Invasive Species and Restoration of California Grasslands and Shrublands

Transcription

1 Invasive Species and Restoration of California Grasslands and Shrublands Edith B. Allen Department of Botany and Plant Sciences Center for Conservation Biology

2 Exotic grass invasions are causing vegetation-type conversion in coastal sage scrub and deserts, with loss of native shrub and forb diversity and changes in ecosystem functioning. Jepson Manual ~7600 species/subspecies (~6000 species). ~1500 are exotic introductions, and ~200 species are invasive (California Invasive Plant Council Exotic, alien, introduced and non-native are synonyms Invasive is defined as a plant that is spreading over large areas and causing negative impacts to plant communities and ecosystems.

3 Origins of California Invasive Plants A large majority are from the Mediterranean, climate similar to California Many (especially annual weedy plants) were introduced accidentally as contaminants in crop seed or animal forage. Others were introduced as ornamentals or for practical uses. Exotic annual grasses and forbs have invaded CA grasslands, oak woodlands, coastal sage scrub, etc. and have caused vegetation-type conversion. California soils tend to be richer in nutrients than Mediterranean soils. This may be one important reason why Mediterranean species are more abundant in California than in their native sites of origin.

4 California purple needle grassland, Exotic annual grasses dominant Santa Rosa Plateau oak savanna Dominated by Mediterranean annual grasses Diverse perennial grassland, Portugal Oak-pine savanna, Portugal, including Grass species that are now in California (brome, wild oats)

5 Objectives 1. Discuss ecosystem impacts of exotic grass invasions in coastal sage scrub and desert scrub (reduced native cover and richness, increased fire risk from high exotic grass production). 2. Examine interactions of invasive grasses with nitrogen deposition, drought, and a newly discovered pathogen of exotic brome grasses. 3. Review restoration methods to control exotic grasses and replace them with native species 4. Summarize actions that California Naturalists can take to restore invaded ecosystems.

6 Nitrogen deposition in California San Francisco Caused by fossil fuel combustion (nitrates) and agricultural fertilization and feedlots(ammonia) Model output (Tonnesen et al. 2007, Fenn et al. 2010) Los Angeles

7 Nitrogen deposition is up to 30 kg N ha -1 yr -1 in the Los Angeles air basin. Most is dry deposition (nitrate and ammonium) that falls during the dry summer. View from Riverside west to Los Angeles. Vegetation in areas with high N deposition have converted to exotic annual grasses.

8 Diversity assessment on a nitrogen deposition gradient in coastal sage scrub (CSS): High nitrogen deposition (20 kg N ha -1 yr -1 ) dominated by exotic annual grasses from Mediterranean (brome, wild oats, barley, fescue) Low N deposition (8.7 kg N ha -1 yr -1 ) dominated by native forbs and shrubs

9 Native forb richness declines under elevated N along a deposition gradient and exotic grass increases in cover Richnes s r 2 = 0.01 N.S. R 2 = 0.73, P = r 2 = 0.73 P < 0.01 N.S N deposition (kg/ha/yr) Native forbs Invasive grass Critical load for richness 80 R 2 = 0.51, P = 0.07 Native forbs Cover R 2 = 0.55, P = 0.06 r 2 = 0.55 P < Invasive grass r 2 = 0.51 P < N deposition (kg/ha/yr)

10 Biomass of exotic grass in control and plus-n treatment. The threshold for fire is 100 g/m 2 ( = 1.0 T/ha) of fine grass fuel (red line). Grass biomass is below threshold in control plots in most years. P is repeated measures probability, * is P < 0.05 by year. g/m 2 P < * * Exotic Grass Biomass * N-Fert Control * * * * * * * * * * 0 burned Fertilized Control

11 Air pollution outside Joshua Tree National Park (12/2004) 12 kg N ha -1 yr -1 Mediterranean splitgrass (Schismus) invasion in creosote bush scrub (12 kg N ha -1 yr -1 ) Native forbs in creosote bush scrub (3 kg N ha -1 yr -1 )

12 Percent Cover exotic herb native herb r 2 R= 2 =.56, 0.56, P = P.05 = N Deposition (kg/ha/yr) r 2 = R.35, 2 = 0.35, P =.16 P = 0.16 Relationships between N deposition, native and exotic cover, and richness along a gradient in creosote bush scrub R 2 = 0.71, P = 0.02 r 2 =.70, P =.02 N.S. Richness N Deposition (kg/ha/yr) r 2 =.44, P =.10 exotic herb native herb Means of 0.5m 2 quadrats at each site, 4/2005

13 Allen et al. 2009) Nitrogen-fertilized plot in Larrea tridentata scrub in a site with clean air (low N deposition)

14 Fire carried by Bromus rubens in desert chaparral 7/2006 Grass fuel load of 1 T/ha will carry fire between shrubs Joshua tree woodland burned in 1999 (2004 photo) unburned

15 Ecological restoration is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed. ( Restoration research to control exotic annual grasses Coastal sage scrub, Riverside County

16 Sheep grazing to control exotic grass. 200 sheep/ha, 2 days, Mar/Apr Grazing removed annual grasses and native forbs. Grass seedbank is short-lived Native forb seedbank persists

17 Stands of native flowers one year following herbicide application. However, invasive grasses recovered after about 5 years, so treatments must be reapplied. Grass-specific herbicide (Fusilade) application

18 Nov 2004 Solarization (tarping), herbicide, mowing in abandoned agricultural land Seeded with native forbs and shrubs Jan. 05 Mowed Feb, Mar Fusilade Feb. 05 Jan 2005 Herbicide Damage Mar 05

19 Results: Apr 2005 Herbicide Control Solarized Mowed Solarization is the best method for restoration

20 Restoration at a site of high N deposition and frequent fire in Riverside: CSS shrubs established in spring 1998 using grass-specific herbicide and hand cultivation. Jun 2005 photo Fire Oct Post-fire May 2009 Plots are dominated by exotic grasses and shrubs have not recovered. Restoration will be successful if site can recover from natural fire regime. (Cione et al 2002)

21 Restored CSS in abandoned farmland, ~ 50 acres at San Jacinto Wildlife Area, low nitrogen deposition. 3/2003 3/2010 3/2004 Goldfields, tidytips, buckwheat, brittlebush, sagebrush 3/2013 Dominated by buckwheat, few native annuals, exotic grasses, no fire

22 Seedbank of exotic grassland, native shrubland with grass understory, and adjacent burned and unburned sites after the October 2003 fire, Shipley Reserve. Average Seedlings per m 2 Species Grassland Shrubland Unburned Burned Exotic Grasses Exotic Forbs Native Forbs Native shrubs Exotic seeds overwhelm native seedbank. Fire reduces exotic grass seedbank, providing a window of opportunity for restoration (Cox and Allen 2008).

23 Lake Skinner spring was dry (~ 5 cm, average is 26 cm); almost no exotic grasses, native forbs formed the only ground cover. Will exotic grasses recolonize?

24 Desert Restoration Experiments: Native forbs (green) in plot sprayed with Fusilade (grass-specific herbicide) around creosote bush. Mediterranean split-grass (senescent) dominant across stand, April 2005

25 Fusilade (grass-specific herbicide) treatment in burned creosote-bush scrub to control exotic grasses. Removal of grasses releases native plants from competition. However, exotic grasses recolonized after a few years. (Steers et al. 2008) control herbicide control

26 Covington Flat 1999 burn 2005 dominated by Bromus rubens 2014 Bromus << 1% cover, native shrubs and grasses recovering

27 What happened to brome grasses? Drought (Salo 2004) and head smut fungus reduce seed bank May % of red brome and cheatgrass brome infected with head smut (Ustilago bullata) April 2014 Bromus << 1% cover at previously burned sites at Joshua Tree NP

28 Head smut (Ustilago bullata) observed on Bromus rubens, B. tectorum, and B. diandrus (sect. Genea)

29 Conclusions 1. Invasive grasses reduce native species richness and increase fire risk in coastal sage scrub and desert scrub. 2. Reductions in N deposition below 10 kg/ha/yr will increase native diversity and reduce fire risk. Need to strengthen air quality standards for nitrogenous pollutants. 3. Many restoration techniques have been used, including fire, herbicides, grazing, and solarization. But grasses always reinvade, especially in areas with high nitrogen deposition. 4. Drought (CSS and desert) and smut fungi (desert) reduced the abundance of brome grasses in 2013/2014. Native forbs were abundant in sites that have long been dominated by exotic grasses. 5. California Naturalists can be involved in restoration efforts and long-term monitoring of vegetation under improving air quality and changing climate conditions.

30 Acknowledgements Mike Bell, Leela Rao, Christopher True, Robert Steers, Sara Jo Dickens, Heather Schneider, Bridget Hilbig, Matt O Neill, Robert Johnson, Tracy Tennant, Sheila Kee, Mike Allen, Richard Minnich (UCR), Mark Fenn, Andrzej Bytnerowicz (USFS), Tom Meixner (U AZ), Jane Rodgers, Josh Hoines (NPS) and many others. Research funded by National Park Service, National Science Foundation, UCR-CNAS Shipley-Skinner Endowment, Riverside County Habitat Conservation Agency