Introduction

Conclusions

Perennial Pepperweed (Lepidum latifolium) has invaded a wide range of habitat types in the far west (Young et al.1998). In San Francisco Estuary, dense infestations have impacted sensitive tidal wetlands and compromised endangered species recovery efforts (USFWS 2013). Conservation concerns due to presence of threatened and endangered plant (soft bird’s-beak; Chloropyron molle ssp. molle) and bird (Ridgway’s rail, Rallus obsoletus) populations (Fig. 1) have delayed weed management.

Adaptive Management of Perennial Pepperweed for Endangered Species and Tidal Marsh Recovery

Brenda J. Grewell 1 Caryn J. Futrell 1, Michael Forbert 2, and Meghan J. Skaer Thomason 1 1USDA-ARS Exotic & Invasive Weed Research Unit, Davis, CA; 2West Coast Wildlands, Pacifica , CA

Glyphosate applied with precision nozzles from backpack foliar spray or hand wick/wipe can be selective for pepperweed. These results demonstrate that careful, science-based adaptive management can be successful for herbicide suppression of invasive weeds in highly sensitive endangered species habitat. The project now serves as a model for responsible weed management for endangered species recovery.

Literature Cited Rinella MJ, JJ James. 2010. Invasive plant researchers should calculate effect sizes, not P-values. Invasive Plant Science & Management 3:106-112. U.S. Fish &Wildlife Service. 2013. Recovery Plan for Tidal Marsh Ecosystems of Northern and Central California. Sacramento, California. 605 pp. Young JA, DE Palmquist, RR Blank.1998.The ecology and control of perennial pepperweed (Lepidium latifolium). Weed Technology 12:402-405.

Fig. 3. Live biomass of perennial pepperweed before glyphosate treatment at two representative dates: A) 4 and B) 27 months after treatment (MAT). C,D) Confidence intervals (75% [bars] and 95% [lines]) estimate effect size of treatment relative to control plots to highlight spatial and temporal variability of management outcome.

Objectives: Provide a model for weed management in sensitive habitat. Monitor and provide experimental results to refine and adjust annual treatments to 1) evaluate efficacy of foliar-applied glyphosate treatments to pepperweed through a spatially-stratified experiment; 2) assess the marsh-wide distribution and abundance of pepperweed and sensitive species by microhabitats to establish conservation zones and stratified treatment approaches.

Fig. 1. Photographs of perennial pepperweed, soft bird’s-beak, and Ridgway’s rail.

Materials and Methods Study Site: Southampton Bay Wetland Natural Preserve, Benicia State Park, 55 km up-estuary from the Golden Gate/Pacific Ocean. Treatment and Conservation Zones: Annual GPS mapping of pepperweed and endangered species occupied habitat were used to stratify herbicide application methods (Fig. 2A,B). Herbicide Applications: Foliar applications of glyphosate solution (isopropylamine salt; 5.3 to 5.5 kg ai/ha) with a nonionic surfactant to pepperweed at flower bud stage using backpack sprayers fitted with wands and Spray Jet 5500 X8 (0.8GPM) spray wand cone/jet nozzle tips for precise applications and drift avoidance (Fig. 2C). Hand wick/wipe applications to pepperweed (LELA) were used within 1m of soft bird’s-beak (CHMO, Fig. 2D). Pre-entry surveys for ground-nesting rails were conducted by ornithologists immediately prior to treatment in Ridgway’s rail habitat conservation zones.

Fig. 5. Marsh-wide results: Occupied area of pepperweed at A) baseline/pretreatment, and B) reduced by 84% by 5 YAT (note: no herbicide applications in rail conservation zone 2011).

Results Management Efficacy: All measured responses were significantly reduced by glyphosate treatments, though the magnitude of treatment effectiveness varied by year and microenvironment. The shoot biomass and stem density of pepperweed were significantly reduced by herbicide treatment (MANOVArm:F2,13=30.26, p=0.0001; biomass response Fig. 3A,B). The magnitude of effect of herbicide application on pepperweed biomass varied depending on year (F2,14=5.3, p=0.0193) and microenvironment (F2,14=4.38, p=0.0333). Calculated effect sizes (Rinella and James 2010) reveal variation in treatment effectiveness was greatest in mid-marsh inundation zones near slough edges, prompting greater applicator attention to these areas (biomass data; Fig. 3C,D). TNC analyses of below ground storage organs revealed dramatic reductions of pepperweed storage reserves for resprouting (F1,14=98.16, p=0.0001; Fig. 4). Marsh-wide GIS-based Assessment: Marsh-wide, ground-based GPS mapping documented an 84% decrease in perennial pepperweed from baseline conditions (Fig. 5), with extant stands reduced to trace cover levels and only minor untreated areas remaining at higher cover. During the project, the total occupied area of the endangered plant population increased by over 200%. Succession of the desirable native plant community followed suppression of pepperweed, and no non-target effects to rare plant or bird populations were detected.

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Fig. 2. Endangered plant conservation plan, red zones A,B) LELA ≤ 1m CHMO: Hand wick/wipe glyphosate; C,D) LELA > 1m CHMO: backpack spray foliar application.

Fig. 4. Total non-structural carbohydrate content (TNC) of subterranean storage organs of perennial pepperweed A) in microhabitats; B) photographs of pepperweed root crowns, roots, and rhizomes from the study site.

Management Efficacy Experiment: Twenty-four stratified random plots (10m x 4m; treatment, control) were established in two pepperweed occupied microhabitats (channel edge, marsh-terrestrial ecotone) in each of three tidal inundation zones (north, mid, bayshore) to evaluate efficacy of foliar-applied herbicide treatments to pepperweed and response of non-target plant community. Baseline and post-treatment (1, 3, 4, 15, 17, 27 MAT) response measures included live percent cover, stem density, shoot biomass, and total non-structural carbohydrate (TNC) concentration in below ground storage organs. Frequency and percent cover of all non-target plant species within plots were recorded four years. Marsh Wide Assessments: GIS maps were produced from annual ground-based GPS surveys of pepperweed patches by area, cover class, tidal inundation zone, microhabitat, and distance relative to endangered species.

Acknowledgements The project was supported by U.S. Fish & Wildlife Service Endangered Species Recovery Branch, Sacramento; California Dept. of Fish & Wildlife Rare Plant Program; and California State Parks - Bay Area District. Christina Freeman (CA State Parks) led the adaptive management team, and Jules Evens (Wildlife Biologist, Avocet Research Associates) provided ornithological census data, rail habitat delineations, and scientific expertise on the adaptive management team.

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