Seed Fate of Warm-Season Perennial
Grasses
Laurie B. Abbott
Bruce A. Roundy
Sharon H. Biedenbender
The relationship between episodic rainfall and episodic
natural recruitment suggests that native and introduced
grass species may respond differently to soil moisture availability patterns (Roundy, 1994). This led us to ask the following research questions. First, how does seed fate response of a species vary with different patterns of rainfall
and soil moisture availability? Second, does seed fate response to soil moisture vary for different species?
Seed fate is the ultimate destiny of a planted germinable
seed. The fate of a seed depends on a combination of environmental cues and genetic characteristics of the species.
Depending on the environmental stimulus, a planted seed
could follow several different trajectories (Fig. 1). If germination requirements are met, a seed could initiate and complete germination. If conditions continue to be favorable,
that germinated seed could then emerge, grow, and establish. If the germination requirements are not met, that same
seed would not germinate, but could remain germinable in
the seed bank. The seed could remain viable and become
dormant under conditions that induce dormancy. Alternatively, environmental conditions or predation could result
in seed or seedling mortality at any of these stages. In this
paper we report studies that focused on seed fate response
at the initial germination stage.
Abstract—Vulnerability to moisture stress during germination,
emergence, and seedling growth stages may be a factor in differential survival of seeded grasses in revegetation projects. The fate
of eight warm-season perennial grasses under field rainfall and
soil moisture conditions was studied for two summers in southern
Arizona. Seeds were retrieved from the field periodically after
rainstorms to determine species-specific responses to wetting and
drying events. Native grasses germinated a significantly greater
portion of their seeds in response to initial rains than the introduced Lehmann lovegrass (Eragrostis lehmanniana). Initial rains
followed by long dry periods resulted in high native grass mortality. In contrast, ungerminated Lehmann lovegrass seeds remain
germinable throughout dry periods following an initial rainfall
event.
The use of native plants has become a common goal for
revegetation projects in recent years. However, in semidesert grasslands, seedings of native warm-season perennial
grasses often fail while plantings of introduced species are
successful (Cox and others, 1982; Roundy and Biedenbender,
1994). For example, in southern Arizona, the introduced
Lehmann lovegrass (Eragrostis lehmanniana) has been
commonly used in rangeland revegetation because of its
reliable establishment (Roundy, this proceedings). Natural populations of native grasses may exhibit an episodic
recruitment pattern. A better understanding of the germination and establishment requirements of these species
could be useful for planning revegetation strategies in this
region.
Precipitation in the Sonoran Desert region is bimodally
distributed, falling primarily in the summer and winter
months. Summer rainfall is episodic, such that both the
distribution of rain and total precipitation are variable.
The amount of soil moisture that is available to a germinating seed follows the same pattern as precipitation and
is episodic and highly variable.
Methods
Field experiments were conducted in the summer months
of 1992 and 1993. The research site was the Santa Rita
Experimental Range, located approximately 65 km south
In: Roundy, Bruce A.; McArthur, E. Durant; Haley, Jennifer S.; Mann,
David K., comps. 1995. Proceedings: wildland shrub and arid land restoration symposium; 1993 October 19-21; Las Vegas, NV. Gen. Tech. Rep.
INT-GTR-315. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station.
Laurie B. Abbott and Sharon H. Biedenbender are Graduate Research
Assistants, and Bruce A. Roundy is Associate Professor; Range Management Program, School of Renewable Natural Resources, University of
Arizona, Tucson, AZ 85721.
Figure 1—Possible fate of a planted germinable seed.
37
Table 1—Warm-season grasses seeded in a seed fate experiment
in southern Arizona.
of Tucson, AZ. The research plots were located on a 2 to 5%
sloped alluvial fan at 1,100 m elevation. Approximately 60%
of the 300 to 400 mm annual precipitation falls during the
summer rainy season. The soil is a Comoro sandy loam
(thermic Typic Torrifluvent) that varies in depth from 0.2 to
2.5 m (Hendricks, 1985). The plots are located in grassland
dominated by Lehmann lovegrass with a few mesquite
(Prosopis juliflora var. velutina) and burroweed (Isocoma
tenuisecta). Trees and shrubs were cleared from the plots
prior to beginning the experiments.
Soil moisture content was measured at depth intervals of
1 to 3, 4 to 6, 12 to 14, and 18 to 20 cm using buried fiberglass soil moisture sensors and measurement methods described by Roundy and others (1992). Five sensors were
buried at each depth interval in each of three blocks. Ambient climatic data (precipitation, air temperature, relative
humidity, wind speed, and incident solar radiation) were
measured at the plots. Measurements were recorded every
minute using Campbell Scientific Inc., CR-10 data loggers,
and stored as an hourly sum for precipitation and as hourly
averages for all other variables.
Seeds were planted twice each summer to examine how
changes in rainfall patterns affected seed fate response.
Seeds were planted on 16 June and 30 July in 1992, and
on 15 June and 2 August in 1993.
The experimental design was a split-plot randomized
block design; the main plot factor was grass species and
the sub-plot factor was retrieval date. A total of eight species were studied. The six native and two introduced species planted are listed in Table 1. Nylon cloth mesh bags
containing 10 pure live seeds of each species were buried
under 3 to 5 mm of soil. Each bag contained seeds from
only one species. For each species, six bags were buried
in each of three blocks (N=18).
The design included two separate retrievals of the bags.
In each retrieval three of the buried bags were removed
and examined to determine the effect of different patterns
of wetting and drying on seed fate response. The first retrieval was performed towards the end of the first wet-dry
cycle. We defined the initial wetting event when the top
3 cm of soil was wet for at least 24 hr after that rain event.
The first retrieval was performed after the top 1 cm of soil
began to dry. The second retrieval followed after a series
of wetting and drying events to see the effect of subsequent
wet-dry cycles on germination and germinability.
Retrieval bags were removed from the soil and brought
to the lab, where they were gently rinsed of soil and carefully opened. The seeds were inspected for germination.
Field-germinated seeds were counted and listed as ‘germinated.’ Ungerminated seeds were placed on blotter paper
in petri dishes, wetted up and placed in a 25 °C constant
temperature incubator. The petri dishes were checked
daily for germinated seeds. Seeds that germinated in the
petri dishes were counted and listed as ‘germinable.’ Seeds
that failed to germinate were listed as ‘dead or dormant.’
Analysis of the relative percentages of germinated, germinable, and dead or dormant seeds allowed us to determine the
seed fate response of each species as a function of rainfall
pattern and soil moisture availability.
Native
cane beardgrass
Arizona cottontop
plains lovegrass
sideoats grama
green sprangletop
bush muhly
Bothriochloa barbinodis
Digitaria californica
Eragrostis intermedia
Bouteloua curtipendula
Leptochloa dubia
Muhlenbergia porteri
Introduced from South Africa
Lehmann lovegrass
Eragrostis lehmanniana
Cochise lovegrass
E. lehmanniana x E. tricophora
Results
Rainfall and Soil Moisture Patterns
The 1992 rainfall patterns and resulting soil moisture
availability were strikingly different than the patterns in
1993. The plots received 96 mm of rainfall in July 1992
(Fig. 2). The rainfall was fairly well distributed, resulting
in relatively high soil moisture for at least seven days following the first rain. In August 1992 the plots received
115 mm of rain following the 30 July planting (Fig. 3).
Rainfall during this period was extremely episodic; nearly
90 mm fell in one 24-hr period. The resulting soil moisture
availability was also sporadic and inconsistent over time.
Therefore, in 1992 soil moisture availability was more consistent in July than in August.
The seasonal distribution of summer rainfall in 1993 was
very different from 1992 (Figs. 4 and 5). The plots received
33 mm of rain during July. The first rainfall was an isolated 11-mm event that resulted in a rapid increase and
Figure 2—Rainfall and volumetric soil moisture
content following the 16 June 1992 planting of
warm-season grasses in southern Arizona. Triangles indicate first and second retrievals of seed
bags. Dashed black line indicates soil water content at –1.5 MPa soil matric potential.
38
Figure 3—Rainfall and volumetric soil moisture content following the 30 July 1992 planting
of warm-season grasses in southern Arizona.
Triangles indicate first and second retrievals of
seed bags. Dashed black line indicates soil
water content at –1.5 MPa soil matric potential.
Figure 5—Rainfall and volumetric soil moisture content following the 2 August 1993 planting of warmseason grasses in southern Arizona. Triangles indicate first and second retrievals of seed bags. Dashed
black line indicates soil water content at –1.5 MPa
soil matric potential.
Seed Fate Response
Within species comparisons of the seed fate response
between the first and second retrieval of each planting
shows the effect of rainfall and soil moisture availability
patterns on seed fate (Table 2). For example, following the
16 June 1992 planting, the percentage of dead or dormant
cane beardgrass seeds increased from 1% to 15% between
the first and second retrievals. In the June 1992, June
1993, and August 1993 plantings the percentage of fieldgerminated Lehmann lovegrass seeds was significantly
lower (P<0.05) in the first retrieval than in the second
retrieval.
Between species comparisons following each planting
can help to show differences in seed fate response relative
to the rainfall and soil moisture patterns. The results for
three native species (cane beardgrass, Arizona cottontop,
and plains lovegrass) and one introduced species (Lehmann
lovegrass) are presented in histogram form to aid in the
comparison (Figs. 6 and 7).
Comparison between species sown 16 June 1992 shows
an overall trend for the native species to germinate a large
portion of their seeds by the first retrieval (Fig. 6). This
germination response was triggered by the initial rainfall
(Fig. 2). This pattern contrasts with Lehmann lovegrass,
which had only 13% of its seeds germinated by the first retrieval. In this case, 59% of the Lehmann lovegrass seeds
were germinable but ungerminated. Soil moisture availability was relatively consistent during July 1992, and by
the second retrieval 35% of the Lehmann lovegrass had
germinated.
Each of the species sown 30 July 1992 shows a trend toward greater seed mortality by the second retrieval (Fig. 7).
The rainfall was very sporadic and soil moisture availability
during this period generally fluctuated from wet to very dry
Figure 4—Rainfall and volumetric soil moisture
content following the 15 June 1993 planting. Triangles indicate first and second retrievals of seed
bags. Dashed black line indicates soil water content at –1.5 MPa soil matric potential.
decrease in soil moisture. The remainder of the rainfall
was distributed over 8 consecutive days. However, daily
totals during that period were of such small magnitude that
the resulting soil moisture availability was punctuated by
short dry periods. In August 1993 the plots received 125
mm of rain (Fig. 5). The 2 August planting was followed
by 7 consecutive days of rain, resulting in 10 days of high
soil moisture.
39
Table 2—Seed fate response of warm-season grasses seeded in 1992 and 1993 on a loamy upland range site in southern Arizona. Values are
percentage of total planted seeds. The first retrieval was toward the end of the first wet-dry cycle; the second was after a series of
wet-dry events.
1992
Planting Date
Retrieval
Response (% )
Species
cane beardgrass
Arizona cottontop
plains lovegrass
sideoats grama
green sprangletop
bush muhly
Lehmann lovegrass
Cochise lovegrass
16 JUNE
F
96
48
42
73
64
54
13*
64
FIRST
G
3
20*
28
6
9
11
59
16
30 JULY
D
F
SECOND
G
1*
32
30
21
27
36
27
20
85
54
69
70
68
56
35*
67
0
3*
2
0
0
0
39
3
D
F
15*
42
29
30
32
44
27
29
75
24
37
57
52
34
4
29*
FIRST
G
3
19
40
3
19
6
59
24*
D
F
SECOND
G
D
22
56
23
40
29
60
37
48*
64
35
18
48
29
38
13
17*
0
2
42
1
0
1
37
3*
36
63
39
51
71
60
49
81*
1993
Planting Date
Retrieval
Response (% )
Species
cane beardgrass
Arizona cottontop
plains lovegrass
sideoats grama
green sprangletop
bush muhly
Lehmann lovegrass
Cochise lovegrass
15 JUNE
F
7*
7
0*
48
7*
6
1*
0
FIRST
G
78*
47*
91*
51*
70*
61*
78
80*
2 AUGUST
D
F
SECOND
G
15
46
9*
1*
23
33*
21
20
80*
28
42*
55
46*
30
21*
50
3*
0*
9*
0*
0*
0*
62
8*
D
F
17
72
49*
45*
54
70*
17
42
83
56
66*
93
79
61*
11*
48
FIRST
G
0
0
11
0
0
0
68
13*
D
F
SECOND
G
D
17
44
23
7
21
39*
21
39
85
62
48*
97
88
78*
38*
61
0
0
15
0
0
0
39
01*
15
38
37
3
12
22*
23
38
1
F- field germinated; G- germinable (growth chamber germinated); D- dead or dormant.
* indicates significant difference (P < .05) in a response category between the first and second retrievals within a planting.
in most 24 hr periods (Fig. 3). Seeds of both cane beardgrass and Arizona cottontop had high germination in response to the initial rain, and subsequent mortality was
probably due to low soil moisture levels that were insufficient to sustain seedling growth and establishment.
Although 11 mm of rain fell during the initial rainfall of
the 15 June 1993 planting, each of the species showed a low
germination response (Fig. 8). Soil moisture following the
initial rainfall was available for only 1 day (Fig. 4) and was
not sufficient to initiate germination. However, by the second retrieval, the percentage of germinable seeds retained
by the native grasses had decreased significantly. Field germination of cane beardgrass, plains lovegrass and Lehmann
lovegrass all increased significantly.
The seed fate responses following the 2 August 1993 planting are typical of a rainfall pattern resulting in consistently
high soil moisture. The native grasses had high germination
in response to the initial rain, while only 11% of Lehmann
lovegrass seeds germinated in the field (Fig. 9). By the second retrieval, Lehmann lovegrass field germination had increased to 38%.
An overall trend emerges in which field germination of
the native species is high in response to an initial rainfall
event that results in moist soil for approximately 48 hr.
In a study on the effect of six different wet-dry periods on
seedling emergence and survival, Frasier and others (1985)
showed that the length of the initial wet period influenced
survival during the following dry period. If the initial
wet period was too short to initiate germination, or if that
wet period was long enough to promote vigorous seedling
growth, then the plant had an increased chance of surviving the ensuing dry period. Thus, a species that germinates
quickly has a decreased ability to survive if moisture availability becomes limited. Therefore, moisture availability
patterns interacting with germination rates of the native
grasses and Lehmann lovegrass may play a major role in
their differential establishment.
Conclusions
The native grass species studied tend to require shorter
periods of soil moisture availability to initiate germination
when compared to Lehmann lovegrass. Once germination
has begun, native grass seedling survival is dependent on
the pattern of soil moisture availability after the initial rain.
These species are subject to early seedling mortality if soil
moisture becomes limited. Therefore, the distribution and
extent of rainfall is an important factor when seeding native grasses.
In contrast, Lehmann lovegrass retains a comparatively
large portion of ungerminated, germinable seeds after the
initial rain event. The tendency for a portion of Lehmann
lovegrass seeds to remain viable and germinable after the
40
Figure 6—Seed fate responses of cane beardgrass, Arizona cottontop,
plains lovegrass and Lehmann lovegrass following the 16 June 1992 planting on a loamy upland range site in southern Arizona.
Figure 7—Seed fate responses of cane beardgrass, Arizona cottontop,
plains lovegrass and Lehmann lovegrass following the 30 July 1992 planting
on a loamy upland range site in southern Arizona.
41
Figure 8—Seed fate responses of cane beardgrass, Arizona cottontop,
plains lovegrass and Lehmann lovegrass following the 15 June 1993 planting on a loamy upland range site in southern Arizona.
Figure 9—Seed fate responses of cane beardgrass, Arizona cottontop,
plains lovegrass and Lehmann lovegrass following the 2 August 1993 planting on a loamy upland range site in southern Arizona.
42
first rain reduces its susceptibility to infrequent rains at
the beginning of the summer rainy season. Species that
produce both early- and late germinating seeds tend to be
favored in variable environments (Venable, 1989). This
pattern of delayed germination is a probable factor in the
successful establishment of Lehmann lovegrass in revegetation projects in southern Arizona.
Frasier, G.W.; Cox, J.R.; and Woolhiser, D.A. 1985. Emergence and survival response of seven grasses for six wetdry sequences. Journal of Range Management. 38: 372-377.
Hendricks, D.M. 1985. Arizona Soils. University of Arizona
Press, Tucson, Arizona.
Roundy, B.A. 1994. Relating seedbed environmental conditions to seedling establishment. In: Monsen, Stephen B.;
Kitchen, Stanley G., comps. 1994. Proceedings—ecology
and management of annual rangelands; 1992 May 18-21;
Boise, ID. Gen. Tech. Rep. INT-GTR-313. Ogden, UT:
U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 295-299.
Roundy, B.A.; Biedenbender, S.H. (1994). Revegetation in
the desert grassland. In: McClaran, M. P.; Van Devender,
T.R., eds. Desert Grasslands, Univ. of Arizona Press,
Tucson, Arizona. In Press
Roundy, B.A.; Winkel, V.K.; Khalifa, H.; Matthias, A.D.
1992. Soil water availability and temperature dynamics
after one-time heavy cattle trampling and land imprinting. Arid Soil Research and Rehabilitation 6: 53-69.
Venable, D.L. 1989. Modeling the evolutionary ecology
of seedbanks. In: Leck, M.A.; Parker, V.T.; Simpson,
R.L., eds. Ecology of Soil Seed Banks, Academic Press,
San Diego, California: 67-87.
Acknowledgments
The authors thank Kevin Williams for his assistance
in the field. This research was supported in part by the
Arizona Agricultural Experiment Station and the Rangeland Competitive Grants Program.
References
Cox, J.R.; Morton, H.L.; Johnsen, T.N.; Jordan, G.L.;
Martin, S.C.; Fierro, L.C. 1982. Vegetation restoration
in the Chihuahuan and Sonoran Deserts of North
America. USDA Agricultural Research Service, Agricultural Reviews and Manuals, ARM-W-28.
43