Papers by Julie Stromberg
Restoration Ecology, 1997
H umans have long been fascinated by the dynamism of free-flowing waters. Yet we have expended gr... more H umans have long been fascinated by the dynamism of free-flowing waters. Yet we have expended great effort to tame rivers for transportation, water supply, flood control, agriculture, and power generation. It is now recognized that harnessing of streams and rivers comes at great cost: Many rivers no longer support socially valued native species or sustain healthy ecosystems that provide important goods and services (Naiman et al. 1995, NRC 1992).
The Southwestern Naturalist, 2020
Eryngium sparganophyllum is an imperiled plant species with a small geographic range in the south... more Eryngium sparganophyllum is an imperiled plant species with a small geographic range in the southwestern United States and northern Mexico and is restricted to wetlands known as cienegas. We compiled information on its population status, habitat, life history, and threats to inform conservation and recovery efforts and to guide future research. Of the six historically documented populations, this species has been extirpated at two and remains extant in four. Extractions of groundwater or diversions of spring discharge are the greatest threats, exacerbated by reduced groundwater inflow to cienegas because of regional drought and temperature increases. Eryngium sparganophyllum is locally common in its known sites, but woody encroachment may be a threat, as field data in Arizona reveal the plants to be more abundant in open sun than under tree canopy. Plants flower within their first year and host a wide range of invertebrate visitors. Experimental techniques confirm seed viability with a morphophysiological dormancy; however, patterns of seedling establishment in the wild remain undocumented. Reintroduction at two sites is being implemented and long-term monitoring of survivorship and fecundity of these transplants is necessary to guide future recovery efforts.
Scientific Investigations Report, 2006
21-40. Graphs showing: 21. Mean maximum vegetation height (flood-plain zone) plus one standard de... more 21-40. Graphs showing: 21. Mean maximum vegetation height (flood-plain zone) plus one standard deviation for sites classified into three hydrology classes for unburned biohydrology study areas, SPRNCA,
Journal of Arid Environments, 2017
Ephemeral streams are abundant in drylands, yet we know little about how their vegetation differs... more Ephemeral streams are abundant in drylands, yet we know little about how their vegetation differs from surrounding terrestrial zones and about their projected response to regional warming and drying. We assessed plant communities at seven ephemeral streams (and terrestrial zones) distributed among three climatic settings in Arizona. Compared to terrestrial zones, riparian zones had similar herbaceous cover but greater woody vegetation volume. They supported more plant species, with several woody taxa restricted to the ephemeral zone (consistent with the idea that herbaceous plants are rain-dependent while riparian trees rely on runoff stored in stream sediments). Their herbaceous communities had high compositional overlap with terrestrial zones and may sustain regional diversity as droughts intensify. Presumably owing to periodic flood disturbance, riparian plant communities had greater evenness than terrestrial zones, many of which were dominated by Eragrostis lehmanniana. Patterns along the climatic gradient suggest that increasing aridity will reduce the number of herbaceous (and total) plant species within riparian zones (110 species per stream in semihumid settings, 88 in semiarid, 48 in arid) and drive compositional shifts from perennials grasses and forbs to annuals. Hotter and drier conditions will drive sharp declines in herbaceous cover, converting riparian savanna to xeroriparian scrubland.
Journal of Arid Environments, 2016
The Central Arizona Project (CAP) is a large canal system that traverses hundreds of ephemeral st... more The Central Arizona Project (CAP) is a large canal system that traverses hundreds of ephemeral stream channels in the Sonoran Desert. This longitudinal barrier alters flow during runoff events, causing water to pond behind the canal's wall. We asked: How has riparian vegetation of the ephemeral streams changed over the course of 35 years in response to canal construction? We compared field data (vegetation volume, woody plant stem density and size, and herbaceous cover) collected in distinct zones located upstream and downstream of the canal to unaltered controls. By ponding water and inducing sedimentation of fine particles, the canal has created areas that support dense vegetation. The wettest zone has the greatest vegetation volume and plant height, and supports densities of Prosopis velutina that are six times greater than in the control zone. Larrea tridentata and other desert shrubs are displaced to the border of the wettest zone, but have greater height and stem diameter than typically occur in the desert owing to increased frequency of soil wetting in the canalassociated anthropogenic-floodplain. This research aids in understanding the impacts of canal barriers on desert riparian vegetation, and can be used to predict future outcomes of proposed canals in desert environments.
Ecological Applications, 1996
Notes, 2002
Recovery Plans delineate reasonable actions that are believed to be required to recover and/or pr... more Recovery Plans delineate reasonable actions that are believed to be required to recover and/or protect listed species. Plans are published by the U.S. Fish and Wildlife Service, sometimes prepared with the assistance of recovery team s, contractors, State agencies, and others. Objectives will be attained and any necessary funds made available subject to budgetary and other constraints affecting the parties involved, as well as the need to address other priorities. Recovery plans do not necessarily represent the views nor the official positions or approval of any individuals or agencies involved in the plan formulation, other than the U.S. Fish and Wildlife Service. They represent the official position of the U.S. Fish and Wildlife Service only after they have been signed by the Regional Director or Director as approved. Approved Recovery plans are subject to modification as dictated by new findings, changes in species status, and the completion of recovery tasks. Some of the techniques outlined for recovery efforts in this plan are completely new regarding this subspecies. Therefore, the cost and time estimates are approximations.
[From CAB Abstracts] Riparian areas in southwestern American deserts support native plant communi... more [From CAB Abstracts] Riparian areas in southwestern American deserts support native plant communities that are more dense, structurally complex, productive and species-rich than those of the adjacent xeric uplands. Riparian areas attract human settlement and, as a result, riparian exotic plants have become an issue. The factors that have facilitated the influx of exotics into riparian corridors and the ways in which riparian ecosystem functions have been affected are discussed (with the impact of saltcedars (Tamarix spp.) as an example). High exotic abundance may be aided by flooding (enhancing dispersal and establishment opportunities), the introduction of grazing animals and other anthropogenic influences, e.g. road development and agriculture. Some woody exotics are now widespread and locally abundant in southwestern riparian areas, including Elaeagnus angustifolia, Ailanthus altissima, Tamarix spp. and Nicotiana glauca. There are probably over 100 naturalized exotic herbaceous species, the most successful of which are probably the grasses, e.g. Cynodon dactylon, Sorghum halepense, Polypogon monspeliensis and Bromus spp. Constructive options for restoring functionality to southwestern desert riparian ecosystems are discussed.
... Recruitment of Fremont cottonwood (Populus fremontii) and Goodding willow (Salix gooddingii) ... more ... Recruitment of Fremont cottonwood (Populus fremontii) and Goodding willow (Salix gooddingii) depends on (1) large prior-year floods to remove herbaceous cover; (2) high spring flows or rains to stimulate germination on floodplains above the zone of frequent flood inundation ...
Restoration Ecology, 2009
Initially introduced to western United States to provide ecosystem services such as erosion contr... more Initially introduced to western United States to provide ecosystem services such as erosion control, Tamarix by the mid-1900s had became vilified as a profligate waster of water. This large shrub continues, today, to be indicted for various presumed environmental and economic costs, and millions of dollars are expended on its eradication. In this review, we examine the role of scientists in driving changes in perceptions of Tamarix from valuable import to vilified invader and (in some instances) back to a productive member of riparian plant communities. Scientists over the years have sustained a negative perception of Tamarix by, among other things, (1) citing outmoded sources; (2) inferring causation from correlative studies; (3) applying conclusions beyond the scope (domain) of the studies; and (4) emphasizing findings that present the species as an extreme or unnatural agent of change. Recent research is challenging the prevailing dogma regarding Tamarix's role in ecosystem function and habitat degradation and many scientists now recommend management shifts from ''pest plant'' eradication to systemic, process-based restoration. However, prejudice against this and other non-native species persists. To further close the gap between science and management, it is important for scientists to strive to (1) cite sources appropriately; (2) avoid reflexive antiexotic bias; (3) avoid war-based and pestilence-based terminology; (4) heed the levels of certainty and the environmental domain of studies; (5) maintain up-to-date information on educational Web sites; and (6) prior to undertaking restoration or management actions, conduct a thorough and critical review of the literature.
Journal of Arid Environments, 2017
The Southwestern Naturalist, 2020
Eryngium sparganophyllum is an imperiled plant species with a small geographic range in the south... more Eryngium sparganophyllum is an imperiled plant species with a small geographic range in the southwestern United States and northern Mexico and is restricted to wetlands known as cienegas. We compiled information on its population status, habitat, life history, and threats to inform conservation and recovery efforts and to guide future research. Of the six historically documented populations, this species has been extirpated at two and remains extant in four. Extractions of groundwater or diversions of spring discharge are the greatest threats, exacerbated by reduced groundwater inflow to cienegas because of regional drought and temperature increases. Eryngium sparganophyllum is locally common in its known sites, but woody encroachment may be a threat, as field data in Arizona reveal the plants to be more abundant in open sun than under tree canopy. Plants flower within their first year and host a wide range of invertebrate visitors. Experimental techniques confirm seed viability with a morphophysiological dormancy; however, patterns of seedling establishment in the wild remain undocumented. Reintroduction at two sites is being implemented and long-term monitoring of survivorship and fecundity of these transplants is necessary to guide future recovery efforts.
Ecological Applications, 1996
On 20 July 2010, the inflatable bladder dams that created the Tempe [Arizona, USA] Town Lake rupt... more On 20 July 2010, the inflatable bladder dams that created the Tempe [Arizona, USA] Town Lake ruptured, emptying the lake and exposing sediments that had been submerged for a decade. Among the 20 species of plants subsequently germinating we found three introduced taxa previously rare, misidentified or unknown in Arizona: Ammannia coccinea (Rottb.), Cyperus michelianus subsp. pygmaeus (Rottb. Asch & Graebn) and Ludwigia erecta (L.) H. Hara.
Nature, 2011
Conservationists should assess organisms on environmental impact rather than on whether they are ... more Conservationists should assess organisms on environmental impact rather than on whether they are natives.
Wetlands, 2010
Riparian fire studies in the American Southwest have focused on flow-regulated rivers and typical... more Riparian fire studies in the American Southwest have focused on flow-regulated rivers and typically show increase in introduced Tamarix, a species with high resprout capacity, and declines in Populus. Effects of fire, however, can vary with environmental setting. We examined riparian fire along the free-flowing Upper San Pedro River (Arizona) by making temporal comparisons supplemented by spatial contrasts between burned and unburned sites. Pre-fire, Populus fremontii and Salix gooddingii were dominant species, with Tamarix sparse in the understory. Species differed in mortality and resprout rates, producing post-fire vegetation change. Mortality was highest for Tamarix, intermediate for Salix, and lowest for Populus, and also varied among size classes. Resprout rate was low for Populus, high for Salix, and also high for the few surviving Tamarix. The net effect was changes in population size structure (relative shifts towards larger Populus but smaller individuals of other species) and forest composition (decreased abundance of Tamarix relative to Populus and Salix), and in spatial distribution of stems. Tamarix’s low ability to survive fire resulted from competitive suppression by Populus and Salix, with their dominance a product of the site’s flow regime. Results indicate that post-fire outcomes within the riparian Southwest are variable and context-dependent.
Diversity and Distributions, 2007
Rivers are conduits for materials and energy; this, the frequent and intense disturbances that th... more Rivers are conduits for materials and energy; this, the frequent and intense disturbances that these systems experience, and their narrow, linear nature, create problems for conservation of biodiversity and ecosystem functioning in the face of increasing human influence. In most parts of the world, riparian zones are highly modified. Changes caused by alien plants — or environmental changes that facilitate shifts in dominance creating novel ecosystems — are often important agents of perturbation in these systems. Many restoration projects are underway. Objective frameworks based on an understanding of biogeographical processes at different spatial scales (reach, segment, catchment), the specific relationships between invasive plants and resilience and ecosystem functioning, and realistic endpoints are needed to guide sustainable restoration initiatives. This paper examines the biogeography and the determinants of composition and structure of riparian vegetation in temperate and subtropical regions and conceptualizes the components of resilience in these systems. We consider changes to structure and functioning caused by, or associated with, alien plant invasions, in particular those that lead to breached abiotic- or biotic thresholds. These pose challenges when formulating restoration programmes. Pervasive and escalating human-mediated changes to multiple factors and at a range of scales in riparian environments demand innovative and pragmatic approaches to restoration. The application of a new framework accommodating such complexity is demonstrated with reference to a hypothetical riparian ecosystem under three scenarios: (1) system unaffected by invasive plants; (2) system initially uninvaded, but with flood-generated incursion of alien plants and escalating invasion-driven alteration; and (3) system affected by both invasions and engineering interventions. The scheme has been used to derive a decision-making framework for restoring riparian zones in South Africa and could guide similar initiatives in other parts of the world.
Wetlands, 2009
Dryland rivers are characterized by high spatio-temporal variability in water resources, with alt... more Dryland rivers are characterized by high spatio-temporal variability in water resources, with alternating wet and dry reaches and fluctuating stream flow rates. To investigate response to this dynamic, riparian plant communities from perennial, temporally intermittent, and ephemeral-flow reaches of the Hassayampa River and an ephemeral tributary (southwestern USA) were characterized over a three year period, and soil seed banks (emergence method) were assessed. Variance through time in cover and diversity of wetland (hydric) species was lowest at perennial sites and highest at ephemeral flow sites. Wetland plants established along the active channel of the ephemeral reach following a 10-year recurrence interval flood, and compositional similarity between perennial and ephemeral sites was high during this wet year. About half of the wetland species growing at ephemeral flow sites were present in floodplain soil seed banks. Wetland species emerged from soils of the ephemeral Hassayampa River (but not of the ephemeral tributary) but their density and diversity declined with distance downstream of the perennial reach. Wetland species in the seed banks of ephemeral-flow sites were a nested subset of those at wetter sites, raising the possibility of dispersal from upstream sites. These results indicate that wetlands develop episodically on ephemeral reaches of spatially intermittent rivers of the Sonoran Desert following high winter runoff, and suggest that the wetland plants arise from local seed banks and off-site sources. The study also suggests that riverine restoration success is sensitive to location within a stream network: upon re-watering, sites closer to perennial reaches will have greater capacity to self-assemble wetland plant communities.
Restoration Ecology, 2009
After a long period in which fuel loads were sparse, fire recently has occurred with high frequen... more After a long period in which fuel loads were sparse, fire recently has occurred with high frequency in the ungrazed riparian zone of the Upper San Pedro River in southern Arizona’s Chihuahuan Desert. We studied four accidental fires that occurred during 1994–2003 (two in different years at the same site). Woody vegetation was contrasted between three burned sites and matched spatial controls, and before and after the most recent fire. Herbaceous vegetation was sampled in multiple years producing a chronosequence of time since fire (from 4 months to 8 years). Riparian fire was associated with reductions in woody plant species diversity and canopy cover. In contrast, fire caused a short-term (2 year) pulse of herbaceous plant diversity, driven by annual species, and persistent increase in herbaceous cover. Path analysis indicated that the increase in herbaceous cover was mediated in part by the reduction in tree canopy cover. Ordination (nonmetric multidimensional scaling) and regression analysis also indicated that canopy cover and/or fire played a role in structuring the herbaceous community, although its effects were secondary to that of hydrologic factors (stream flow rate, seasonal flood size). By converting riparian forests to grasslands and savannahs, fire may be shifting structure of the Upper San Pedro floodplain vegetation closer toward conditions present during past centuries when fire was frequent in the upland desert grasslands and embedded riparian corridor.
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Papers by Julie Stromberg