a |
| 0.25-1.0 | 161 a | 176 a | 69 a |
| 1.0-2.0 | 257 a | 205 a | 34 b |
| 2.0-3.0 | 107 a | 117 a | 7 b |
| 3.0-4.0 | 26 a | 14 ab | 4 b |
| >4.0 | 47 a | 24 b | 17 c |
| Total | 769 a | 679 a | 276 b |
Longterm Change in Woody Plant Composition
Photographs from the early 1900's indicate
that juniper was a small portion of the woody
vegetation of the Texas A&M Research Station.
By the 1940's it had become a significant
portion of the plant community. In 1949 most
junipers were removed from the station by hand
cutting. Following the drought of the 1950's
Merrill and Young (1959) evaluated the impact
of the drought on woody species mortality and
survival. The pre-drought composition of
woody plant cover was live oak 64%, shin oak
26%, Texas persimmon 4% and juniper 1%.
The remaining 5% was composed of several
species including hackberry, algerita, elbowbush
and catclaw acacia (Acacia greggii Gray). Live
oak suffered a 54% mortality and shin oak 30%.
At the end of the drought the composition had
shifted slightly to 52% canopy cover for live
oak, 33% for shin oak, 8% for Texas
persimmon and 2% for juniper. Large junipers
suffered 90% mortality while plants less than 2
m in height experienced no death loss. Overall
there was a 56% reduction in canopy cover of
all woody plants. Thus, extended droughts can
significantly influence the abundance and
composition of the woody vegetation, and
appears to favor increases in juniper.
Interpretation of aerial photographs from
1955, 1966, 1973 and 1985 provide an
assessment of longterm change in Ashe juniper
numbers and canopy cover for selected sites on
the Texas A&M Research Station at Sonora.
The number of canopies increased from
approximately 15/ha in 1955 to 82/ha in 1985.
The majority of this increase occurred between
1973 and 1985. Percent juniper cover increased
from less than 1% in 1955 to over 12% in 1985
(Table 4). The majority of this increase also
occurred from 1973 to 1985 which agrees with
the period of time of greatest increase in growth
of individual trees described above. Fuhlendorf
(1992) found similar levels of increase in Ashe
juniper using interpretation of aerial
photographs on an entire grazing exclosure on
the Station. Smeins and Merrill (1988)
documented an overall increase in total woody
plant cover of 12 to 32% from 1949 to 1983
across several grazing regimes. Juniper
increased from 5% of the composition to 35%
over that time period which accounted for the
majority of the total increase.
Table 4. Density (no/ha) and percent cover of juniper canopies at 4 dates over a 30 year period. Different letters denote significant differences (p < 0.05) (From Blomquist 1990).
| Year | ||||
| 1955 | 1966 | 1973 | 1985 | |
| Density of Canopies | 14 a | 25 a | 30 a | 82 b |
| Percent Cover | 1 a | 3 a | 4 b | 12 c |
Samples from several pastures across the
Sonora Station collected in 1993 showed all
pastures, regardless of grazing regime or
stocking rate, to have about the same level of
woody canopy cover (40%) and Ashe juniper
contributed >50% of the composition. The only
exception was again the historically heavily goat
browsed pastures that had lower total cover
17% with about the same proportion for
juniper.
The ability of juniper to increase in canopy
cover is either the result of increase in size of
existing plants or recruitment of new plants.
Minimal increase in canopy cover from 1955 to
1973 suggests that slow growth rate of the small
plants and limited recruitment kept total canopy
cover low. However, by 1973 sufficient plants
had reached adequate size to rapidly increase
their growth rate and by seed production which
resulted in increased recruitment potential.
Collectively this resulted in rapid canopy cover
increase from 1973 to 1985 that continues
today.
Age/Size Relationships
Age/size relationship were determined for 75
trees varying in height from <0.5 m to over 4 m
collected from several different pastures on the
Sonora Research Station in 1991 (Fuhlendorf
1992). Trees were cut at ground level with a
chainsaw and cross-sections were removed from
the base to determine annual ring counts. Ring
counts, as estimates of age, were then related to
plant height and canopy diameter for each tree
to determine age/size relationships (Table 5).
The trees varied in age from 11 to 77 years.
An attempt was made to remove all junipers
from the study site in 1949 by hand cutting, thus
the maximum age of trees should be about 42
years old. However, field vegetation surveys
(Merrill and Young 1959) and photographs of
the station in the early 1950's indicate the
presence of Ashe juniper trees, some of
substantial size, which suggests that all plants
were not removed in 1948. Other woody
species were not removed, and the high
densities of these species could explain the
failure to remove all junipers, since the other
species may have provided escape cover for
junipers.
Analysis of trees less than 40 years old showed those with a long history of heavy browsing by goats were significantly smaller in size at a given age than those from pastures with a history of light browsing (Table 5). Trees between 10 and 40 years of age had generally one-half the canopy diameter and heights in heavily browsed pastures compared to pastures with a history of light browsing. As trees became larger, the influence of utilization of goats decreased and those trees over 40 years of age showed no differences between grazing histories. Pruning of the juniper by the goats apparently is the cause of these differences. Therefore, heavy browsing by goats reduces the canopy growth rate of individual trees, as well as maintaining lower densities and cover within a pasture.
Table 5. Average canopy diameters (m) and height (m) of Ashe juniper for 4 age classes on the Texas A&M Research Station, 1991. Different letters indicate significant differences between grazing histories. (from Fuhlendorf 1992)
| Pastures |
| Age
Class (yrs) |
Heavy
Browse |
Light
Browse | |
| Canopy Diameter | 10-20 | .66 a | 1.18 b |
| 21-30 | 1.43 a | 2.43 b | |
| 31-40 | 1.41 a | 2.69 a | |
| >40 | 5.56 a | 5.41 a | |
| Height | 10-20 | .82 a | 1.52 b |
| 21-30 | 1.48 a | 2.35 b | |
| 31-40 | 1.51 a | 2.67 a | |
| >40 | 4.46 a | 4.37 a |
Overstory/Understory Relationships
Ashe juniper plants from small saplings to
large trees were selected within ungrazed
exclosures, moderate-deferred rotation pastures
and heavily-continuously grazed pastures to
determine the influence of past grazing history
and juniper canopy on herbaceous production
(Blomquist 1990). Samples were collected
from near the stem base, the edge of the canopy
and in open areas away from the canopy (Figure
4).
The zone from the stem base to near the canopy
edge typically has little herbaceous production
or it is very low, generally less than 5 gm/m2.
Hence, as the juniper plant gets larger, more and
more ground is lost to production of herbaceous
forage and the forage produced is often
inaccessible to grazing animals because of the
dense, low growing juniper foliage. The dense
shade of the juniper canopy as well as the deep
accumulation of juniper leaf litter under larger
trees appear to be the primary constraints to
herbaceous production (Fuhlendorf and Smeins
1997). The species diversity of herbaceous
species is also greatly reduced by increased
canopy cover and litter accumulation of
juniper(Fuhlendorf 1992, Yager 1993).
There is little evidence that the accumulated
litter of Ashe juniper in anyway alters the
chemical nature of the soil as it relates to
growth and development of other plants (Yager
1993). Soil chemical and physical properties
are in fact "improved" by the presence of the
juniper litter (Marshall 1995). No allelopathic
effects have been shown to be produced by the
litter. The major impact of the litter seems to be
its physical presence and its alteration of
hydrologic properties of the area under the
canopy. The thick litter layer is a difficult
physical medium for seeds of other species to
germinate and grow in mainly because moisture
either runs off (due to the hydrophobic nature of
the litter) or it dries out very quickly after a
precipitation event, which prevents seedling
growth from reaching the mineral soil (Yager
1993).
When heavy goat browsing has been a factor
in a pasture, a browse line is created on the
juniper which alters their influence on
herbaceous production and cover as compared
to unbrowsed plants (Fuhlendorf 1992). The
understory of browsed plants has more light
penetration, less litter accumulation and
generally more herbaceous production and
species diversity than unbrowsed plants. A
fairly dense cover of cool season grasses, such
as Texas wintergrass (Stipa leucotricha Trin. &
Rupr.), is often prevalent under browsed
canopies partially reducing the influence of
Ashe juniper on herbaceous production and
diversity. Figure 5 is a schematic side view diagram which illustrates
the interaction of the browse line, litter depth
and understory herbaceous foliar cover for a
tree with a history of heavy browsing verses one
that has a history of no browsing (Fuhlendorf
1992).
Successional/Disturbance Relationships
Observation and most studies indicated that Ashe juniper has great potential to re-establish
itself on sites from which it has been removed
and in the absence of fire to spread to sites that
it historically did not occupy. It would appear
that much of the Edwards Plateau could succeed
to nearly closed canopy stands in the absence of
periodic disturbances, such as fire. Of course,
grazing regimes, fire regimes, weather variation
and other factors interact to influence the rate
and pattern of these changes. Longterm models
of grassland/woodland vegetation dynamics are
presented by Fuhlendorf (this volume).
Transformation from a grassland to a woodland
dominated landscape occurs when fires are
absent, regardless of grazing treatment. This
change is associated with a threshold, where
change is slow and reversible initially but after
25-50 years (depending upon grazing and
weather) become rapid and non-reversible with
typical management fires. Emergent processes
related to these changes and effects on
hydrology, wildlife, livestock and other
resources and uses are discussed in other
chapters of this volume (Thurow, Owens and
Ansley, Rollins and Armstrong, Kothman and
others).
Summary And Implications
1. Ashe juniper is a small, non-sprouting, evergreen, dioecious tree that occurs primarily on limestone substrates from Central America to Missouri with its center of abundance within the Hill Country of Texas.
2. Ashe juniper has always been abundant on portions of the Edwards Plateau but it is generally agreed that its habitat range and overall abundance has increased since European settlement/or at least during the past 50 years
3. Elimination of naturally occurring fires, overgrazing, soil erosion, droughts, increase in atmospheric carbon dioxide and other factors have interacted to contribute to its increase.
4. Ashe juniper produces flowers during the winter, matures fruits during summer and early fall and disperses seeds in fall and winter. At 1.5 m height (10 to 20 years of age) seeds are produced and as plants increase in size they regularly produce large quantities of seeds.
5. Fruits (seeds) are consumed by nearly all birds and mammals, including domestic livestock, and generally pass unaltered, except for berry removal from the seeds through the digestive tract and can be dispersed in the feces some distance from the source plant. Overland flow of water can also disperse large quantities of seeds.
6. Seed longevity is limited and by the end of two years few seeds remain viable in the field. They are longer lived under laboratory conditions
7. Seedling establishment appears to be possible on nearly all sites and in almost all plant communities. Continuous deep soil with a vigorous, continuous herbaceous plant community can slow the rate of ingress but apparently not prevent it, particularly if periodic fires and/or intensive browsing does not occur. It appears that protection from large herbivores and the presence of high grass cover increases the survivorship of emerged seedlings.
8. Most seedlings occur under mature, large female trees either because of a favorable establishment environment or reduced herbivory impacts.
9. The canopy cover and density of Ashe juniper is significantly influenced by grazing history. Intense browsing by goats, deer and other wildlife can reduce plant numbers and maintain plants in smaller size classes for a given age compared to unbrowsed plants. Communities that are not browsed or grazed frequently have the highest densities and cover of woody plants.
10. Increased size and numbers of Ashe juniper plants in a community greatly alters the production, composition, and diversity of other woody and herbaceous species. Canopy shade and the accumulation of deep litter are primary contributors to these influences. Leaf litter primarily exerts a physical (moisture limiting) affect on other species and not a chemical (allelopathic) effect.
11. Increase in juniper dominance is associated with a threshold where return to grassland dominance becomes difficult and expensive after approximately 25-50 years, depending upon grazing and weather patterns. Managing juniper plants before they produce seeds is an important management strategy.
12. This can be accomplished in many ways (see later chapters) but fire seems the most reasonable ecological and economical method. This, of course, has to be integrated with proper grazing management strategies.
13. Managing Ashe juniper at a landscape level
to provide its values where desired and to
reduce its impacts where needed is the
preferred, though difficult, approach to its
management.
Literature Cited
Adams, R.P. and B.L. Turner 1970. Chemosystematic and numerical studies of natural population of Juniperus ashei Buch.. Taxonomy 19:728-751.
Blomquist, K.W. 1990. Selected life history and synecological characteristics of Ashe juniper on the Edwards Plateau of Texas. M.S. Thesis, Dep. Rangeland Ecology and Management, Texas A&M Univ., College Station, TX.
Bray, N.L. 1904. The timber of the Edwards Plateau of Texas: its relation to climate, water supply and soil. U.S. Department of Agriculture, Bureau of Forestry Bulletin No. 49. 30 p.
Brown, D.E. (Editor) 1982. Biotic Communities of the American Southwest - United States and Mexico. Desert Plants 4:1-342.
Bryant V.M. and H.J. Shafer 1977. The late Quaternary paleoenvironment of Texas: A model for the archeologist. Bulletin of Texas Archeological Society 48:1-25.
Cartwright, W.J. 1966. The cedar choppers. Southwestern Historical Quarterly 70:247-255.
Chavez-Ramirez, F. 1992. The role of birds and mammals in the dispersal ecology of Ashe juniper on the Edwards Plateau, Texas. M.S. Thesis, Dep. Wildl. Fish. Sci., Texas A&M Univ, College Station, TX.
Chavez-Ramirez, F. and R.D. Slack 1994. Effects of avian foraging and post-foraging behavior on seed dispersal patterns of Ashe juniper. Oikos 71: 40-46.
Chavez-Ramirez, F. and R.D. Slack 1993. Carnivore fruit-use and seed dispersal of two selected plant species of the Edwards Plateau, Texas. Southwestern Naturalist 38:141-145.
Correll D.S. and M.C. Johnston 1970. Manual of Vascular Plants of Texas. Texas Research Foundation, Renner, TX. 1881 p.
Dalrymple, R.L. 1969. Prescribed grass burning for Ashe juniper control. Progress Report, Noble Foundation, Inc. Ardmore OK. 2p.
Emanuel, W.R., H.H. Shugart, and M. Stevenson 1985. Climatic change and the broad-scale distribution of terrestrial ecosystem complexes. Climate Change 7:29-43.
Foster, J.H. 1917. The spread of timbered areas in central Texas Journal of Forestry 15:442-445.
Fuhlendorf, S.D. 1992. Influence of age/size and grazing history on understory relationships of Ashe juniper. M.S. Thesis, Dep. Rangeland Ecology and Management, Texas A&M Univ., College Station, TX.
Fuhlendorf, S.D. and F.E. Smeins 1997. Browsing and tree size influence on understory of Ashe juniper. Journal of Range Management (in press).
Gehlbach, F.R. 1988. Forests and woodlands of the Northeastern Balcones Escarpment. pp. 27-77 In. Edwards Plateau Vegetation - Plant Ecological Studies in Central Texas. Edited by B.B. Amos and F.R. Gehlbach. Baylor Univ. Press, Waco. 144p.
Hall, M.T., J.F. McCormick and G.G. Fogg 1961. Hybridization between Juniperus ashei, Buckholz and Juniperus pinchotii Sudworth in southwestern Texas. Butler University Botanical Studies 14:9-28.
Holthuijzen, A.M.A. and T.L. Sharik 1984. Seed longevity and mechanisms of regeneration of eastern red cedar (Juniperus virginiana L.). Bulletin of Torr. Botany Club 2:153-158.
Huss, D.L. 1954. Factors influencing plant succession following fire in Ashe juniper woodland types in Real county, Texas. M.S. Thesis, Dep. Rangeland Ecology and Management, Texas A&M Univ., College Station, TX.
Kinucan, R.J. and F.E. Smeins 1992. Soil seed bank of a semi-arid grassland under three long-term (36 years) grazing regimes. American Midland Naturalist 128:11-21.
Kroll, J.C. 1980. Habitat requirements of the Golden-cheeked warbler: Management implications. Journal of Range Management 33:60-65.
Marshall, S. B. 1995. Biogeochemical consequences of livestock grazing in a juniper-oak savanna. M.S. Thesis, Dep. Rangeland Ecology and Management, Texas A&M Univ., College Station, TX.
Marsh, W.M. and Marsh, N.L. 1993. Juniper trees, soil loss, and local runoff processes. (eds.) C.M Woodruff, W.M. Marsh and L.P. Wilding. In Soils, Landforms, Hydrologic Processes, and Land-use Issues-Glen Rose Limestone Terrains, Barton Creek Watershed, Travis County, Texas. Field Report and Guidebook. Soc. Independent Prof. Earth Scientists, Central Texas Chap., Austin, TX.
Merrill, L.B. and V.A. Young 1959. Effect of drought on woody plants. Texas Agriculture Progress 3:9-10.
Owens, M.K. and T.G. Schliesing 1995. Invasive potential of Ashe juniper following mechanical disturbace. Journal of Range Management.
Reinecke, R.K. 1996. Ashe juniper seed production and germination, seedling dynamics and response of live oak/ juniper mottes to summer fire. M.S. Thesis, Dep. Rangeland Ecology and Management, Texas A&M Univ., College Station, TX.
Riskind, D.H. and D.D. Diamond 1988. An Introduction to Environments and Vegetation. pp. 1-15. In. Edwards Plateau Vegetation - Plant Ecological Studies in Central Texas. Edited by B.B. Amos and F.R. Gehlbach. Baylor Univ. Press, Waco. 144p.
Rudloff, E.V. 1968. Gas-liquid chromatography of terpenes. Part XVI. The volatile oil of the leaves of Juniperus ashei Buchholz. Canadian Journal of Chemistry 46:679-683.
Rykiel, E.J. and T.L. Cook 1986. Hardwood red cedar clusters in the postoak savanna of Texas. Southwestern Naturalist 31:73-78.
Smeins, F.E. 1980. Natural role of fire on the Edwards Plateau. pp. 4-16. In Prescribed burning of the Edwards Plateau of Texas. Edited by L.D. White, Texas Agricultural Extension Service, College Station, TX. 74p.
Smeins, F.E. and L.B. Merrill 1988. Long-term Change in a Semi-arid Grassland. In. Edwards Plateau Vegetation - Plant Ecological Studies in Central Texas. Edited by B.B. Amos and F.R. Gehlbach. Baylor Univ. Press, Waco. 144p.
Texas State Soil and Water Conservation Board 1991. A comprehensive study of Texas watersheds and their impacts on watter quality and water quantity. Texas State Soil and Water Conservation Board, Temple TX.
Van Auken, O.W. 1988. Woody Vegetation of the Southeastern Escarpment and Plateau. pp. 43-56 In. Edwards Plateau Vegetation - Plant Ecological Studies in Central Texas. Edited by B.B. Amos and F.R. Gehlbach. Baylor Univ. Press, Waco. 144p.
Van Auken, O.W. 1993. Size distribution patterns and potential population change of some dominatn woody species of the Edwards Plateau region of Texas. Texas Journal of Science 45: 199-210.
Vines, R.A. 1960. Trees, Shrubs and Woody Vines of the Southwest. University of Texas Press, Austin 1104 p.
Weniger, D. 1984. The Explorer's Texas - The Lands and Waters. Eakin Press, Austin. 224 p.
Wink, R.L. and H.A. Wright 1975. Effects of fire on a juniper community. Journal of Range Management 26:326-329.
Wright, H.A., F.M. Churchill, and W.C. Stevens 1975. Effects of prescribed burning on sediment, water yield, and water quality from dozed juniper lands in Central Texas. Journal of Range Management 29:294-298.
Yager, L. Y. 1993. Canopy, litter and allelopatic effects of Ashe juniper (Juniperus ashei, Buchholz) on understory vegetation. M.S. Thesis, Dep. Rangeland Ecology and Management, Texas A&M Univ., College Station, TX. 109 p.
APPENDIX
| Length | 1 meter (m) = 3.28 feet |
| 1 centimeter (cm) = 0.394 inches | |
| 1 millimeter (mm) = 0.0394 inches | |
| Area | 1 square meter (m2) = 10.76 square foot |
| 1 hectare (ha) = 2.47 acres | |
| Weight
per area |
1 kilogram per hectare (kg/ha)
= 0.893 pounds per acre |
1. 1 Taxonomic nomenclature follows Hatch et
al. (1990)
2. All data are presented in metric units; See
the appendix at end of chapter for conversion to
english units.
3. Other names applied to the species: Post
Cedar, Rock Cedar, Mexican Juniper, Mountain
Cedar, Blueberry Juniper (Cedar), Texas Cedar,
Sabino, Enebro, Tascate, Taxate, Cedro.
4. Taxonomic nomenclature follows Ransom
(1981).
