Current Research


Using a mechanistic understanding of plant resource use to improve invasive species management in rangeland systems

Collaborators: Carolyn Malmstrom (Michigan State University), Kevin Rice (UC Davis)

Funder: USDA NRI: Biology of Weedy and Invasive Species in Agroecosystems

The main goal of this project is to increase our ability to understand and manage invasion dynamics in rangelands.  We are particularly interested in controlling medusahead (Taeniatherum caput-medusae) and barbed goatgrass (Aegilops triuncialis) invasion into mixed annual grassland systems. This project:

  1. Establishes the timing and magnitude of plant resource use (nutrients and water) for different species, and mixtures of species
  2. Establishes the timing and magnitude of resource availability in soils
  3. Explores the impacts of the timing of grazing on resource availability and thus plant competitive interactions
  4. Explores the impacts of resource manipulations on plant competitive interactions.
  5. Explores how landscape patterns of invasion are related to soil resource distribution.

This occurs on planted plots in controlled settings, as well as on naturally occurring invaded and uninvaded patches on private ranches.



Understanding rangeland seedling dynamics: a tool to increase forage quality and quantity while providing multiple ecosystem services

Collaborator: Chuck Vaughn (HREC), Hopland Research & Extension Center

Funders: USDA NRI Managed Ecosystems (has funded the core of this study), Kearney Foundation of Soil Science (has funded comparative observational and manipulative studies at two additional sites, allowing us to explore links between plant populations and ecosystem processes along a precipitation gradient).

Range managers face increasing pressure to develop management practices that maximize forage quantity and quality while conserving native species, enhancing water quality, storing soil carbon, and minimizing weeds and erosion. Managing ecosystems for multiple goals involves careful evaluation of tradeoffs, thresholds, and feedbacks associated with multiple ecosystem processes (Eviner and Chapin 2001). Not only will management practices greatly benefit from an enhanced understanding of ecological dynamics, but managing ecosystems for multiple goals is the ultimate scientific test of our integrated understanding of various ecosystem components. Currently, in most rangeland systems, Residual Dry Matter (RDM), the amount of old aboveground tissue remaining at the beginning of a new growing season, is widely used as benchmark for levels of sustainable grazing, largely based on its role in shaping plant community composition, determining forage productivity, and minimizing erosion and nutrient loss (Bartolome et al. 2002). However, all of these ecosystem functions are influenced by many mechanisms (Eviner and Chapin 2003), and focusing on just one of these as a management tool is likely to restrict our ability to manage rangeland systems for multiple functions under various environmental and management conditions.

Our work in California annual grasslands suggests that a key driver of plant productivity and nutrient recycling has been overlooked—high seed production and subsequent self-thinning of grasses (Eviner and Firestone 2007, Eviner and Vaughn in prep). Seedling thinning accounts for 37-63% of annual internal N cycling, and greatly differs from senesced litter (RDM) in its quality and timing of N release. 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.

There are a few key goals of this project:

  1. Document the fates, fluxes, and timing of N and C derived from senesced litter vs. seedling thinning. In particular, do plants get most of their N from dead litter or dying seedlings?

Hopland Research and Extension Center has just completed the rebuilding of its lysimeter facility for this project (see picture). To one set of lysimeters, we will add litter labeled with 15N and 13C, and unlabeled seeds. To another set of lysimeters, we will add labeled seeds and unlabeled litter. The fate and fluxes of N and C derived from these different sources will be followed at 4 time points throughout the growing season. We will assess the contribution of these different sources to: leachate, trace gas loss, plant uptake, microbial biomass, soil available pools, total soil pools.

  1. Determine how environmental and management conditions alter the absolute and relative quantities of litter and seed density/seedling thinning, and the relative importance of these inputs in explaining variability in multiple ecosystem processes and services (NPP, N cycling and retention, C cycling and storage, water infiltration, weed control, erosion control).

Plant biomass, seed production, and seedling thinning vary greatly over space and time due to different environmental conditions and management practices. At twelve sites differing in environmental and management conditions, we are monitoring yearly seed and litter production. Every 6 weeks throughout the growing season, we are monitoring seedling density and biomass, litter biomass, and nutrient inputs from both of these sources. We are determining to what extent variability in litter and seedling dynamics explain variability in: plant growth, N cycling, N retention, erosion potential, weed presence, soil water infiltration, and water holding capacity. Two 5m x 5m plots are set up at each site. One represents natural conditions, the 2nd is a small mammal exclosure, designed to elucidate the potentially large role of these granivores in mediating plant and ecosystem dynamics.

  1. Manipulate RDM vs. seed inputs and determine the impacts of these inputs on multiple ecosystem processes and services (NPP, N cycling and retention, C cycling and storage, water infiltration, weed control, erosion control).

We will establish plots that differ in the absolute and relative inputs of RDM and seeds in order to determine how the amount  and ratio of these inputs impact various ecosystem processes.

  1. Train students to collaborate with land managers in researching and applying scientific knowledge.


Related project: Effects of seed density and composition on plant growth and nutrient content

Collaborator: Maria Uriarte (Columbia U.)

In a greenhouse setting, we manipulated the density and identity of neighbors to test these impacts on plant productivity, nutrient uptake, and plant seed production. Density treatments included: low density (similar to mature plant density, thus minimizing thinning), and high density (typical field densities that allow for substantial amounts of thinning). High density treatments doubled plant productivity, N content, and seed production. This suggests that the nutrients provided by seedling thinning have strong impacts on range productivity, and that these impacts are likely to persist into subsequent field seasons. (Eviner and Uriarte, in prep.)


Plant species effects on multiple ecosystem processes: variations due to time, environmental conditions, and neighboring species

Collaborators: Audubon California, Mary Cadenasso (UC Davis), Christine Hawkes (U. Texas Austin), Carolyn Malmstrom (Michigan State U), Kevin Rice (UC Davis), Susan Ustin (UC Davis)

Funder: Kearney Foundation of Soil Science, California Nitrogen Assessment Team funded through the Packard Foundation

Plant community composition can alter most soil properties and processes, and thus vegetation manipulations have been commonly used to provide critical soil services in managed landscapes. While there is a strong conceptual background for predicting the average impacts a plant community has on soil, managers need to employ site-specific understanding because the effects of a given plant community change across environmental conditions and management practices.

In this project, we seek to develop a mechanistic understanding of plant impacts on multiple soil processes, and how these impacts change:

across space:

  • soil depth
  • gradients of water and nutrients
  • soil type, aspect
  • management practices
  • vegetation patch size
  • location along transition zones between communities

over time:

  • daily (due to hydraulic lift)
  • seasonally
  • since plant establishment (building up on plant effects)
  • since removal (legacy of previous vegetation)

These goals will be pursued using experimental and observational approaches from plot to landscape scales. In all of these approaches, we will investigate the spatial and temporal scales outlined above as they relate to soil water, carbon and nitrogen dynamics:

  1. The foundation for a mechanistic understanding of plant effects on soils will be developed through experimental plots planted with monocultures and mixtures of California grassland plants. These plots will be exposed to varying levels of water, nutrients, and grazing. The conceptual framework developed here will be used as a guide to understand landscape-level heterogeneity (below), and will be improved upon by integrating plot-scale with landscape-scale studies.
  2. Moving from controlled manipulations of environmental factors to landscape heterogeneity in multiple factors, we are evaluating the ecosystem effects of previous restoration sites (planted with a similar mix of native perennial grasses) versus adjacent unrestored sites (consisting of a similar mix of annual exotic grasses across sites). These restored sites vary from 0-20 years old, and are situated across the landscape under various soil, slope, aspect, and climate conditions.


Impacts of precipitation change, nutrient deposition, and grazing management on community interactions between native grasses, exotic forage, and exotic noxious weeds in California grasslands

Collaborators: Carolyn Malmstrom (Michigan State U.), Kevin Rice (UC Davis)

Funders: Hatch and Multi-State Funds, USDA NRI: Biology of Weedy and Invasive Species in Agroecosystems

Plant composition and ecosystem dynamics in semi-arid systems are strongly impacted by the timing and quantity of precipitation. Current precipitation patterns vary across sites and years in California grasslands, and climate change models predict that California grasslands will either experience wetter, longer growing seasons, or shorter drier seasons. In this study, we investigate:

  1. How predicted shifts in precipitation impact plant community composition and diversity, particularly the presence of noxious weeds (recent invaders), annual forage species (invaded 200-300 years ago), and native grasses and forbs. In particular, we are interested in how these patterns are influenced by initial plant composition, nitrogen deposition, and timing of grazing.
  2. What mechanisms underlie plant community responses? We will investigate the mechanisms driving community responses, focusing on the role of species differences in demographic traits and phenology of resource uptake.
  3. How do plant species shifts influence the timing and magnitude of water, carbon, and nutrient fluxes under different precipitation regimes? Do these plant effects feedback to further alter vegetation dynamics?


Prescribed grazing to restore rangeland soil quality, plant diversity, water quality and agricultural productivity

Collaborators: Ken Tate (Lead PI), Leslie Roche, Justin Derner, Mark Lubell, Toby O’Geen, Mel George, Bethany Cutts, Alexis Robertson, DJ Eastburn, Sierra Foothills Research & Extension Center

Funder: USDA Range Research Program and USDA Western Sustainable Agriculture Research and Extension Program

We are involved in a large collaborative project that investigates the effects of the season and intensity of cattle grazing on the provision of multiple ecosystem services in California’s rangelands. This controlled experimental study is coupled with observational ranch-scale monitoring, as well as with social science research determining the social-cultural-economic- institutional factors driving how range managers assess and use grazing management information.

For full project details, visit this link.




Effects of elevated carbon dioxide and/or ozone on litter chemistry and decomposition dynamics in soybean-corn rotations

Collaborators: Ross Fitzhugh (U. Illinois), Bart Hoorens, Rod Venterea (USDA ARS)


Increases in atmospheric carbon dioxide (CO2) and tropospheric ozone (O3) concentrations are two important components of global environmental change. To date, studies of the ecosystem-level effects of CO2 have focused primarily on natural ecosystems, while studies in agricultural ecosystems have focused on crop yield and quality, to the neglect of ecosystem-level variables such as nitrogen (N) cycling.  Controls over biogeochemical cycling in managed systems can be very different from natural systems.  It is important to understand the unique responses of agricultural ecosystems to environmental change because agriculture is a significant component of the landscape, and agricultural practices can have serious environmental effects.

Overall, this project investigates how crop traits are altered by elevated carbon dioxide and/or ozone, and how these impact soil biogeochemical dynamics. Our lab’s focus is to investigate changes in litter chemistry and litter decomposition.


Plant effects on soils feedback to alter success of restoration and invasion

Collaborators: Christine Hawkes (U. Texas-Austin), Sarah Hoskinson (UCD), Hopland Research & Extension Center

For 10 years, we have cultured soil in lysimeter tanks by growing: (1) exotic annual grasses, (2) the native grass, Nassella pulchra, and (3) a mix of the native and exotic grasses. Over this time, we have found that exotic grasses vs. Nassella strongly differ in their impacts on the timing, location, amount, and form of soil resources (Hawkes et al. 2005, Eviner and Hawkes, in prep.), and in the composition of the microbial community (Hawkes et al. 2006). These changes in soil resources match each plant type’s requirements from the soil, suggesting that these plants change the soil to benefit themselves. We are investigating:

  1. To what extent do species’ effects on soil feedback to alter competitive dynamics between natives and exotics?
  2. The relative importance of the multiple feedback mechanisms in mediating competitive dynamics.
  3. The extent to which different soil properties need to be altered to enhance restoration success.


Plant impacts on soils persist even after vegetation has been replaced by other species

Collaborator: Hopland Research & Extension Center

It has been well established that plant species can differ in their effects on almost every aspect of ecosystem structure and function, but there is relatively little understanding of how reversible these effects are, and how the nature and duration of these legacy effects vary across different soil processes. We are investigating this in plots that had been maintained as monocultures of 8 different species for 4 years. Within two years after weeding ceased, vegetation composition was identical across all plots. We have been sampling soils annually in these plots for over 7 years to monitor how carbon and nitrogen dynamics differ due to previous vegetation composition.


Graduate student projects

Current graduate students in the lab are involved in research projects that are independent of those listed above. These include:

  1. Evan Batzer: The impacts of nitrogen deposition on spatial patterns of vegetation composition and diversity in California annual grasslands.
  2. Sarah Gaffney: Effects of native grassland restoration on suppression of weeds, and the impacts of plant-soil interactions on restoration success.
  3. Julia Michaels: The impacts of current and past livestock grazing on vegetation composition and beta-diversity in California’s vernal pools.
  4. Ben Waitman: Effects of nitrogen deposition on interactions between plants, ectomycorrhizae, and soil processes.

Past graduate student projects include:

  1. Kelly Garbach: Spatial dimensions of the ecosystem services provided by conservation plantings in agroecosystems in Costa Rica, as part of the payment for ecosystem services program.
  2. Jill Baty: Effects of plant species on the role of soil foodwebs in litter decomposition, and how global change factors will alter these impacts.
  3. Marguerite Mauritz: Community interactions in Coastal Sage Scrub as mediated by plant-soil-water interactions and fire.
  4. Tracy Erwin: Understanding ecotypic differences among populations of the rare species, Cordylanthus palmatus, and how these can be applied to conservation and restoration management.
  5. Liz Goebel: Controls over distribution of remnant stands of native grasses, and importance of grass-oak interactions in maintaining native oak savannah communities.
  6. Sarah Hoskinson: Effects of the proportion of plant species in mixture on ecosystem processes and plant-soil feedbacks.