Post on 29-Mar-2021
Can summer watering to flush annuals reduce exotic cover in
restored grasslands?
Kristina Wolf Young Restoration Lab
California Society for Ecological Restoration Technical Session:
Upland Habitat Restoration and Management Wednesday May 14th, 2014
The Mediterranean Challenge • > 22.7 million acres of CA native grasslands1
invaded by Mediterranean grasses & forbs2 • Native perennials can take 20+ years to
dominate after restoration3 • Restored sites susceptible to reinvasion3
• Challenge: highly invasive & competitive1
– “Jump” on resource pulses more quickly4
– Deplete resources5
– Early germination and rapid growth 6-9
– Disturbance facilitates reinvasion4 (1) Seabloom et al. 2003 (2) Mooney & Drake 1986 (3) Munson & Lauenroth 2012 (4) Davis & Pelsor 2001 (5) Lowe et al. 2003 (6) Greippson 2011 (7) Fargione et al. 2003 (8) Hooper & Dukes 2010 (9) Wainwright & Cleland 2013
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The Mediterranean Challenge
3 adapted from Wainwright et al. 2012
Timing of germination/growth after dry season
Peak abundance in growing season
The Mediterranean Challenge • > 22.7 million acres of CA native grasslands1
invaded by Mediterranean grasses & forbs2 • Native perennials can take 20+ years to
dominate after restoration3 • Restored sites susceptible to reinvasion3
• Challenge: highly invasive & competitive1
– “Jump” on resource pulses more quickly4
– Deplete resources5
– Early germination and rapid growth 6-8
– Disturbance facilitates reinvasion4 (1) Seabloom et al. 2003 (2) Mooney & Drake 1986 (3) Munson & Lauenroth 2012 (4) Davis & Pelsor 2001 (5) Lowe et al. 2003 (6) Greippson 2011 (7) Fargione et al. 2003 (8) Hooper & Dukes 2010
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An Annual Advantage • Seasonal priority advantage may promote
annuals, and reduce native plant success1-2
• Alter environmental factors to promote natives3
– Water resources – Nutrient availability – Competition
• Timing and availability of summer water
(1) Reynolds et al. 2001 (2) Wainwright et al. 2012 (3) Funk et al. 2008 5
Effects of summer watering • Positive correlation between perennial grass
cover and warm season rainfall1
• Annual grasses may be “flushed out” with summer watering2 – Summer watering reduced exotic annual cover
through January in an invaded coastal scrub • Pre-season watering in September caused annual
grass germination, but none survived to onset of natural rains
(1) Clary 2008 (2) Wainwright et al. 2012 6
Pre-season watering reduced exotic cover through January (from Wainwright et al. 2012).
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Under ambient rainfall, exotic annual grasses far
outnumber native grasses, and peak abundance is
considerably higher than natives.
When flushed in the summer, exotic annual
grass germination is considerably lower than
natives.
But do these effects last?
Effects of summer watering • Positive correlation between perennial grass
cover and warm season rainfall1
• Annual grasses may be “flushed out” with summer watering2 – Summer watering reduced exotic annual cover
through January in an invaded coastal scrub • Pre-season watering caused annual grass germination,
but none survived to onset of natural rains
• Commonly done in a pre-restoration setting • Could annual flushing work in already
restored (but re-invaded) grasslands? (1) Clary 2008 (2) Wainwright et al. 2012
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Flushing the weed seedbank
Does summer watering… 1) trigger exotic annual grass germination? 2) promote release of perennial grass
dormancy?
And in the following growing season: 3) reduce overall exotic annual cover? 4) increase native perennial grass cover?
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Methods
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• Restored 2010 – Elymus glaucus
Methods
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• Restored 2010 – Elymus glaucus – Elymus triticoides
Methods
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• Restored 2010 – Elymus glaucus – Elymus triticoides – Stipa pulchra
Methods
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• Restored 2010 – Elymus glaucus – Elymus triticoides – Stipa pulchra
• 6 treatments (5 watering, 1 control) • 6 replicates • 36 1-m2 plots
† treatments spanned a 4 week period between August 25th and September 21st 2012
Treatments Treatment† Total Water (cm) Number of Watering Days
Control 0 0
4 days/week, 1 week 4.4 4
4 days/week, 2 weeks 8.8 8
4 days/week, 4 weeks 17.6 16
4 days/week, Week 4 4.4 4
4 days/week, Week 4
Twice per day 8.8 4
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† treatments spanned a 4 week period between August 25th and September 21st 2012
Treatments Treatment† Total Water (cm) Number of Watering Days
Control 0 0
4 days/week, 1 week 4.4 4
4 days/week, 2 weeks 8.8 8
4 days/week, 4 weeks 17.6 16
4 days/week, Week 4 4.4 4
4 days/week, Week 4
Twice per day 8.8 4
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Timing Frequency
Amount
Monitoring • Weekly annual counts and perennial cover
from August – November • Annual and perennial cover the following year
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0
200
400
600
800
1000
1200
0
10
20
30
40
0 2 4 6 8 10
Exotic C
oun
ts C
over
(%
)
Monitoring Event
4 weeks April % Exotic Cover
Aug 2012 Baseline
Apr 2013 Aug 2013 Nov 2012
//
Ambient Rainfall Response
Summer % Native
Cover Max Treatment Response
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Counts & Cover
0
200
400
600
800
1000
1200
0
10
20
30
40
0 2 4 6 8 10
2 weeks
Ave
rage
Cov
er (
%)
Monitoring Event
Average Seedling C
ount
0
200
400
600
800
1000
1200
0
10
20
30
40
0 2 4 6 8 10
4 days, Week 1
0
200
400
600
800
1000
1200
0
10
20
30
40
0 2 4 6 8 10
4 days, Week 4
0
200
400
600
800
1000
1200
0
10
20
30
40
0 2 4 6 8 10
4 days, twice/day Week 4
0
200
400
600
800
1000
1200
0
10
20
30
40
0 2 4 6 8 10
4 weeks
0
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1200
0
10
20
30
40
0 2 4 6 8 10
Control
// //
// //
// //
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†
Max Treatment Response
Friedman chi-squared = 16.3942, p-value = 0.005804
Ambient Rainfall Response
Friedman chi-squared = 26.9126, p-value = 0.000059 19
Friedman chi-squared = 2.7114, p-value = 0.7444
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Spring Exotic Cover
Friedman chi-squared = 2.6111, p-value = 0.7597
Summer Native Cover
Implications
• Keeping soil moist in between waterings was critical to pre-season flush of exotic germinants
• Flushing of annual grass seedlings may result in a short-term (seasonal) reduction in exotic seedlings, but not an ultimate reduction in cover the following growing season
• Perennial grass cover did not increase the following year
Part Two! • Would MORE water reduce exotic cover long-term? • Refine treatments
• 6 replicates per treatment • 36 1-m2 plots
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Summary of watering treatments for 2012 and 2013 experiments. † treatments spanned a 4-week period, Aug. 25 to Sept. 21, 2012 ‡ watering treatments applied in the fourth week of August and September 2013
2012† Water (cm) # Watering
Days 2013‡ Water (cm)
# Watering Days
Control 0 0 Control 0 0
4 days/week, 1 week 4.4 4 Twice for one day 2.2 1
4 days/week, 2 weeks 8.8 8 Twice for 2 days 4.4 2
4 days/week, 4 weeks 17.6 16 Twice for 4 days 8.8 4
4 days/week, Week 4 4.4 4 Three times for 4 days 13.2 4
4 days/week, Week 4
Twice per day 8.8 4 Four times for 4 days 17.6 4
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Russell Ranch, Davis
Spring % Native
Cover 0
5
10
15
20
25
30
35
40
45
50
1 3 5 7 9 11 13
Three times for 4 days
Ave
rage
Cov
er (
%)
Monitoring Event
Aug 2013 Baseline
May 2014
Nov 2013 mid-Sept 2013
//
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Ambient Rainfall Response
Spring % Exotic Cover
Max Treatment Response
August
0
10
20
30
40
50
1 3 5 7 9 11 13
Control
Grass Cover (%)
0
10
20
30
40
50
1 3 5 7 9 11 13
Twice for 1 day
Ave
rage
Cov
er (
%)
Monitoring Event
0
10
20
30
40
50
1 3 5 7 9 11 13
Twice for 2 days
0
10
20
30
40
50
1 3 5 7 9 11 13
Twice for 4 days
0
10
20
30
40
50
1 3 5 7 9 11 13
Three times for 4 days
0
10
20
30
40
50
1 3 5 7 9 11 13
Four times for 4 days
//
//
// //
//
//
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p-values 1 0.06 0.08 0.004 <0.001 0.003 <0.001 <0.001 <0.001 <0.001
Native Perennial Response
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September December
Native Perennial Response
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Spring
p-value 0.57
p-values 1 1 < 0.0001 < 0.0001 < 0.0001
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Pre-Season Exotic Seedling Flush
p-values 1 1 0.03 0.02 < 0.001
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Exotic Annual Response Max Treatment
Response
1 1 1 0.52 1
Ambient Rainfall Response
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Exotic Annual Response Spring
On average, 7% more exotic cover in previously flushed plots*
*p-value 0.06
Possible explanations
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Simplest explanation: there are so many
seeds, that one year of flushing doesn’t make
a difference
What else?
Why no difference (or higher) exotic cover? Larger B. diandrus
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p = 0.91 Avg 0.13 g/plant larger in flushed plots*
*p = 0.068
Only 3 reps Probably need more
samples to verify
Possible explanations
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• Simplest explanation: there are so many seeds, that one year of flushing doesn’t make a difference
• Larger plants? Potential trend for larger ripgut brome, but not soft chess – Need more samples
Why would watering in the summer make later-germinating exotics larger, more numerous, or
as numerous as in control plots?
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• (Over)Compensation (size or number) – Fertilization effect/resource pulse – Death of seedlings after flush provides N?
• Seedling death as a N source “Seedling thinning … results in the inputs of labile litter with low structural material, and thus the N released from dying seedlings during times of high competition is likely to be readily available to surviving plants. This suggests that self-thinning is the ultimate time-release fertilizer.” 1
• Resources that invasive plants can capitalize on more quickly than natives2
35 (1) http://www.plantsciences.ucdavis.edu/plantsciences_faculty/eviner/main/current_research.htm (2) Davis & Pelsor 2001
Flushing the weed seedbank
Does summer watering… 1) trigger annual grass germination? Yes 2) promote release of dormancy in perennial
grasses in the summer? Yes
And in the following growing season: 3) reduce overall exotic annual cover? No 4) increase native perennial grass cover? No
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• Need a LOT of water to get a good flush (more than what is commonly used).
• Over a broad range of watering regimes, overall exotic cover was not reduced long term.
• Natives green up in the summer with supplemental water, but they did not benefit long term.
• Post-restoration strategy? Other options?
Take-home messages
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• Need a LOT of water to get a good flush (more than what is commonly used).
• Over a broad range of watering regimes, overall exotic cover was not reduced long term.
• Natives green up in the summer with supplemental water, but they did not benefit long term.
• Post-restoration strategy? Other options?
Take-home messages
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~188,000 gallons/acre for 4 times*4 days
Literature Cited • Clary, J. 2008. Rainfall seasonality determines annual/perennial grass balance in vegetation of Mediterranean Iberian. Plant
Ecology 195:13-20. • Davis, M. A., and M. Pelsor 2001. Experimental support for a resource-based mechanistic model of invasibility. Ecology Letters
4:421-428. • Eviner, V. 2014. Understanding rangeland seedling dynamics: a tool to increase forage quality and quantity while providing
multiple ecosystem services. Eviner lab, University of California Davis. Accessed 5 May 2014 online at http://www.plantsciences.ucdavis.edu/plantsciences_faculty/eviner/main/current_research.htm.
• Fargione, J., C. S. Brown, and D. Tilman 2003. Community assembly and invasion: An experimental test of neutral versus niche processes. Proceedings of the National Academy of Sciences of the United States of America 100:8916-8920.
• Funk, J. L., E. E. Cleland, K. N. Suding, and E. S. Zavaleta 2008. Restoration through reassembly: Plant traits and invasion resistance. Trends in Ecology & Evolution 23:695-703.
• Greippson, S. 2011. Restoration Ecology. Jones & Bartlett Learning, LLC., Sudbury, MA. • Hooper, D. U., and J. S. Dukes 2010. Functional composition controls invasion success in a California serpentine grassland.
Journal of Ecology 98:764-777. • Lowe, P. N., W. K. Lauenroth, and I. C. Burke 2003. Effects of nitrogen availability on competition between Bromus tectorum and
Bouteloua gracilis. Plant Ecology 167:247-254. • Mooney, H. A., and J. A. Drake 1986. Ecology of biological invasions of North America and Hawaii. Springer-Verlag, New York. • Munson, S. M., and W. K. Lauenroth 2012. Plant community recovery following restoration in semiarid grasslands. Restoration
Ecology 20:656-663. • Reynolds, S. A., J. D. Corbin, and C. M. D. D’Antonio. 2001. The effects of litter and temperature on the germination of native
and exotic grasses in a coastal California grassland. Madroño 48:230–235. • Seabloom, E. W., W. S. Harpole, O. J. Reichman, and D. Tilman 2003. Invasion, competitive dominance, and resource use by
exotic and native California grassland species. Proceedings of the National Academy of Sciences of the United States of America 100:13384-13389.
• Wainwright, C. E., and E. E. Cleland 2013. Exotic species display greater germination plasticity and higher germination rates than native species across multiple cues. Biological Invasions 15:2253-2264.
• Wainwright, C. E., E. M. Wolkovich, and E. E. Cleland 2012. Seasonal priority effects: Implications for invasion and restoration in a semi-arid system. Journal of Applied Ecology 49:234-241.
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