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Papers by Vanessa D Tobias

Many estuarine ecosystems and the fish communities that inhabit them have undergone substantial c... more Many estuarine ecosystems and the fish communities that inhabit them have undergone substantial changes in the past several decades, largely due to multiple interacting stressors that are often of anthropogenic origin. Few are more impactful than droughts, which are predicted to increase in both frequency and severity with climate change. In this study, we examined over five decades of fish monitoring data from the San Francisco Estuary, California, U.S.A, to evaluate the resistance and resilience of fish communities to disturbance from prolonged drought events. High resistance was defined by the lack of decline in species occurrence from a wet to a subsequent drought period, while high resilience was defined by the increase in species occurrence from a drought to a subsequent wet period. We found some unifying themes connecting the multiple drought events over the fifty-year period. Pelagic fishes consistently declined during droughts (low resistance), but exhibit a considerable am...

Water hyacinth is a major invasive species that has modified ecosystem functioning in the Sacrame... more Water hyacinth is a major invasive species that has modified ecosystem functioning in the Sacramento-San Joaquin Delta (hereafter, Delta). Studies in lakes and rivers have shown that water hyacinth can alter water quality. In tidal systems, such as the Delta, water moves back and forth through the water hyacinth patch so water quality directly outside the patch in either direction is likely to be directly influenced by the patch. In this study, we asked whether the presence or treatment of water hyacinth with herbicides resulted in changes in water quality in this tidal freshwater system. We combined existing datasets that were originally collected for permit compliance and for long-term regional monitoring into a dataset that we analyzed with a before-after control-impact (BACI) framework. This approach allowed us to describe the effects of presence as well as treatment of water hyacinth, while accounting for seasonal patterns in water quality. We focused on temperature, dissolved ...

Wetlands Ecology and Management, 2015

Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one ... more Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one million waterbirds annually. Although the landscape has been altered for well over a century by increasing levels of urbanization, and by the historic establishment of evaporator ponds for salt production, it remains heavily used by waterbirds. The area also hosts the west coast’s largest tidal wetlands restoration project; the South Bay Salt Pond Restoration Project is implementing a plan to convert thousands of acres of salt ponds into tidal and managed wetland habitat. While the restoration to tidal marsh will increase habitat for many species, it also will reduce the overall habitat available for waterbirds. Through adaptive management, the Project is committed to maintaining historic levels of waterbirds in this landscape. To inform these efforts, the USFWS, San Francisco Bay Bird Observatory, USGS and UC Davis partnered to assess changes in bird population abundance and community c...

Communications in Soil Science and Plant Analysis, 2014

Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one ... more Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one million waterbirds annually. Although the landscape has been altered for well over a century by increasing levels of urbanization, and by the historic establishment of evaporator ponds for salt production, it remains heavily used by waterbirds. The area also hosts the west coast’s largest tidal wetlands restoration project; the South Bay Salt Pond Restoration Project is implementing a plan to convert thousands of acres of salt ponds into tidal and managed wetland habitat. While the restoration to tidal marsh will increase habitat for many species, it also will reduce the overall habitat available for waterbirds. Through adaptive management, the Project is committed to maintaining historic levels of waterbirds in this landscape. To inform these efforts, the USFWS, San Francisco Bay Bird Observatory, USGS and UC Davis partnered to assess changes in bird population abundance and community c...

Many factors, such as rising sea levels and human alterations, threaten coastal wetlands in the U... more Many factors, such as rising sea levels and human alterations, threaten coastal wetlands in the United States and around the world. To reverse some wetland loss, dredge material, tidal flow, or river flow can be used to create new wetlands or existing wetlands can be managed to increase plant productivity. Identifying the causes of limited production can improve management plans by suggesting possible remedies. Managing and restoring marshes depends on understanding which stress factors limit growth of key marsh-building plants. Spartina patens is a common marsh-building species of grass in brackish marshes along the Gulf of Mexico and Atlantic coasts of North America and it is often the target of management and restoration plans. S. patens was grown under six flooding levels in a field experiment. S. patens plants grown at lower elevations had consistently lower biomass, in contrast to S. alterniflora, which has been shown to exhibit peak biomass at intermediate levels of flooding. Critical values of elemental concentrations in plant tissue are widely used to diagnose mineral deficiencies in agricultural crops and are just beginning to be developed aid wetland management and restoration. For leaf tissue harvested in summer, [Mn] < 256 ppm and [Ca] < 0.3% indicated that plants grew at lower elevations and had limited biomass. The results suggest that concentrations of Mn and Ca in the leaf tissue of S. patens could form part of an indicator to monitor below-ground productivity of marshes. Although low concentrations of these elements were associated with the smaller root and shoot biomass of plants grown at low elevations, variability in the relationship suggests that additional factors may need to be considered. Marsh managers should monitor soil elevation loss carefully if they choose to drain marshes to increase plant production to prevent excessive loss of soil elevation.

Background/Question/Methods Perennial pepperweed (Lepidium latifolium, LELA) is an aggressively i... more Background/Question/Methods Perennial pepperweed (Lepidium latifolium, LELA) is an aggressively invasive crucifer that is a major concern for coastal management. This plant invades a wide variety of habitats including saline wetlands, riparian areas, and upland rangelands. Traditional management actions such as hand removal and mowing are impractical for controlling pepperweed in wetland habitats. Understanding the physical factors that influence invasion and the efficacy of herbicides to control LELA in saline environments will improve management plans. We mapped LELA occupancy in a saline marsh at San Pablo Bay National Wildlife Refuge (San Francisco Bay, California) before and after spraying with imazapyr. We also collected environmental data including elevations, locations of channels, and vegetation types from freely-available sources. Pixels for each dataset were 2 m by 2 m. We combined LELA locations and environmental data into two generalized additive models (GAMs): one desc...

Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one ... more Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one million waterbirds annually. Although the landscape has been altered for well over a century by increasing levels of urbanization, and by the historic establishment of evaporator ponds for salt production, it remains heavily used by waterbirds. The area also hosts the west coast’s largest tidal wetlands restoration project; the South Bay Salt Pond Restoration Project is implementing a plan to convert thousands of acres of salt ponds into tidal and managed wetland habitat. While the restoration to tidal marsh will increase habitat for many species, it also will reduce the overall habitat available for waterbirds. Through adaptive management, the Project is committed to maintaining historic levels of waterbirds in this landscape. To inform these efforts, the USFWS, San Francisco Bay Bird Observatory, USGS and UC Davis partnered to assess changes in bird population abundance and community c...

Wetlands Ecology and Management, Oct 9, 2015

Perennial pepperweed (Lepidium latifolium) is an aggressively invasive species that spreads by ve... more Perennial pepperweed (Lepidium latifolium) is an aggressively invasive species that spreads by vegetative growth and seeds. Common methods for removal such as hand-pulling and mowing are impractical in brackish marsh environments. We evaluated the effects of two herbicide treatments (imazapyr and imazapyr + glyphosate) against a non-herbicide control (flower head removal) on invasive pepperweed and native vegetation in three habitats (bay edge, channel edge, and levee) in brackish marshes. Both herbicide treatments produced significantly better control of pepperweed than the control, but imazapyr alone took two years of treatment to produce levels of control that were similar to one year of the imazpyr + glyphosate treatment. Both herbicide treatments also reduced native cover, but the effects were more severe in plots treated with imazapyr + glyphosate than in plots treated with imazapyr alone. Effects on pepperweed were similar across the three habitats, but impacts on native vegetation were less severe in bay edge environments. Managers should consider the tradeoffs when choosing a treatment plan for pepperweed: the quick reduction of pepperweed achieved by the combination of imazapyr and glyphosate may come at the expense of creating opportunities for reinvasion by causing bare ground and/or patches of litter that are slowly recolonized by native species.

Marsh loss is a problem in many areas around the world. In order to combat the problem, scientist... more Marsh loss is a problem in many areas around the world. In order to combat the problem, scientists and managers need tools to determine its cause and evaluate the effectiveness of management techniques. Current methods for estimating productivity and identifying factors that limit productivity are too time-consuming or expensive for wide-spread, regular use, however. In coastal Louisiana, where Spartina patens (Ait.) Muhl is the most common plant, restoration seeks to slow wetland loss rates that averaged approximately 77.4 km2/year between 1978 and 2000. We used the chemical composition of leaf tissue from S. patens grown under controlled conditions to create a simple and inexpensive tool to identify salinity stress and nutrient limitation. By growing S. patens at varying nitrogen availability and salinity levels, we found that C:N ratios and Na concentrations can be used to classify factors that limit production in S. patens.

Many estuarine ecosystems and the fish communities that inhabit them have undergone substantial c... more Many estuarine ecosystems and the fish communities that inhabit them have undergone substantial changes in the past several decades, largely due to multiple interacting stressors that are often of anthropogenic origin. Few are more impactful than droughts, which are predicted to increase in both frequency and severity with climate change. In this study, we examined over five decades of fish monitoring data from the San Francisco Estuary, California, U.S.A, to evaluate the resistance and resilience of fish communities to disturbance from prolonged drought events. High resistance was defined by the lack of decline in species occurrence from a wet to a subsequent drought period, while high resilience was defined by the increase in species occurrence from a drought to a subsequent wet period. We found some unifying themes connecting the multiple drought events over the fifty-year period. Pelagic fishes consistently declined during droughts (low resistance), but exhibit a considerable am...

Water hyacinth is a major invasive species that has modified ecosystem functioning in the Sacrame... more Water hyacinth is a major invasive species that has modified ecosystem functioning in the Sacramento-San Joaquin Delta (hereafter, Delta). Studies in lakes and rivers have shown that water hyacinth can alter water quality. In tidal systems, such as the Delta, water moves back and forth through the water hyacinth patch so water quality directly outside the patch in either direction is likely to be directly influenced by the patch. In this study, we asked whether the presence or treatment of water hyacinth with herbicides resulted in changes in water quality in this tidal freshwater system. We combined existing datasets that were originally collected for permit compliance and for long-term regional monitoring into a dataset that we analyzed with a before-after control-impact (BACI) framework. This approach allowed us to describe the effects of presence as well as treatment of water hyacinth, while accounting for seasonal patterns in water quality. We focused on temperature, dissolved ...

Wetlands Ecology and Management, 2015

Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one ... more Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one million waterbirds annually. Although the landscape has been altered for well over a century by increasing levels of urbanization, and by the historic establishment of evaporator ponds for salt production, it remains heavily used by waterbirds. The area also hosts the west coast’s largest tidal wetlands restoration project; the South Bay Salt Pond Restoration Project is implementing a plan to convert thousands of acres of salt ponds into tidal and managed wetland habitat. While the restoration to tidal marsh will increase habitat for many species, it also will reduce the overall habitat available for waterbirds. Through adaptive management, the Project is committed to maintaining historic levels of waterbirds in this landscape. To inform these efforts, the USFWS, San Francisco Bay Bird Observatory, USGS and UC Davis partnered to assess changes in bird population abundance and community c...

Communications in Soil Science and Plant Analysis, 2014

Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one ... more Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one million waterbirds annually. Although the landscape has been altered for well over a century by increasing levels of urbanization, and by the historic establishment of evaporator ponds for salt production, it remains heavily used by waterbirds. The area also hosts the west coast’s largest tidal wetlands restoration project; the South Bay Salt Pond Restoration Project is implementing a plan to convert thousands of acres of salt ponds into tidal and managed wetland habitat. While the restoration to tidal marsh will increase habitat for many species, it also will reduce the overall habitat available for waterbirds. Through adaptive management, the Project is committed to maintaining historic levels of waterbirds in this landscape. To inform these efforts, the USFWS, San Francisco Bay Bird Observatory, USGS and UC Davis partnered to assess changes in bird population abundance and community c...

Many factors, such as rising sea levels and human alterations, threaten coastal wetlands in the U... more Many factors, such as rising sea levels and human alterations, threaten coastal wetlands in the United States and around the world. To reverse some wetland loss, dredge material, tidal flow, or river flow can be used to create new wetlands or existing wetlands can be managed to increase plant productivity. Identifying the causes of limited production can improve management plans by suggesting possible remedies. Managing and restoring marshes depends on understanding which stress factors limit growth of key marsh-building plants. Spartina patens is a common marsh-building species of grass in brackish marshes along the Gulf of Mexico and Atlantic coasts of North America and it is often the target of management and restoration plans. S. patens was grown under six flooding levels in a field experiment. S. patens plants grown at lower elevations had consistently lower biomass, in contrast to S. alterniflora, which has been shown to exhibit peak biomass at intermediate levels of flooding. Critical values of elemental concentrations in plant tissue are widely used to diagnose mineral deficiencies in agricultural crops and are just beginning to be developed aid wetland management and restoration. For leaf tissue harvested in summer, [Mn] < 256 ppm and [Ca] < 0.3% indicated that plants grew at lower elevations and had limited biomass. The results suggest that concentrations of Mn and Ca in the leaf tissue of S. patens could form part of an indicator to monitor below-ground productivity of marshes. Although low concentrations of these elements were associated with the smaller root and shoot biomass of plants grown at low elevations, variability in the relationship suggests that additional factors may need to be considered. Marsh managers should monitor soil elevation loss carefully if they choose to drain marshes to increase plant production to prevent excessive loss of soil elevation.

Background/Question/Methods Perennial pepperweed (Lepidium latifolium, LELA) is an aggressively i... more Background/Question/Methods Perennial pepperweed (Lepidium latifolium, LELA) is an aggressively invasive crucifer that is a major concern for coastal management. This plant invades a wide variety of habitats including saline wetlands, riparian areas, and upland rangelands. Traditional management actions such as hand removal and mowing are impractical for controlling pepperweed in wetland habitats. Understanding the physical factors that influence invasion and the efficacy of herbicides to control LELA in saline environments will improve management plans. We mapped LELA occupancy in a saline marsh at San Pablo Bay National Wildlife Refuge (San Francisco Bay, California) before and after spraying with imazapyr. We also collected environmental data including elevations, locations of channels, and vegetation types from freely-available sources. Pixels for each dataset were 2 m by 2 m. We combined LELA locations and environmental data into two generalized additive models (GAMs): one desc...

Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one ... more Background/Question/Methods The San Francisco Bay estuary provides critical habitat for over one million waterbirds annually. Although the landscape has been altered for well over a century by increasing levels of urbanization, and by the historic establishment of evaporator ponds for salt production, it remains heavily used by waterbirds. The area also hosts the west coast’s largest tidal wetlands restoration project; the South Bay Salt Pond Restoration Project is implementing a plan to convert thousands of acres of salt ponds into tidal and managed wetland habitat. While the restoration to tidal marsh will increase habitat for many species, it also will reduce the overall habitat available for waterbirds. Through adaptive management, the Project is committed to maintaining historic levels of waterbirds in this landscape. To inform these efforts, the USFWS, San Francisco Bay Bird Observatory, USGS and UC Davis partnered to assess changes in bird population abundance and community c...

Wetlands Ecology and Management, Oct 9, 2015

Perennial pepperweed (Lepidium latifolium) is an aggressively invasive species that spreads by ve... more Perennial pepperweed (Lepidium latifolium) is an aggressively invasive species that spreads by vegetative growth and seeds. Common methods for removal such as hand-pulling and mowing are impractical in brackish marsh environments. We evaluated the effects of two herbicide treatments (imazapyr and imazapyr + glyphosate) against a non-herbicide control (flower head removal) on invasive pepperweed and native vegetation in three habitats (bay edge, channel edge, and levee) in brackish marshes. Both herbicide treatments produced significantly better control of pepperweed than the control, but imazapyr alone took two years of treatment to produce levels of control that were similar to one year of the imazpyr + glyphosate treatment. Both herbicide treatments also reduced native cover, but the effects were more severe in plots treated with imazapyr + glyphosate than in plots treated with imazapyr alone. Effects on pepperweed were similar across the three habitats, but impacts on native vegetation were less severe in bay edge environments. Managers should consider the tradeoffs when choosing a treatment plan for pepperweed: the quick reduction of pepperweed achieved by the combination of imazapyr and glyphosate may come at the expense of creating opportunities for reinvasion by causing bare ground and/or patches of litter that are slowly recolonized by native species.

Marsh loss is a problem in many areas around the world. In order to combat the problem, scientist... more Marsh loss is a problem in many areas around the world. In order to combat the problem, scientists and managers need tools to determine its cause and evaluate the effectiveness of management techniques. Current methods for estimating productivity and identifying factors that limit productivity are too time-consuming or expensive for wide-spread, regular use, however. In coastal Louisiana, where Spartina patens (Ait.) Muhl is the most common plant, restoration seeks to slow wetland loss rates that averaged approximately 77.4 km2/year between 1978 and 2000. We used the chemical composition of leaf tissue from S. patens grown under controlled conditions to create a simple and inexpensive tool to identify salinity stress and nutrient limitation. By growing S. patens at varying nitrogen availability and salinity levels, we found that C:N ratios and Na concentrations can be used to classify factors that limit production in S. patens.