Can summer watering alter seasonal priority effects and reduce the annual grass weed challenge in a restored grassland?

This post will describe the background, rationale, methods, and some preliminary results for a series of summer watering experiments that I conducted over two years to determine if summer watering in an already restored grassland can produce a strong enough weed flush (exotic annual grasses) to reduce weed challenges in the subsequent growing season. Please see my other post ( with results for two watering experiments, as described in the powerpoint link provided. If you have any questions, please feel free to contact me at kmwolf[AT] (replace [AT] with the @ symbol).

Background and Rationale

Over 9.2 million hectares (> 22.7 million acres) of California’s native grasslands have been replaced by exotic plants (1), mainly annual Mediterranean grasses and forbs (2). These annual plants present a significant restoration challenge in California, with this largely attributed to their invasive and competitive abilities (1). Restored sites may be highly susceptible to invasion by exotic annuals until native perennials can dominate. However, recovery in these systems may take up to 20 or more years after seeding with native perennial grasses, and may still fall short of native cover in undisturbed systems by more than 50%, making them more susceptible to re-invasion (3). Exotic grasses may rapidly deplete available resources – particularly water – before perennial grasses have a chance to recover and spread (4). Therefore, successful restoration requires a shift in this competitive balance in favor of desired native perennial grasses.

Life history, growth form, and phenology are important in success of different species that are vying for water, and strategies for capturing this resource early may mean the difference between death and long-term success (56). Specifically, “seasonal priority effects” in exotic annual grasses, or their ability to take early advantage of water and resources, may reduce native plant success (78). Ecologists may promote native plant restoration or recovery and reduce impacts or extent of exotic invasions by altering environmental filters such as timing and amount of water, nutrient availability, and biotic interactions (9). In California’s Mediterranean climate, manipulation of water resources may create early circumstances favorable to native grasses, and reduce competition from exotic annual grasses in the later growing season. In a greenhouse study simulating climate regimes of eastern Spain, Clary et al. (10) found that drought-stressed Mediterranean perennial grasses recovered faster than a Mediterranean shrub, suggesting that the perennial grass was able to opportunistically make use of rare summer rainfall, contributing to its competitive success over time. Further, a survey of annual and perennial grass cover over the Mediterranean Iberian Peninsula found a positive correlation between perennial grass cover and warm season rainfall (11).

In addition to conferring an advantage to perennial grasses with access to water during California’s droughty summers, exotic annual grasses may be flushed out with summer watering to reduce the annual seedbank and decrease competitive pressure on perennial grasses in the subsequent growing season. Much of the supposed competitive advantage that exotic annuals have over native grasses has been attributed to earlier germination and rapid early growth of annual grasses (12). This is consistent with studies in which early phenology of exotic annual species accounted for competitive exclusion of native species (1314). Indeed, Wainwright et al. (8) found summer watering to be a viable strategy for reducing exotic annual grass cover in an invaded southern California coastal sage scrub in San Diego County. Thus, late summer watering may stimulate germination of annuals, while cessation of watering prior to ambient rains may result in death prior to setting seed, thereby depleting the seedbank. Summer watering may also give perennial grasses a competitive advantage by allowing for growth during the usually dormant season, storage of carbohydrates, and a stronger competitive edge over annuals in the subsequent growing season.

Objective: In this study I manipulated seasonal priority effects via manipulation of timing and amounts of summer watering to test whether this simple, cost-effective intervention could confer a competitive advantage to native perennial grasses.


Site Selection.  The summer 2012 study was conducted at the Experimental Ecosystem (U.C. Davis). Two additional sites managed by U.C. Davis were added for the summer 2013 study: a Russell Ranch site lightly grazed seasonally by cattle, and an ungrazed site located on Long-Term Research in Agricultural Sustainability (LTRAS) land. Results of the Experimental Ecosystem and Russell Ranch experiments are described at Preliminary results from the third experiment at LTRAS are described on another post (pending). All sites are owned and managed by U.C. Davis, and restoration projects were designed, implemented, and managed by Andrew Fulks (Putah Creek Riparian Reserve Manager) and Jean-Phillip Marie (Putah Creek Riparian Reserve Steward).

Results.  In the summer of 2012 my goal was to: a) stimulate early perennial growth, and/or b) deplete the exotic grass seedbank; both results should increase relative success of native perennials. The treatments that appeared most promising were all 2+ times daily watering. For the amount of water applied, these treatment were by far the most effective. Although all watering treatments stimulated growth in the resident perennial grasses, these were the only treatments that also stimulated noteworthy germination and die-back of exotic annuals; no other treatment provided these simultaneously desirable results.

Treatments.  The 4-day twice-daily watering treatment and a control were tested again in 2013 at both locations, but additional levels were also applied (Table 1) in both August and September. First, to determine if more days of water were applied than necessary to obtain a similar response, two treatments with only one and two days of twice-daily watering treatments were applied. Second, to determine if more watering per day would result in a greater plant response, 3- and 4-times daily watering treatments were applied over a four day period (see Table 1 for treatment summaries). The amount of water applied at each watering event remained the same, at approximately 10 liters per 1 m2 plot, or 1 cm of water per watering event.

2012 Treatments Water (cm) Code 2013 Treatments Water (cm) Code
Control 0 C Control 0 C
Week 1: 4-day 4.4 4dA 1 day, 2 watering events 2.2 1-2
Weeks 1-2: 4 days 8.8 2w 2 days, 2 watering events 4.4 2-2
Weeks 1-4: 4 days 17.6 4w 4 days, 2 watering events 8.8  4-2§
Week 4: 4 days 4.4 4dB 4 days, 3 watering events 13.2 4-3
Week 4: 4 days, 2 watering events 8.8 4dC§ 4 days, 4 watering events 17.6 4-4

† treatments spanned a 4 week period between August 25th and September 21st

‡ watering treatments will be applied during the fourth week in September 2013

§ treatments 4dC (in 2012) and 4-2 (in 2013) were equivalent

Response Variables.  Total and relative cover of annual exotic and native perennial by species was recorded at each monitoring event. While all species of forbs and grasses were recorded, analyses focused on exotic annual and native perennial grasses, unless one or more forb species appeared to be a strongly influencing factor at any of the sites. In the growing season, three subsamples each of the two dominant exotic annual grass species (Bromus hordeaceus and Bromus diandrus) were harvested in a subset of experimental plots at the Russell Ranch location to determine if plant size was reduced or increased in plots receiving watering treatments. All data were analyzed in R (15).

Conclusion In brief, while summer watering could induce a substantial seedling flush of exotic annual grasses, this did not translate into a long-term reduction in the total exotic cover the following growing season, and required prohibitively large quantities of water.


  1. 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.
  2. Mooney, H. A., and J. A. Drake 1986. Ecology of biological invasions of North America and Hawaii. Springer-Verlag, New York.
  3. Munson, S. M., and W. K. Lauenroth 2012. Plant community recovery following restoration in semiarid grasslands. Restoration Ecology 20:656-663.
  4. 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.
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  6. Alscher, R. G., and J. R. Cumming 1990. Stress responses in plants. Wiley-Liss, New York.
  7. Reynolds, S. A., J. D. Corbin, and C. M. 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.
  8. 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.
  9. 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.
  10. Clary, J., R. Savé, C. Biel, and F. D. Herralde 2004. Water relations in competitive interactions of Mediterranean grasses and shrubs. Annals of Applied Biology 144:149-155.
  11. Clary, J. 2008. Rainfall seasonality determines annual/perennial grass balance in vegetation of Mediterranean Iberian. Plant Ecology195:13-20.
  12. Greippson, S. 2011. Restoration Ecology. Jones & Bartlett Learning, LLC., Sudbury, MA.
  13. 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.
  14. Hooper, D. U., and J. S. Dukes 2010. Functional composition controls invasion success in a California serpentine grassland. Journal of Ecology 98:764-777.
  15. R Core Team 2012. R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0. URL

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