Transcript Schneider2Spr2012x

With changing global precipitation patterns, will the growth of prairie plants be more
constrained by reduced rainfall frequency or reduced rainfall volume?
Adam Schneider*, Molly Kreiser, Greg Nelson ● Faculty Mentor: Tali Lee
Department of Biology-University of Wisconsin Eau Claire
*[email protected]
Introduction
Results
11.4
Volume per
event (mL)
300
600
150
↓ Freq. Freq.
/ ↓ Vol.
Reduced
and Vol.
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
24.0
0
1
6
Measurements
• Volumetric % soil moisture (immediately before and one day after each
watering event)
• Nitrogen stable isotope ratios (atom % 15N vs 14N)
• Total number of leaves per plant that were expanding, expanded,
senescing, or fully senesced (during weeks 5, 7, and 13)
• Total accumulated shoot and root biomass
• Leaf gas exchange (net photosynthetic CO2 uptake and stomatal
conductance of H2O) using a portable infrared gas exchange analyzer
between watering treatments (week 11)
15
10
Lupinus
30
**
20
10
5
0
Mean Days Since Last Reduced-Frequency Watering
Control
↓Frequency
↓Volume
↓Vol. / ↓Freq.
↓Frequency
Watering Treatment
↓Volume
↓Vol. / ↓Freq.
Watering Treatment
20.0
Figure 3. (A) Reduced water treatments did not affect the number of leaves produced by Agropyron or
Lupinus. However, (B) 50% reduced rainfall volume increased the proportion of leaves in senescence of
18.0
16.0
Measuring soil moisture
14.0
Lupinus, although Agropyron senescence was not significantly affected. Leaves were counted at weeks 5,
7, and 13. Error bars indicate standard error; asterisks indicate a significant difference from the control (p<0.05).
12.0
10.0
Effects on Root, Shoot, and Total Biomass
6.0
Agropyron
Lupinus
3.5
3.0
0.0
1
6
7
Mean Days Since Last Reduced-Frequency Watering
11
Root to Shoot Ratio
0
Figure 1. Effect of watering treatments on soil moisture of pots containing
single individuals of Agropyron repens and Lupinus perennis. Soil moisture was
measured in all pots immediately before and one day after any watering event.
Points represent the mean ± SE of eleven replicates over eight watering cycles
(n=88). Orange bars indicate mean soil moisture field data at six and twelve days
after a >1 cm rain event in experimental grasslands at Cedar Creek Ecosystem
Science Reserve in east-central Minnesota (May-July, 2000-2010).
0.9
A
A
3.5
A
AB
AB
3.0
2.5
2.5
2.0
2.0
B
1.5
20
1.5
1.0
1.0
0.5
0.5
0.0
0.0
Control
↓ Freq.
↓ Vol.
↓ Freq./
↓ Vol
Control
9.0
9.0
8.0
8.0
7.0
7.0
A
B
0.8
C
C
0.7
0.6
6.0
A
5.0
AB
3.0
2.0
2.0
0.4
1.0
1.0
0.3
0.0
0.0
Control
0.2
↓ Freq.
↓ Vol.
Watering Treatment
0.1
0
↓ Frequency
↓ Volume
↓ Freq. / ↓ Vol.
Watering Treatment
Figure 2. Compared to the control, Lupinus showed a reduction in the
proportion of N derived from fixation of 13% in a reduced volume situation
and a 22% reduction in a reduced frequency and volume situation.
Treatments not connected by the same letter are significantly different (p<0.05).
↓ Freq./
↓ Vol
10
-10
0
-10
-20
-30
-40
-50
-60
C
↓ Freq.
↓ Vol.
↓ Freq./
↓ Vol
Watering Treatment
Figure 4. Biomass accumulation was significantly reduced in Lupinus
but not in Agropyron under decreased water availability. The ratio of
accumulated root to shoot biomass did not change in Lupinus, but there
appeared to be a weak antagonistic effect between decreased volume and
decreased frequency in Agropyron. Treatments not connected by the same
letter are significantly different (p<0.05). Bars represent the mean ± SE of
eleven replicates.
-20
-30
-40
*
*
*
-50
-60
-70
-80
-70
*
↓ Freq. /
↓ Vol.
Figure 5. Percent changes in leaf
B
10
0
Lupinus
Watering Treatment
A
Control
*
B
Agropyron
Watering↓Treatment
↓ Frequency
Volume
4.0
B
↓ Freq. /
↓ Vol.
5.0
3.0
0.5
↓ Vol.
6.0
AB
4.0
↓ Freq.
A
A
B
Control
A 30
% Change in Photosynthesis
4.0
Effects on Leaf Physiology
and Water Use Efficiency
% Change in Conductance
8.0
2.0
After watering
Experiment Design
• Plants grown in cylindrical pots (5.6 cm dia., X 30 cm; 3.0 L)
• Soil collected from experimental grasslands at Cedar Creek Ecosystem
Science Reserve in east-central Minnesota.
• Single individuals of Agropyron repens and Lupinus perennis grown
under standard greenhouse conditions for fourteen weeks; 14 hour
photoperiod.
• Water availability was manipulated by reducing the total water volume
by 50%, reducing the frequency of watering events by 50% or both
(N=88, Table 1).
o Plants were well watered until they developed two true leaves, at
which point they were thinned to one per pot and treatments began.
o Watering treatments were determined from a pilot study to simulate
rain volumes, frequencies and resulting soil moisture availabilities
experienced at Cedar Creek Ecosystem Science Reserve.
• Each pot was fertilized 1.70 mg of NH4NO3 enriched with 15N (0.3850
atom% 15N) four times to determine the proportion of N derived from N2
fixation in Lupinus by isotope analysis.
20
22.0
1
Materials and Methods
**
25
0
11
7
B
Lupinus
% Change in proportion
of leaves in senescence
18.0
Total Number of Leaves Produced
Volumetric Soil Moisture (%)
Reduced
Frequency
↓ Frequency
Reduced
Volume
↓ Volume
20.0
Agropyron
30
40
Effects on Legume Nitrogen Fixation
300
Statistical Analyses
• Data were analyzed using ANOVA (JMP 8.0.2.2).
22.0
Total Dry Biomass (g)
Control
↓Frequency
↓Volume
↓Frequency
/ ↓Volume
Average days
between watering
5.7
11.4
5.7
A
Control
Control
physiology of plants grown with
reduced water availability compared
to the control. Effect on leaf net
photosynthetic rates was variable
(A); however, due to a decrease in
conductance (B), the water use
efficiency (CO2 assimilation per unit
water loss) increased significantly in
Agropyron under combined water
stress conditions whereas Lupinus
had a more variable response (C).
Error bars represent standard error;
asterisks indicate a significant
difference between treatments and
the control (** = p<0.05; * = p<0.10).
Discussion and Conclusions
The watering treatments strongly affected the soil water content of each pot though remained realistic to what a native Upper-Midwest prairie
plant may experience during a growing season in terms of both water availability and rainfall frequency (Table 1, Fig. 1).
Specific responses varied by species, but in general, reduced rainfall volume was found to be more limiting than reduced rainfall frequency. It
is well established that N fixation by legumes is sensitive to drought, though research has been mostly limited to agricultural species (Serraj et. al.
1999). In our study, reducing watering frequency did not affect N fixation, whereas reducing in total water volume significantly reduced the
proportion of plant N derived from fixation in Lupinus. Moreover, imposing both forms of reduced water availability resulted in a synergistic
decrease in N fixation (Fig. 2). This suggests that it is only the reduction in rainfall—not necessarily the frequency of rainfall events—that is mostly
responsible for inhibiting legume N fixation.
Similar responses were found when leaf senescence and biomass were measured. Lupinus showed an increase in the proportion of senesced
leaves and a reduction in total dry biomass with decreased water availability, but these effects were similarly only significant in cases where the
total volume of water received was reduced (Fig. 3B, 4). The proportion of Agropyron leaves senescing at a given time and total dry biomass
were not significantly different from the control in any of the treatments, suggesting that this species is better adapted to drought (Fig. 3B, 4).
Watering treatments reduced leaf net photosynthetic rates in Lupinus only. However, due to a greater reduction in stomatal conductance
(water loss) than Lupinus, Agropyron increased its water use efficiency for reduced volume and reduced volume and frequency treatments
compared to the control (Fig. 5). This is consistent with the prevailing generality that grasses are better adapted to drought than forbs (Fay et. al.
2003).
For every growth parameter studied, a 50% reduction in watering frequency did not significantly affect the plant as long as there no
overall reduction in total water volume. However, if volume was reduced by 50%, Lupinus showed a reduction in growth and nitrogen
fixation and Agropyron showed changes in leaf physiology (water use efficiency) to compensate. Reductions in volume and frequency
together showed additive or synergistic effects. In the future, predictions of vegetation responses to climate change could be
improved by considering these two dimensions of water availability independently.
-90
↓ Frequency ↓ Volume
C
C 100
% Change in Water Use Efficiency
Table 1. Volume and frequency of water given to
plants in each of four treatments.
Effects on Leaf Production and Senescence
Agropyron
24.0
Percent N derived from Fixation
Water Treatments
Before watering
Effects on Soil Moisture
Volumetric Soil Moisture (%)
Water deficits are a common limiting factor of plant growth. In North
America, rainfall gradients are a key determinant of ecosystem type and
primary productivity, particularly among grasslands (Fay et. al., 2003).
Rainfall patterns are one of a number of climate factors that are expected to
change substantially in the coming decades. Predictions for the Upper
Midwest show only a slight increase in total rainfall, but a dramatic change
in the distribution of rainfall, including severe June-August droughts, and
larger spring rain events (Winter & Eltahir, 2012; Hayhoe et. al., 2009).
Many studies have looked at the effects of drought on plants, but few
have independently compared the two ways a plant might experience
reduced water availability: fewer rain events, or less water per event. A
decade-long field study of a tallgrass prairie in Kansas found that for some
species, reducing the frequency of rainfall events while keeping total water
volume constant reduced soil water availability as much as reducing the
rainfall amount. Also, grass species were found to compensate for
changing water availability through physiological adjustments (Fay et. al.,
2003). However, the way these findings relate to other community types,
species, and functional groups is uncertain. And, while field studies allow
more natural conditions, growth in controlled conditions can provide
additional information, such as access to below-ground plant parts.
In this greenhouse experiment we investigated responses of the
legume Lupinus perennis and the grass Agropyron repens to 50%
reductions in rainfall frequency, total rainfall volume, or both (Table 1). Our
objective was to contrast the effects of reduced rainfall amount with a
more extreme distribution of rainfall on plant function by measuring
leaf senescence, biomass accumulation, photosynthetic and stomatal
conductance rates, and for Lupinus, the amount of nitrogen (N) derived
from symbiotic N2 fixation.
↓ Freq. /
↓ Vol.
**
80
60
**
40
20
0
-20
Watering Treatment
↓ Frequency ↓ Volume
↓ Freq. /
↓ Vol.
Watering Treatment
Acknowledgements
Funding sources:
• UW-Eau Claire Differential Tuition
• UW-Eau Claire Office of Research and
Sponsored Programs
• Blugold Fellowship (A. Schneider)
Also, special thanks to Susan Barrott and
Cedar Creek Ecosystem Science Reserve
for soil moisture data, stable isotope
analysis, and soil for the experiment.
References
Fay, P.A., Carlisle, J.D., Knapp, A.K., Blair,
J.M, Collins, S.L., 2003. Productivity
responses to altered rainfall patterns in a
C4-dominated grassland. Oecologia,
137(2), 245-251.
Hayhoe, K., VanDorn, J., Naik, V., Wuebbles,
D., 2009. Climate Change in the Midwest:
Projections of Future Temperature and
Precipitation. Cambridge, MA: Union of
Concerned Scientists.
Serraj. R., Sinclair, T.R., Purcell, L.C., 1999.
Symbiotic N2 fixation response to drought.
Journal of Experimental Botany 50:143155
Winter, J.M., Eltahir, E.A.B., 2012. Modeling
the hydroclimatology of the midwestern
United States. Part 2: future climate.
Climate Dynamics, 38:595-61.