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RELATING SEEDBED
ENVIRONMENTAL CONDITIONS
TO SEEDLING ESTABLISHMENT
Bruce A. Roundy
ABSTRACT
Laboratory evaluations of germination and seedling
growth are often conducted under static environmental
conditions, such as static temperature and water potential. However, these responses are difficult to interpret
in terms of field establishment due to the dynamic environmental conditions of field seedbeds. Concurrent
measurement of field environmental conditions and biologic responses (such as germination, root growth, seedling survival) may be necessary to fully understand why
seedings succeed or fail, what the establishment requirements are for different plant materials, and what biological characteristics are adaptive for establishment under
certain environmental conditions.
Most revegetation studies measure only precipitation and
seedling density, cover, or forage production. Concurrent
and continuous measurement of seedbed water and temperature conditions and seedling emergence may help explain why certain species and seedbed treatments are successful, while others are not. Seedbed environmental data
may help construct laboratory experiments that are more
representative of field conditions for evaluating successful
establishment characteristics. A challenge for revegetation
scientists is to construct seedling establishment models
to predict responses under different climatic and weather
conditions.
MEASURING SEEDBED
ENVIRONMENT
INTRODUCTION
The current emphasis on using native species for revegetation necessitates an understanding of their establishment requirements. Traditionally, revegetation studies
have been empirical, evaluating the density, cover, and
production of different species and plant materials on different seedbeds, soils, and years. This empirical testing
has produced revegetation guidelines, principles, and specific site recommendations for revegetation of semiarid
rangelands (Roundy and Call1985). However, this approach has usually not provided the information necessary
to understand why certain plant materials and techniques
succeed or fail on different sites or in different years (Call
and Roundy 1991). Consequently, we still do not know
why many native plants are hard to establish and what
environmental conditions are required for their successful
establishment.
Plant improvement scientists have emphasized the importance of characterizing the establishment environment
and selecting for adaptive establishment characteristics
(Johnson and others 1981; Wright 1975). However, it has
been difficult to identify morphological and physiological
characteristics that can be consistently used to develop selection criteria for improved plant establishment (Johnson
and others 1981). These researchers have attempted to
evaluate plant materials under simulated drought conditions using specialized techniques and growth chambers.
Although the plant materials have often shown differential
response to these simulated drought conditions, it has often been difficult to show that superior plant materials in
the laboratory also have superior establishment in the
field.
An important first step is to quantify the environmental
conditions such as the soil moisture and temperature conditions of the seedbed. Only a few of the hundreds of revegetation studies have concurrently measured soil moisture and temperature conditions during seedling establishment. Most of these studies have sought to determine
the benefits of seedbed modification for revegetation.
McGinnies (1959) sampled surface soil moisture gravimetrically every 2 to 4 days in April and May to determine the benefit of different furrow depths for maintaining available soil water for germination on a silt-loam soil.
Herbel (1972) measured temperature and soil water potential at 1-3 em to show advantages of pitting, furrowing,
and mulching in New Mexico during summer rainfall.
Springfield (1972) reported decreased soil temperature
and increased time of soil water availability under various
mulches in New Mexico. Evans and Young (1987) pioneered studies of seedbed environmental conditions, as
modified by microtopography and plant litter, in relation
to the establishment of cheatgrass (Bromus tectorum) and
other weedy range plants. They used large equipment
and strip-chart recorders to continuously measure soil
moisture, temperature, and humidity.
Electronic microloggers are now available to continuously read a variety of sensors. Data can be easily transferred to a computer and handled with spreadsheet or
data base software. Pin-point soil temperatures can be
easily measured by small thermistors or thermocouples,
but the measurement of soil water content or potential at
the microscale of most seeds and seedlings is not possible
due to the large size of the sensors. Gypsum blocks and
fiberglass soil cells are commonly 2 em in width or diameter. Gypsum blocks are sensitive from -0.1 to -1.5 MPa
of matric potential while fiberglass soil cells are sensitive
Paper presented at the Symposium on Ecology, Management, and Restoration oflntermountain Annual Rangelands, Boise, ID, MaylS-22, 1992.
Bruce A Roundy is Associate Professor of Range Management, School
of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721.
295
to water contents from saturation to air dry. Soil cells
should be individually calibrated for greatest accuracy.
Fiberglass cells and gypsum blocks can be continuously
read by electronic microloggers and are either less expensive, more reliable, or measure a smaller volume than
psychrometers, neutron probes, or the promising timedomain reflectrometry (TDR) technology (Call and
Roundy 1991).
Smaller soil water sensors are needed to best determine
seed-soil water relations relative to germination and seedling growth. Most seeds are appropriately buried 0.5 to
1.5 em deep, but most moisture sensors are difficult to
bury more shallow than a 1-cm depth where they sense
a 1- to 3-cm interval. Small changes in soil water conditions near seeds on the surface or buried at a shallow
depth may be difficult to detect with current sensors or
sampling methods but could greatly affect germination
responses (Harper and others 1965; Winkel and others
1991).
An advantage of continuously measuring soil moisture
and temperature conditions is that it may help to quantify
how effective certain seedbed modifications are under different weather conditions. Jackson and others (1991)
used thermocouples and gypsum blocks to determine the
effects of mulch and catchment berms for establishment
of desert saltbush CAtriplex polycarpa) on an abandoned
farmland site in the Sonoran desert. After spring rains,
mulch reduced 1-cm temperature peaks by 9 °C andresulted in continuously available soil water at 1- to 3-cm
depths for 20 days, while nonmulched plots had intermittently available water (fig. 1). Berms and mulch also
lengthened the period of subsurface water availability
that was associated with higher establishment and vigor
of desert saltbush.
Continuous monitoring of soil water content using fiberglass cells has shown that seedbed disturbances such as
cattle trampling and land imprinting may lengthen the
available water period enough to improve seedling establishment during moderately wet, but not dry, summer
rainy seasons in the Southwest (Roundy and others 1992).
Disturbed seedbeds in 1988 were associated with a
shorter period when soil water was unavailable and also
higher seedling emergence than undisturbed seedbeds
(fig. 2). In 1989, available water periods were too short
for all treatments to permit successful seedling establishment. This study suggests that the period and timing of
available soil water (water content above some corresponding matric potential) may be more critical to seedling establishment and of more interpretive value than
the actual soil matric potential at a given time.
Monitoring of soil moisture and temperature along with
specific canopy manipulations allowed Sumrall and others
(1991) to determine that increased seedling recruitment
of Lehmann lovegrass (Eragrostis lehmanniana) after
burning was due to modification of the microenvironment
associated with canopy removal. Removal of the lovegrass
canopy resulted in increased diurnal temperature fluctuations and presumably red light at the seedbed, both of
which stimulate Lehmann lovegrass germination (Roundy
and others 1992a, c). Increased emergence of lovegrass
on burned and mowed plots was not associated with
greater soil water availability since these plots dried out
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Figure 1-5oil water tension (MPa) at 6 a.m.
at 1-3 em (top) and 13-15 em (bottom) depths
above catchment berms on abandoned farmland in the Sonoran desert. Cultivation and especially mulching delayed increases in water
tension after rain and increased establishment
of desert saltbush (Jackson and others 1991 ).
more quickly than those with an intact canopy. This
information is valuable because it suggests strategies for
replacing this exotic grass with shade-tolerant native
grasses. The lovegrass could be killed with an herbicide
and used as a mulch to inhibit its own germination and
conserve soil water for native grass establishment.
BIOLOGICAL RESPONSES TO
ENVIRONMENT
Microenvironmental monitoring may allow us to define
most probable soil temperature and moisture scenarios.
This may allow us to not only conduct laboratory tests of
germination and seedling growth under more representative environmental conditions, but also to determine what
biological responses are most likely to succeed under particular seedbed conditions.
For example, laboratory germination responses under
actual field temperature curves may be more representative of field germination responses for some species, compared to responses under constant or abruptly alternating
temperatures (fig. 3). This may be especially true for germination tests under cooler temperatures.
296
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Figure 3-Diumal bare soil temperatures at 1 em
in the semidesert grassland in southern Arizona.
Data are from means of two sites when the soil was
wet (matric potential > -o.1 MPa) during the summer rainy season (Roundy and others 1992b, c;
Sumrall and others 1991 ).
1989
Heavily trampled
Imprint basin
2
6
10
14
18
22
26
actual field temperature conditions on breaking of dormancy and germination responses as determined in the
laboratory may provide important clues to the establishment requirements of some species and may have enhanced interpretive value for explaining field emergence.
An understanding of actual seedbed temperature and
moisture dynamics is necessary to determine biological
responses that result in successful establishment in certain environments. For example, ability to germinate
and develop roots under cool temperatures has been identified as a major reason that cheatgrass is successful on
Intermountain rangelands (Harris and Wilson 1970). Researchers have concluded that successful perennial grass
competitors would also need to germinate and develop
roots during the winter and early spring to be able to compete with cheatgrass for stored soil moisture during the
spring growth period (Aguirre and Johnson 1991).
However, rapid root growth may not necessarily be
most associated with ability of warm-season grasses to
establish during the summer rainy season in the semidesert grassland in southern Arizona. In fact, introduced
grasses such as Cochise (Eragrostis lehmanniana x E.
tricophera) and Lehmann lovegrass are more easily established in revegetation projects, but have less and slower
root growth than other grasses such as sideoats grama
(Bouteloua curtipendula) and blue panicgrass (Panicum
antedotale) (Simanton and Jordan 1986). These warmseason grasses elongate their subcoleoptile internodes to
produce adventitious roots at the soil surface (Hyder and
others 1971; Winkel 1990). They generally require 9 to 11
days of continuously available soil moisture to develop adventitious roots (Winkel 1990). This suggests that prolonged available moisture after germination or the ability
to tolerate drought until consistent rainfall occurs is necessary for these species to successfully establish.
Laboratory root growth studies (Winkel1990) and field
moisture measurements (Roundy and others 1992b)
30
July
Figure 2-Periods of available water (volumetric
water content greater than a corresponding
matric potential of -o.1 MPa) at a 1-3 em depth
as indicated by crosshatching, for different seedbed treatments on a loamy upland range site in
southern Arizona during two summer rainy seasons. Seedbed disturbance slightly increased
the period of available water and was associated
with greater seedling emergence in 1988 but not
1989 (Roundy and others 1992b).
Abrupt temperature alternations between maximum
and minimum temperatures may result in a longer time
above a given threshold temperature in a 24-hour period
than would occur under field temperature alternations
between the maximum and minimum. Abruptly alternating temperature tests could result in higher germination
estimations in the laboratory for cooler or widely fluctuating temperature extremes than would actually occur in
the field. On the other hand, abrupt temperature alternations may help to break dormancy of some species and
increase germination compared to gradual alternations.
Continuous concurrent measurement of soil temperature and moisture permits determination of actual diurnal temperature curves when water is available in the
seedbed. These curves can easily be programmed into
"ramping" growth chambers to permit germination testing
under actual field temperature conditions. The effects of
297
suggest that if germinating rains are not followed by subsequent rains, the soil drying front will proceed faster
than seminal root growth and seedlings will desiccate
(fig. 4). Although sideoats grama seminal roots grow
faster than those of Cochise lovegrass, they are not fast
enough to stay ahead of the soil drying front in the absence of rain.
Just why the exotic lovegrasses are more easily established than native grasses in the semidesert grassland is
the subject of an ongoing "seed fate" study where we are
trying to determine field germination and seedling survival responses in relation to specific rainfall events and
subsequent dry periods. Frasier and others (1985, 1987)
have shown that sideoats grama may germinate rapidly
under a short wet period and thereby be vulnerable to
seedling desiccation during the subsequent dry period.
The introduced lovegrasses may be slower to germinate
under a short wet period (Frasier and others 1985, 1987)
and more likely to survive by requiring a longer wet period to germinate. They may avoid the initial summer
rainstorms in July, which are usually followed by a dry
period, and save their germination for more consistent
rainfall in late July and August.
[
t
Seedbed water and
temperature models
1. Establishment predictability
3. Selection criteria
4. Site limitations
Figure 5-Relating seedbed environmental
dynamics to plant establishment requirements through construction of seedling establishment models could advance the
predictability of revegetation science.
This example illustrates the need to relate meteorological conditions and actual field temperature-moisture conditions to biological establishment responses. Because
of all the different possible meteorological scenarios, this
problem should be addressed by development of seedling
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Biological response
models
2. Establishment requirements
ESTABLISHMENT MODELS
-
Meteorological
Models-scenarios
16
Time (days)
Figure 4-Maximum seminal root depth as a function of time since seed wetting for three warmseason grasses (Winkel1990). Small dashed
lines represent the time that soil matric potentials
decrease to s -1.5 MPa at a given depth after the
end of a 2-day rainstorm on two typical sandy
loam soils during the summer rainy season in
southern Arizona (data from study of Roundy and
others 1992b and unpublished data, University of
Arizona, Tucson). In the absence of subsequent
rain, the soil drying front may exceed the rate of
seminal root elongation.
298
establishment models. These models have not been developed, but would have three basic parts (fig. 5): (1) a meteorological model to determine and set weather conditions of known probabilities; (2) an environmental model
to estimate seedbed water and temperature conditions
as a function of soil characteristics and meteorological inputs; and (3) a biological response model to estimate establishment responseB such as germination, root and
shoot growth, and seedling survival in relation to temperature and moisture dynamics and thresholds .
Meteorological models and weather generators are
available. Probabilities of different weather patterns over
different relevant time scales need to be determined. A
number of temperature and moisture models are available
(Chung and Horton 1987; Lascano and van Bavel1983)
but most have not been evaluated or validated with
rangeland seedbed environmental data, which have been
lacking in the past (Wight and Hanson 1987). Biological
response data must be developed under more representative sets of field temperature and moisture dynamics to
determine representative germination and growth rates
and appropriate thresholds.
Using environmental models to drive biological response models necessitates a greater understanding of
the relationship between temperature and moisture dynamics and germination and seedling growth than we
now have for most species. However, development of
revegetation science in this direction may allow us to be
much more realistic and predictive of revegetation success, without years of extensive trial-and-error field
testing. We should be able to better determine plant
requirements for establishment, site suitability for different plant materials, appropriate selection criteria for
enhanced establishment, and the value and necessity of
various seedbed modifications for different climates and
weather conditions (fig. 5). This mechanistic approach
may also be helpful in determining compatible species
for establishment of more diverse plant communities.
temperature profiles. Soil Science Society of America
Journal. 47: 441-448.
McGinnies, W. J. 1959. The relationship of furrow depth
to moisture content of soil and to seedling establishment on a range soil. Agronomy Journal. 51: 13-14.
Roundy, B. A.; Call, C. A. 1988. Revegetation of arid
and semiarid rangelands. In: Tueller, P. T., ed. Vegetation science applications for rangeland analysis and
management. Boston: Kluwer Academic Publishers:
607-635.
Roundy, B. A.; Taylorson, R. B.; Sumrall, L. B. 1992a.
Germination responses of Lehmann lovegrass to light.
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Roundy, B. A.; Winkel, V. K.; Khalifa, H.; Matthias, A. D.
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Roundy, B. A.; Young, J. A.; Sumrall, L. B.; Livingston, M.
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Simanton, J. R.; Jordan, G. L. 1986. Early root and shoot
elongation of selected warm-season perennial grasses.
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Springfield, H. W. 1972. Using mulches to establish
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