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Soil Seed Banking in Pinyon-Juniper Areas
With Differing Levels of Tree Cover,
Understory Density and Composition
Clare L. Poulsen
Scott C. Walker
Richard Stevens
Abstract-With removal of competitive pinyon-juniper overstory,
endemic vegetation is released allowing germinating seed within
the soil seed bank to establish. Density of seedlings is closely
correlated with the density and composition of the understory
community and tree cover. Considerable effort and costs are generally put into seeding areas following tree removal. If the amount and
type of viable seed in the soil could be accurately estimated, seeding
costs and effort could be adjusted. Studies were conducted on soil
seed banking at two pinyon-juniper sites in central Utah. At each
site, areas were studied ranging from closed pinyon-juniper stands
with minimal understory to open stands with excellent understory
communities. Seed density and composition within the soil varied
with tree cover and understory community differences.
As pinyon -j uni per stands mature, canopy cover increases,
and a decrease in the understory community density and
composition occurs. The correlation between decreasing
understory species and increasing tree canopy cover on the
soil seed bank is poorly understood. By quantifying composition and viability of seeds existing in the soil seed bank, more
effective treatment of pinyon-juniper stands could occur.
Studies have looked at various influences and aspects of
the soil seed bank; fire (Valbuena and Trabaud 1995);
grazing (Kinucan and Smeins 1992); grasslands (Cofin and
Lauenroth 1989); forest lands (Chambers and others 1991);
and cultivated fields (Dessaint and others 1991). These
studies show that a soil seed bank with sufficient quantity
and viability of seeds can provide the needed seed reserves
for community regeneration. However, it has not been identified how tree canopy cover affects the number of species
and volume of viable seed in the soil seed bank. This study
was established to identify the density of seeds in a soil seed
bank at different percent canopy cover of pinyon-juniper.
In: Monsen, Stephen B.; Stevens, Richard, comps. 1999. Proceedings:
ecology and management of pinyon-juniper communities within the Interior
West; 1997 September 15-18; Provo, UT. Proc. RMRS-P-9. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Rocky Mountain Research
Station.
Clare L. Poulsen, Scott C. Walker are Research Biologists and Richard
Stevens was Project Leader (retired), Utah Division of Wildlife, 540 N. Main
32-7, Ephraim, UT 84627.
USDA Forest Service Proceedings RMRS-P-9. 1999
Methods _ _ _ _ _ _ _ _ __
Two study sites were on Utah Division of Wildlife land
6 miles north ofEphraim, UT. The area is 1,800 min elevation
with Amtoft flaggy loam and Quaker silty clay loam soils.
Annual precipitation for the area is 32.1 cm. The sites had
an average slope of 18 percent. The southern site has a west
aspect and the northern site has a north aspect. Each study
site contained three areas with distinctive pinyon-juniper
canopy cover classes; (1) >60 percent tree canopy cover with
depleted understory community; (2) >30 percent to <60
percent tree canopy cover with moderately depleted understory community; and (3) <30 percent tree canopy cover with
nondepleted understory community present. These three
cover class areas were adjacent to each another with similar
slope, soils, and exposure.
Within each canopy cover class area on each site one 25 x
25 m plot was identified. Within each plot five 25 m long
transects were randomly established. At every meter mark
along each transect one 0.25 m 2 quadrat was placed. Within
each quadrat species density and cover was determined for
grasses, forbs, and shrubs. Five soil samples were randomly
obtained along each transect for a total of 25 samples per
plot. Each soil sample consisted ofthe removal of all soil and
material from a 7 x 7 x 2.5 cm deep area (Cabin 1996 and
Garcia 1995). Soil samples were washed through a series of
three sieves (4 mm, 2 mm, and 250 Jlm) (Gross 1990). Sieved
soil samples were dried for 24 hours in a heated seed
germinator. Sieved samples were ocularlyinspected using a
lOx microscope. Filled seed was then removed and identified to species or group and recorded. Every tree in each plot
was measured for height and crown diameter. From the
diameter, the area was calculated for each tree. Percent tree
canopy cover per plot was calculated by total area of all
trees in a plot divided by total plot area.
Understory and seed bank species were placed in groups
by life form to simplify the analysis. The understory composition, vegetation life form mean density, and seed density
was analyzed using a one-way analysis ofvariance (ANOVA).
Life form groups were analyzed separately to ensure that the
outcome reflected changes in composition not overall abundance. The understory composition was collected by using a
modified Daubenmire (1959) cover class. The seven classes
are: (1) 0.01 to 1 percent, (2) 1.1 to 5 percent, (3) 5.1 to 25
percent, (4) 25.1 to 50 percent, (5) 50.1 to 75 percent, (6) 75.1 to
95 percent, (7) 95.1 to 100 percent. Cover class midpoints of
0.5,3,15,37.5,62.5,85, and 97.5 were used to calculate the
mean cover. Results for vegetation life form per ha and seed
density per m 2 were calculated and analyzed.
141
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J...
Q)
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90
0
()
80
Q)
a. 70
c: 60
::l
"
c: 50
800
40
400
J...
0
~
J...
Q)
a.
600
a.
c:
Q)
C
Q)
Q)
200
10
0
J...
Q)
~
==
t/)
c:
a. 30
+of
c:
20
Q)
(.)
ca
J:
depleted
•
mod. depleted
Percent Cover
nondepleted
~
0
Density per ha
Figure 1-Percent pinyon-juniper cover and numbers of trees associated
with the three understory vegetation classes.
Results ----------------------------------Percent tree cover and number oflive trees per ha for the
depleted, moderately depleted, and nondepleted understories were 69 percent cover with 864 trees per ha; 51 percent
cover with 576 trees per ha; and 8 percent cover with 128
trees per ha respectively (fig. 1). The understory percent
cover was significantly (P < 0.05) higher in the nondepleted
site as compared to the depleted site (fig. 2). Litter and rock
cover was lower in the areas w here the canopy was open and
understory vegetation was highest. Shrub, grass, and forb
cover was significantly higher with fewer trees present.
In the depleted understory 31 species were present. The
most common species were annual and perennial forbs. Both
moderately depleted and nondepleted understory had 27
species each. The vegetation shifted from annual and perennial forbs in the depleted understory to perennial grasses
and shrubs in the moderately depleted and nondepleted
understories (fig. 3). The most common perennial forbs
present with a high pinyon -j uni per canopy were phlox (Phlox
40
A
J...
30
Q)
>
0
(J
20
c:
Q)
+of
(.)
J...
Q)
a.
10
o
depleted
•
Shrubs
D
Rock
mod. depleted
Forb/grass
•
nondepleted
D Litter
Bare ground
Figure 2-Percent cover of understory composition in three understory
vegetation classes. *Columns for same understory component within a
series with same letter are not significantly different (P < 0.05).
142
USDA Forest Service Proceedings RMRS-P-9. 1999
ctI
.c 7000
.....
ci 6000
'-
CI)
c. 5000
>-
-
'ii)
4000
CI)
3000
c:
-
0
c: 2000
ctI
a::
c: 1000
ctI
CI)
0
::E
•
depleted
Annual Forbs
mod. depleted
•
Perennial Grasses •
Perennial Forbs
nondepleted
D
Annual Grasses
Shrubs
Figure 3-Mean plant density of understory species by life form per 0.1 ha.
*Columns for same life form within a series with same letter are not
significantly different (P < 0.05).
spp.), goldenweed (Haplopappus spp.), and cryptantha
(Cryptantha spp.). With a more open tree canopy, astragalus
(Astragalus spp.), globe mallow (Sphaeralcea coccinea), and
primrose (Oenothera spp.) were the prominent perennial
forbs. With a closed canopy cover broom snakeweed
(Gutierrezia sarothrae) was the common shrub. The shrub
component shifted to low rabbitbrush (Chrysothamnus viscidi{lorus), shadscale (A trip lex confertifolia), and big sagebrush (Artemisia tridentata) with a more open tree canopy.
Seed density in the soil seed bank was significantly greater
in the moderately depleted and nondepleted understory
compared to the depleted understory (fig. 4). The greatest
density of seeds was in the moderately depleted understory
with 7,240 seeds per m 2 (table 1). The depleted and
nondepleted areas had 3,884 and 6,236 seeds per m 2 respectively. Seeds for 15, 12, and 14 species were found in the soil
seed bank for the depleted, moderately depleted, and nondepleted communities respectively. The greatest number of
grass seeds was in the nondepleted understory (fig. 4 and
table 1). Perennial forbs, annual forbs, and tree seeds were
the most frequent in the moderately depleted community.
Density of understory vegetation with number and seeds
found in the soil was highly correlated with an R2 = 0.905
(fig. 5).
6000~------------~~~--------~--------------~
N
E
5000+-------------~
'CI)
~
4000+-------------~­
CI)
CI)
~ 3000
o
'-
CI)
~
2000
~
z
1000
o
•
mod. depleted
Annual Forbs
Perennial Forbs
Perennial Grasses •
Trees
D
Annual Grasses
Figure 4-Extrapolated total number of seeds in the soil seed bank by life
form per m2 • *Columns for same life form within a series with same letter
are not significantly different (P < 0.05).
USDA Forest Service Proceedings RMRS-P-9. 1999
143
Table 1-Number of species and number of seeds per species in the soil seed bank by life form
and understory vegetation class. Percent of total is included in parentheses.
Life form
Life form class
Understory vegetation class
Depleted
Mod. depleted
Nondepleted
- - - - - - - Number of species (percent)- - - - - - -
Annual forbs
Perennial forbs
Annual grasses
Perennial grasses
Woody
AF
PF
AG
PG
W
6
1
1
5
2
Total
(40)
(6)
(6)
(33)
(13)
3
2
1
4
2
15
Individual species present
(25)
(17)
(8)
(33)
(17)
4
2
1
6
1
12
(29)
(14)
(7)
(43)
(7)
14
- - - - - - - - Number of seeds (percent)- - - - - - - -
Bur buttercup
Mustard species
Composite species
Cryptantha
Cheatgrass
Bluebunch wheatgrass
Indian ricegrass
Needle-and-thread
Sanberg bluegrass
Squirreltail
Western wheatgrass
Juniper
Pinyon
Other species
AF
AF
PF
PF
AG
PG
PG
PG
PG
PG
PG
W
W
1,892
1,420
0
112
280
16
40
0
4
12
0
72
16
20
Total
3,884
(49)
(37)
5,352
340
224
12
496
296
20
0
308
0
16
168
4
4
(3)
(7)
(0.4)
(1 )
(0.1 )
(0.3)
(2)
(0.4)
(5)
(74)
(5)
(3)
(0.2)
(7)
(4)
(0.3)
3,364
976
4
4
1,212
352
68
4
168
4
16
48
0
16
(4)
(0.2)
(2)
(0.1)
(0.1)
7,240
(54)
(16)
(0.1)
(0.1)
(19)
(6)
(1)
(0.1)
(3)
(0.1)
(0.3)
(0.8)
(0.3)
6,236
6000
+
5000
C\I
E
;;;r-
'Cl)
D.
4000
"'C
/"
Cl)
Cl)
en
3000
~
0
'Cl)
.c
E
2000
::J
Z
1000
o W#
o
/V
7
V
V
/"
V
V
+
+
V
,
V+
1000
./
/
V
/
V
V
2000
3000
4000
5000
6000
7000
Mean Plant Density per 0.1 ha
Figure 5-Understory species density per 0.1 ha and number of seeds per m2in the soil seed
bank regression (R2 = 0.902).
144
USDA Forest Service Proceedings RMRS-P-9. 1999
Conclusions -------------------------------
References ____________________
A direct correlation was found between pinyon-juniper
canopy cover, density and composition of understory vegetation, and number of seeds in the soil seed bank. As tree
cover increased, the understory vegetation and number of
seeds in the soil seed bank decreases. Further research is
needed to look more closely into the correlation between the
canopy cover, understory vegetation and soil seed bank.
Refinements are also needed in techniques to reduce the
time requirements to separate soil and seed. As work continues in this area, the understanding of the correlation
between tree canopy cover and soil seed bank will allow the
use of more effective seed mixes. Seeding rates and cost of
seeding should decrease with increasing abundance of seed
in the soil seed bank.
Cabin, R J. 1996. Genetic comparisons of seed bank and seedling
populations of a perennial desert mustard, Lesquerella fendleri.
Evolution. 500: 1830-1842.
Chambers, J. C.; MacMahon, J. A.; Hafner, J. H. 1991. Seed
entrapment in alpine ecosystem: effects of soil particle size and
diaspore morphology. Ecology. 72:1668-1678.
Cofin, D. P.; Lauenroth, W. K. 1989. Spatial and temporal variation
in the seed bank of semiarid grassland. American Journal of
Botany. 76: 53-59.
Dessaint, F.; Chadeuf, R; Barralis, G. 1991. Spatial pattern analysis of weed seeds in the cultivated soil seed bank. Journal of
Applied Ecology. 28: 721-730.
Garcia, M. A. 1995. Relationships between weed community and
soil seed bank in a tropical agroecosystem. Agri. Eco. And Envrio.
50: 139-146.
Gross, K. L. 1990. A comparison of methods for estimating seed
numbers in the soil. Journal of Ecology. 78: 1079-1093.
Kinucan, R J.; Smeins, F. E. 1992. Soil seed bank of a semiarid
Texas grassland under three long-term (36-years) grazing regimes. American Midland Naturalist. 128: 11-21.
Valbuena, L.; L. Trabaud. 1995. Comparison between the soil seed
banks of a burnt and unburnt Quercus pyrenaica Willd. Forest.
Vegetatio. 119: 81-90.
Acknowledgments
Funds were provided through Federal Aid on wildlife
restoration project W-82-R, project 3 and Rocky Mountain
Research Station, Forest Service, Provo, UT.
USDA Forest Service Proceedings RMRS-P-9. 1999
145