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

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0 2 4 6 8 10

Exotic C

oun

ts C

over

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)

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

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ount

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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

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45

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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

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1 3 5 7 9 11 13

Twice for 1 day

Ave

rage

Cov

er (

%)

Monitoring Event

0

10

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50

1 3 5 7 9 11 13

Twice for 2 days

0

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1 3 5 7 9 11 13

Twice for 4 days

0

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1 3 5 7 9 11 13

Three times for 4 days

0

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1 3 5 7 9 11 13

Four times for 4 days

//

//

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//

//

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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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