ABSTRACT OF THE DISSERTATION

Invasive Plant-Soil Feedbacks and Ecosystem Resistance and Resilience:

A Comparison of Three Vegetation Types in California

By Sara Jo Dickens

Doctor of Philosophy, Graduate Program in Plant Biology

University of California, Riverside, December 2010

Dr Edith B. Allen, Chairperson

Ecosystem processes are strongly dependant on plant-soil feedbacks. The invasion of exotic plant species can result in the introduction of novel traits capable of de-coupling native plant-soil feedbacks and leading to altered nutrient cycling and availability and microbial community composition. In general the degree to which an invading species will impact the system it invades is dependant on how much it differs from plant species native to that system. However, there are examples in which invasion of an exotic plant similar to natives has led to significant alterations of ecosystem processes. The objective of this work was to examine the impacts of a single suite of exotic annual plants invading three very different vegetation types in southern California, grasslands, coastal sage scrub and chaparral. I predicted that invasion of exotic annuals would have greater impacts on shrubland systems than grasslands due to the greater disparity in plant traits between the exotic annuals and the shrub species. Comparisons of invasion impacts, however must consider factors other than vegetation type such as soil parent material, pH and soil moisture as mechanisms by which a native system may be more or less resistant to changes associated with invasion and recover following native vegetation re-establishment. In order to examine vegetation type resistance to invasion and soil resilience of these systems, I analyzed soils for total carbon and nitrogen, extractable phosphorus and nitrogen, nitrogen mineralization, soil respiration and microbial community composition using phospholipid fatty acid analysis. Long and short-term restorations were sampled concurrently with the invaded and native vegetation types to assess resilience of soils. Regardless of vegetation type nitrate was reduced by invasion, seasonality of sampling was a greater driver of microbial community composition than invasion or restoration and abiotic factors proved to be important to microbial species composition and soil nutrient availability. Individual chemical pools and functional groups of microbes responded differently in each vegetation type. However, the degree to which invading species differed from natives did not predict the level to which invasion would impact the system.

Historically, over 4,000 acres of the Orange County Central & Coastal Sub region NCCP/HCP Reserve System have been invaded by a number of exotic plant species including several annual grasses, artichoke thistle (Cynara cardunculus, CYCA), mustards and other exotic forbs. Invasion has resulted in displacement of many desirable plant species and overall degradation of the reserve’s target plant community, coastal sage scrub. To control the spread of CYCA, the Nature Reserve of Orange County (NROC) established a management program involving spot application of herbicide (Fusillade and Round-up) to individual plants. Since 1994, thousands of acres have been treated annually. Additionally, NROC has chemically and mechanically managed other exotic, invasive species such as Ricinus communis (castor bean) and Nicotiana glauca (tree tobacco). While these weed control efforts reduce target invasive species, whether the native plant community is recovering is not necessarily clear, nor is the long-term sustainability of control efforts.

Following degradation, recovery of a native system rarely follows the reverse path of degradation (Fig. 1). Instead alternative paths that may or may not have a trajectory towards the original or goal state can occur (Bradshaw; Cramer et al 2008). In highly degraded areas, sites may become stuck in an alternative or novel state. These novel or alternative trajectories and states play a key role in recovery and restoration and may be dependent on management action. A common example of this is the replacement of a targeted exotic species by a new exotic following control of the original target exotic (Denslow & D’Anonio 2005; Flory & Clay 2009). Within the NROC community, there has been concern that such a replacement occurred following the long-term CYCA removal program such that Brassica nigra (black mustard, BRNI) replaced CYCA. Assessing recovery after control measures is a powerful tool to aid in restoration decision-making and ultimately guiding future management actions for the reserve system.

Goals and Objectives:

Our objectives were to address a number of critical issues relevant to restoration planning at NROC, including:

 Task 1: Provide an updated description of the vegetation community at 109 sites within the Reserve System previously sampled in 1998, 2007 and 2008.

 Task 2: Teasing apart trends due to management efforts from other trends due to environment or land use change (e.g., grazing cessation in the early 1990s).

 Task 3: Determining thresholds in both native and weed species abundance that will allow for further unassisted recovery.

 Task 4: Identifying areas where sites are “stuck” (i.e., not recovering) and need additional intervention (e.g., seeding, mowing, soil amendments).

By capitalizing on past vegetation monitoring and additional analysis of monitoring data, we hoped to reveal the mechanisms underlying the complex dynamics of vegetation recovery over time and inform restoration decision-making within the NCCP/HCP Reserve System.

 Project Start Date: April 1, 2013

Project Completion Date: December 31, 2014

Contracted Organization: University of California, Berkeley