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.

2. 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.

3. 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.

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

 

 

 

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:

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?

 

 

 

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.

 

 

 

Graduate student projects

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

1. David Mitchell: Impacts of soil amendments on soil conditions, soil microbial communities, Phytophthora, and riparian restoration success
2. Landin Noland: Comparing goals and ecological impacts of cultural burns, prescribed burns and wildfires
3. Isaiah Thalmayer: Woody seedling restoration success, as influenced by soil inoculations from drier vs. wetter sites.

Past graduate student projects include:

1. Jill Baty: Effects of plant species on the role of soil foodwebs in litter decomposition, and how global change factors will alter these impacts.
2. Evan Batzer: The impacts of nitrogen deposition on spatial patterns of vegetation composition and diversity in California annual grasslands.
3. Tracy Erwin: Understanding ecotypic differences among populations of the rare species, Cordylanthus palmatus, and how these can be applied to conservation and restoration management.
4. Sarah Gaffney: Effects of native grassland restoration on suppression of weeds, and the impacts of plant-soil interactions on restoration success.
5. 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.
6. Liz Goebel: Controls over distribution of remnant stands of native grasses, and importance of grass-oak interactions in maintaining native oak savannah communities.
7. Sarah Hoskinson: Effects of the proportion of plant species in mixture on ecosystem processes and plant-soil feedbacks.
8. Marguerite Mauritz: Community interactions in Coastal Sage Scrub as mediated by plant-soil-water interactions and fire.
9. Julia Michaels: The impacts of current and past livestock grazing on vegetation composition and beta-diversity in California’s vernal pools.
10. Ben Waitman: Effects of nitrogen deposition on interactions between plants, ectomycorrhizae, and soil processes.