The restoration of urban creek ecosystems is tremendously challenging: human community needs , heavy pollutant inputs, hydrologic alterations, and frequent disturbance complicate management and make many noble restoration goals infeasible . One significant challenge that many restoration projects face is that of invasive non-native plant species , an issue that can be particularly pronounced in urban areas due to the connectivity of urban centers in an increasingly globalized world . Restoration of native flora is frequently cited as a goal in restoration projects , but can be exceedingly difficult. Removing invasive species without effectively establishing other desired species leaves a “weed-shaped hole” that non-natives can quickly re-colonize , . Though the hope behind earnest non-native removal efforts is that native species will re-colonize the area once niche space becomes available, the evidence that this occurs without further intervention is limited , particularly if the native species have been extirpated from the area and thus propagule material does not exist. Funk et al. propose the concept of “limiting similarity” to reduce the possibility of re-invasion by non-natives . The idea is that non-invasive native species that have similar functional traits to non-natives are expected to be better competitors and prevent the reinvasion of non-native species . ‘Functional traits’ are species’ attributes relating to how the species takes up resources and its effect on the resource pool in the ecosystem .
The limiting similarity concept encourages practitioners to fill the ‘weed-shaped hole’ with native species that will prevent non-native invaders from accessing resources in the ecosystem. Nitrogen is a critically important resource in ecosystem management. Nitrogen deposition has been implicated in facilitating invasion of nutrient poor California ecosystems by non-native plant species, rolling benches for growing particularly near urban areas with abundant fertilizer use and combustion-powered machinery . Furthermore, nitrogen has the potential to cause eutrophication of downstream waterways if it is provided in excess by urban runoff . This work builds off of Cadenasso et al. in that I suggest urban riparian restoration plantings as a method to prevent nitrogen pollution of the watershed. In this article I operationalize limiting similarity in the context of a working, volunteer-based restoration project on the University of California – Berkeley campus. Plant functional traits were measured to filter the regional species pool to a set of native plant species best suited to achieve desired project goals, namely to prevent re-invasion by non-native ivy species , and to prevent nitrogen pollution of the creek and riparian habitat. Within the broader trait-filtering framework, I hone in on the selection of native species with high rates of nitrogen uptake, as determined by a stable isotope tracer analysis. Enhancing riparian nitrogen uptake has the potential to both slow the rate of nitrogen delivery to the stream and help prevent re-invasion of riparian habitat by nonnatives . Finally,this research serves as an example of the sort of collaboration encouraged by Palmer , in which campus scientists inform the work of an ‘on-the-ground’ restoration program, which can then provide feedback with regard to the success of different approaches.
Strawberry Creek is an urbanized watercourse that runs east to west through Berkeley , California, from the Berkeley Hills to the San Francisco Bay . The creek has two forks that converge near the west entrance to the UC Berkeley campus . The 4.7 km^2 watershed drained by the creek is relatively undisturbed in the hills east of the campus, but is for the most part heavily urbanized, with impervious surfaces becoming the norm as the creek flows west through the flat lands of Berkeley . The creek flows in underground culverts for the majority of its path, including immediately east and west of the UCB campus. This study focuses on the reaches of the creek within the confines of the UCB main campus, to match the spatial scope of the work done by the partner restoration program. The establishment of the university along the banks of Strawberry Creek led to substantial degradation of its aquatic and riparian habitat. Trash dumping, sewage discharges, and campus lab waste made the creekatoxic siteformostofthe20thcentury . The creek’s course and riparian habitat were substantially modified to prevent flooding of campus buildings, which has led to significant incision and channelization .In the late 1980s,the Strawberry Creek Restoration Program was born, which led to substantial water quality improvement and native fish reintroduction to the creek . Understory habitat at Strawberry Creek is dominated by English and canary ivy , both non-native, invasive species. In recent years, the SCRP has shifted its focus to student-led, volunteer-driven understory vegetation management; perhaps the program’s biggest impact has been the removal of vast swaths of ivy from the shores of the creek. Other invasive species like periwinkle and panic veldtgrass have also been removed, which has resulted in largely unoccupied understory habitat for substantial stretches of Strawberry Creek.
To date, re-colonization of this habitat by native plant species has not occurred, and re-invasion of these habitats by weedy species occurs frequently . Ivy frequently returns to sites from which it was removed, usually as a result of incomplete removal of root biomass. The SCRP has recently increased its native plant output in an attempt to reintroduce native species to the banks of Strawberry Creek; the Program’s interest in discovering which native plant species will do best in this urbanized ecosystem guides this research. Volunteers with the SCRP helped clear nonnatives and plant native species at all of the sites mentioned below.Nine functional traits were measured on 38 plant species native to Alameda County, following from the methods in Cornelissen et al. . The regional species pool was narrowed to 38 species through a variety of considerations, most notably through the elimination of native species for which I did not have access to propagule material or species that did not grow well in the SCRP’s on-campus nursery . The 38 species were almost entirely understory species, a function of the SCRP’s focus on understory management. Species were selected from a variety of different habitat types to minimize the possibility of ‘pre-selecting’ species assumed to do well at Strawberry Creek. In addition to the native species, functional traits were measured on two non-native species: canary and English ivy. These species were included to discern the relative differences in traits between the native and nonnative species, an important prediction of limiting similarity. Ages and propagation methods were standardized across functional groups, to the extent practicable ; the SCRP’s long-standing nursery program had some gaps in records, making it difficult to determine the exact age or geographic origin of some individuals.The selected traits relate to diverse aspects of species morphology . When possible, ratios were used instead of raw values to minimize the effect of any age differences. All trait measurements were taken on plants grown in nursery settings . Trait measurements were taken on five replicate individuals for each plant species, cannabis dry racks then averaged across the replicates. The five replicates were spaced across five blocks in the nursery and species position within each block was randomized to minimize neighbor effects or the effects of divergent growing conditions.Plant nitrogen uptake is a focal point of this research, but was treated differently from the traits listed above. I aimed to discover how nitrogen uptake rates vary across species of different growth forms and geographic origins . A nitrogen-15 stable isotope tracer analysis was conducted to address these questions. For this analysis, five species representing four functional groups were given 15N-labeled ammonium chloride injections . These species were also included in the broader trait-based filtering study. Individuals used for the nitrogen uptake analysis were all of the same age and were all sourced from the Strawberry Creek watershed.
Our interest in controlling these factors, in addition to cost constraints, motivated the choice to evaluate nitrogen uptake only on representative species from each functional group, rather than test all species. As above, propagation methods were standardized within functional groups. In addition to four native species, the nitrogen uptake rate of canary ivy was also analyzed,to allow for the comparison of native and non-native uptake rates. Five replicate individuals of each species were given 15N injections. This work follows from James & Richards in terms of quantity of nitrogen delivered to the system. I assumed that in an urban setting, nitrogen will most likely be delivered to the riparian corridor in ‘pulse’ events carrying large amounts of nitrogen, e.g. rainstorms. However, I modified the methods in James & Richards to adjust for the size of the SCRP’s planting containers and different percent enrichment of 15N. In total, I added 2 mg of 15N in a 176.7 mL solution with de-ionized water to each plant. The solution was delivered via 18 injections in a circle around the base of the plant,to a depth of 10 cm.I attempted to label all parts of the soil column uniformly, injecting the solution into the soil at a slow and steady pace as I moved the syringe up through the soil column . The plants were harvested 13 days after injection.I chose to wait for a relatively long period of time between injection and harvest because this analysis was carried out in the non-growing season . The decision to perform the injections in the winter was pragmatic, based on this project’s timeline. Plants were kept dry for the week prior to injection and were not watered for the 13 days following injection. After the 13 days had passed, the plants were harvested and all plant biomass was dried and weighed. Leaf samples and root samples were collected for each plant; each sample was then ground and homogenized. Roots and leaves remained separate throughout this process. Approximately 5.5 mg of each sample was then weighed into tin capsules, yielding 50 samples: 5 species x 5 replicates x 2 samples/individual. These samples were combusted in an elemental analyzer, and isotopic ratios were analyzed by a mass spectrometer, yielding leaf and root 15N content for each individual plant.The California rice [Oryza sativa L.] growing region comprises approximately 200 000 ha in the Sacramento Valley. The region, which is among the highest-yielding in the world for rice production , is characterized by hot, dry, summers with abundant sunshine, and supports a single crop per year . The rice cropping system is almost exclusively water-seeded, wherein pre-germinated seed is sown by aircraft into flooded fields. Seeds sink to the soil surface and peg down roots, emerging from the water after several days. Floodwaters are generally kept to 10 to 20 cm depth for the entire season. Water seeding was widely adopted in the region in the 1920s as a means to suppress competitive grass weeds , and has been the predominant method of rice cultivation in California ever since . Continuous use of water seeding has resulted in a small spectrum of weed species that are well-adapted to the system, and are very competitive with rice . California rice has a limited number of available herbicides, due to the high cost of development and registration , as well as strict regulations based on concerns of herbicide drift from aerial applications that may damage neighboring orchards . Herbicide resistance has been a major biologic and economic issue in rice for decades . The lack of diversity of registered active ingredients means that once resistance to a particular mode of action arises, it can spread rapidly within and among fields as there may be few alternative herbicides to control the resistant populations. Efforts to promote herbicide resistance mitigation in CA largely focus on rotation of the limited number of available herbicides, while the cropping strategy itself has remained largely static.Most cultural methods for weed and resistance management in California are modifications of the dominant water seeded system . One such method used by some growers is a stale seedbed. In this method, rice seedbeds are prepared as usual and flushed with water to promote weed germination. Non-selective herbicides are used as a burndown treatment , and afterward the fields are flooded and seeded as usual. This method can be a useful strategy to manage weeds that are resistant to rice herbicides, as well as reduce weed seedbanks overall. However, due to the time needed to reflood and seed fields, stale seedbed use can delay rice planting, shortening the growing season and potentially depressing yields .