Ecotypic Responses of Indiangrass (Sorghastrum nutans)
to Varying Water Regimes
Kimberly F. Elsenbroek, Elizabeth M. Bach, and Sara G. Baer
Department of Plant Biology & Center for Ecology, Southern Illinois University Carbondale
Climate change is expected to occur (IPCC 2007), although
the degree and trajectory of change are highly uncertain for
the Midwest (Harris et al. 2006). Climate change models for
this region are also uncertain, as the Midwest could
experience an increase or decrease in amount of and
variability in precipitation. Dominant prairie grasses occur
across a precipitation gradient from western Kansas to
Illinois and these dominant grasses regulate the function of
native and restored prairie (Baer et al. 2002, Smith &
Knapp 2003). Understanding the response of dominant
species from different regions and climate is needed to
identify which population sources or ecotypes are best
adapted to different precipitation regimes to predict sources
that should be used to restore prairie under different climate
scenarios. Ultimately, the long-term success of a restoration
may depend on selecting population sources best suited for
future conditions (Lande & Shannon 1996, Lesica &
Allendorf 1999).
Indiangrass seeds were collected from three sites that
receive approximately 400 mm (Hays, KS), 800 mm (Konza
Prairie, KS), and 1200 mm (southern Illinois) of precipitation
annually. The seeds were germinated in vermiculite and 6
seedlings were planted in pots with a 1:1 sand/soil ratio
according to 12 treatments resulting from a combination of
4 ecotype sources (Hays, Konza, IL and mixed) and 3
watering treatments (300, 600, and 900 mm). Each pot
was thinned to 3 seedlings. Mixed ecotype pots contained 1
seedling from each source. Each treatment was replicated
5 times (n=60). Greenhouse lights were on for 8 hrs/d and
temperatures ranged from 21-29oC. The experiment was
conducted for 12 weeks and plants were watered every 4
days.
The overall objective of this project was to demonstrate
whether single ecotype or mixed population sources of a
dominant prairie grass, Indiangrass (Sorghastrum nutans),
perform differently under a range of watering regimes.
H1: Ecotypes from dry climates (Hays and Konza Prairie
sources from Kansas) will perform better (i.e., higher
photosynthesis rates and biomass production) in the lowest
watering treatment and ecotype from.
H2: Mixed ecotypes will perform better under drought
conditions than single ecotypes from the wetter climate
(i.e., southern Illinois sources).
P=0.03
Hays
Konza
Illinois
Mixed
6
A,x
5
B,x
B,x
B,x
C,x
A,x
A,y
A,x
A,y
BC,x
3
AB,y
2
Data Analyses
Net photosynthesis rates and plant biomass data were
analyzed using the mixed model procedure in SAS (SAS
Inst. 2003), with number of plants/pot as a random effect.
Acknowledgements: John Miller, Dana Carpenter, Meredith Mendola and Ryan Klopf provided
valuable assistance with this project.
B
15
15
10
10
5
5
0
0
Hays
Konza
Illinois
Population Source
East
Hays
West
Konza
Illinois
Population Source
East
Figure 3. Average net photosynthesis rates ( standard error) in pots
planted with one ecotype were highest in plants from Kansas (Hays and
Konza) across all watering treatments. Means with the same letter were not
significantly different (α = 0.05).
0
300
600
900
Target Rainfall Amount (mm)
Figure 1. Average aboveground biomass (± standard
source of Indiangrass in all watering treatments. Letters
differences among ecotypes in each treatment; letters
differences between watering treatments within an ecotype.
same letter were not significantly different (α=0.05).
error) of each
A-C represent
x-y represent
Means with the
• Aboveground biomass in pots containing all sources
(“mixed pots”) or just plants from Hays were the least
variable across the three watering treatments (Fig. 1).
• At the end of the experiment each plant was clipped,
dried at 55oC, and weighed to calculate the aboveground
biomass. Roots were separated from the soil, washed,
dried at 55oC, and weighed to determine belowground
biomass. Total biomass was calculated from the sum of
above and belowground biomass.
20
A
1
• Most differences in above and belowground biomass
among the sources occurred in the 300 and 600 mm
watering treatments (Figs. 1 & 2), but were not
consistent among the sources in the 300 and 600 mm
watering treatments.
• Net photosynthesis was measured after 12 weeks of
growth using the LiCor 6400. Photosynthetically active
radiation (PAR) in the LiCor leaf chamber was set to
approximate conditions in the greenhouse.
n.s.
A
20
Mixed Ecotypes/Pot
25
P<0.001
West
A,y
• Above and belowground biomass were similar among
the sources collected across the precipitation gradient
exposed to the highest watering treatment (Figs. 1 & 2).
Plant Responses
Objectives & Hypotheses
7
4
One Ecotype/Pot
25
Net Photosynthesis Rate
( mol m-2 s-1)
Design and Treatments
Results
• Belowground biomass was most consistent in the “mixed
ecotype” pots across all watering treatments (Fig. 2). In
the lowest watering treatment, Konza and IL sources had
higher belowground biomass than Hays or mixed
sources. Hays plants had the highest belowground
biomass in the 600 mm watering treatment (Fig. 2).
7
Belowground Biomass (g/pot)
Less than 5% of the tallgrass prairie ecosystem remains
undisturbed from agriculture and other human impacts.
Tallgrass prairie is widely restored throughout the Midwest,
however there are uncertainties about the persistence of
these restorations if climate changes. Geographic and
climatic conditions should be considered when selecting
species and population sources for use in restoration
(Hufford & Mazer 2003). Seeds are often collected locally
because plants are considered adapted to the nearby
environment (Lesica & Allendorf 1999). However,
adaptation to changing environmental conditions is a slow
process, and use of non-local native species has been
proposed for consideration in
restoration if rapid
environmental change may occur (Harris et al. 2006).
Methods
Aboveground Biomass (g/pot)
Introduction
P=0.02
Hays
Konza
Illinois
Mixed
6
5
A,x
A,x
4
B,x
C,x
B,y
A,y
A,y
A,y
A,y
B,y
3
AB,y
2
A,y
1
0
300
600
900
Target Rainfall Amount (mm)
Figure 2. Average belowground biomass (± standard
source of S. nutans in all watering treatments. Letters
differences among ecotypes in each treatment, letters
differences between watering treatments within an ecotype.
same letter were not significantly different (α=0.05).
error) of each
A-C represent
x-y represent
Means with the
• Net photosynthesis rates (Anet) were higher in both sources from KS
than IL across all watering treatments. Anet did not differ among the
watering treatments.
• All sources exhibited similar Anet when planted together in the
mixed ecotype pots. Overall Anet in the mixed pots was lower than
the other ecotypes growing separately (average Anet in Hays,
Konza, IL, and mixed ecotype pots across all watering treatments
was 18.24, 17.31,13.79,11.65 µmol/m2/s, respectively).
Conclusions
• Few studies have documented ecotypic variation in
native prairie grasses (but see McMillan 1959, 1965).
Differences in physiological performance of Indiangrass
collected from different source populations grown in a
common soil and controlled environment demonstrates
ecotypic variation in this commonly co-dominant prairie
grass exists.
• Biomass did not increase with an increase in water
supply as predicted due to high productivity in the lowest
water treatment. This may be explained by the high
water use efficiency of prairie grasses (Knapp et al.
1998, 2002) and adaptation to drought in this species
across the precipitation gradient.
• The least variation in biomass among ecotypes of
Indiangrass occurred in the highest watering treatment.
Thus, ecological consequences for ecotypic variation in a
dominant species may vary with climate.
• Using broad collections of species has been
recommended for restorations where suitability of
population sources is unknown due to novel or changing
conditions (Harris et al. 2006). The mixed ecotype pots
showed no variation among sources in biomass across
watering treatments or photosynthesis when all sources
were planted together. These results suggests that
combining ecotypes may stabilize productivity under a
wider range of precipitation regimes, but field tests are
needed to predict whole ecosystem response.