Premise of research. Despite a lack of experimental support, enhanced drought tolerance is one of the recurring hypotheses suggested to explain trace element hyperaccumulation. Trace elements may ...have osmolytic and/or antioxidative properties that help hyperaccumulating plants reduce water loss or its resultant damage. Selenium in particular seems like a promising candidate because of its known antioxidative and drought protectant properties in nonaccumulators, but it has not been tested in hyperaccumulators.
Methodology. Here we investigate the drought tolerance hypothesis in a controlled greenhouse setting using a full-factorial design with seedlings of a selenium hyperaccumulator in the genus Astragalus and a nonaccumulating congener.
Pivotal results. While selenium increased plant survivorship and longevity and drought reduced them, we found no evidence that selenium improved the drought tolerance of either species.
Conclusions. Drought tolerance appears to not be a mechanism that would explain the evolution of hyperaccumulation in Astragalus, although it may still operate in some as-yet untested circumstances.
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Most studies assessing vegetation response following control of invasive Tamarix trees along southwestern U.S. rivers have been small in scale (e.g., river reach), or at a regional scale but with ...poor spatial-temporal replication, and most have not included testing the effects of a now widely used biological control. We monitored plant composition following Tamarix control along hydrologic, soil, and climatic gradients in 244 treated and 172 reference sites across six U.S. states. This represents the largest comprehensive assessment to date on the vegetation response to the four most common Tamarix control treatments. Biocontrol by a defoliating beetle (treatment 1) reduced the abundance of Tamarix less than active removal by mechanically using hand and chain-saws (2), heavy machinery (3) or burning (4). Tamarix abundance also decreased with lower temperatures, higher precipitation, and follow-up treatments for Tamarix resprouting. Native cover generally increased over time in active Tamarix removal sites, however, the increases observed were small and was not consistently increased by active revegetation. Overall, native cover was correlated to permanent stream flow, lower grazing pressure, lower soil salinity and temperatures, and higher precipitation. Species diversity also increased where Tamarix was removed. However, Tamarix treatments, especially those generating the highest disturbance (burning and heavy machinery), also often promoted secondary invasions of exotic forbs. The abundance of hydrophytic species was much lower in treated than in reference sites, suggesting that management of southwestern U.S. rivers has focused too much on weed control, overlooking restoration of fluvial processes that provide habitat fr hydrophytic and floodplain vegetation. These results can help inform future management of Tamarix-infested rivers to restore hydrogeomorphic processes, increase native biodiversity and reduce abundance of noxious species.
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BFBNIB, FZAB, GIS, IJS, INZLJ, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZRSKP
The science‐practice gap is often cited as a limitation to successful restoration outcomes; however, the existence of such a gap in information exchange is rarely measured. Here, we quantify the gap ...by focusing on common recommendations from both scientists (i.e. researchers) and managers (i.e. practitioners, land managers) on what is needed for successful restoration. We surveyed 45 managers associated with 244 invasive species (Tamarix spp.) removal projects across the southwestern U.S. to determine the degree to which they have utilized four strategies advocated by scientists: (1) collaborate widely, (2) monitor beyond cursory visual methods, (3) use a variety of information sources, and (4) consider project goals beyond invasive species removal. Half of these managers were also interviewed to assess managers' perceptions of the role of science in restoration. Twenty‐three scientists specializing in Tamarix‐related research in this region were also surveyed to assess how much they understood and/or shared the concerns of land managers. We found that managers were following scientists' recommendations and that managers' perceptions of the role of science in land management did not have any bearing on the management actions taken. Scientists reported being influenced by managers, and the concerns of scientists and managers were more overlapping than expected. Boundary organizations and river‐wide partnerships were often cited as important in facilitating effective communication between land managers and scientists. A lack of funding for monitoring and for longer‐term projects was cited by both groups as a limitation to incorporating scientists' recommendations into restoration.
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Spatial modeling has proven to be useful in understanding the drivers of plant populations in the field of ecology, but has yet to be applied to understanding variation in biocontrol impact. In this ...study, we employ multi-scale analysis (Moran’s Eigenvector Maps) to better understand the variation in tree canopy exposed to defoliation by a biocontrol beetle (
Diorhabda
spp.). The control of the exotic tree
Tamarix
in riparian areas has long been a priority for land managers and ecologists in the American southwest.
Diorhabda
spp. was introduced as a bio-control agent beginning in 2001 and has since become an inseparable part of
Tamarix
-dominated river systems in the southwest. Between 2013 and 2016 tamarisk dieback was assessed at 79 sites across Grand County, Utah, arguably the epicenter of
Diorhabda
impact in the U.S. Canopy cover of
Tamarix
was between 73 and 81% at these sites, with the percent that was live cover fluctuating by year with a minimum of 42%. Using a traditional general linear model, we found that readily and commonly measured environmental factors could explain only up to 26% of the variation in
Tamarix
live canopy each year. The number of defoliations was correlated with an increase rather than a decrease in percent live canopy, suggesting compensatory growth. Spatial structure alone explained 22–40% of variation. We found fine scale spatial structure at less than 10 km and broad scale spatial structure from 10 to 30 km. Combining both traditional and novel spatial statistical methods we increased that percentage to 43–63%, depending on year. These results suggest that scientists and land managers must look beyond commonly measured environmental variables to explain non-random biocontrol impact in this system. In particular, this study points to the potential for biotic interactions and variation in flood cycles for further exploration of the identified spatial structure.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
We investigated the relative role of manager traits and decisions for explaining the impact of riparian restoration. To do this, we used the difference in vegetation between post-restoration and ...controls for 243 pairs of sites to create a success index. We then determined how much variability in success could be explained by physical variables that directly impact vegetation (environment and weed removal) versus human variables (characteristics of the people who managed those sites and their management decisions). More than 60% of the variability in vegetation change could be explained, with human variables increasing adjusted R-square values of physical-only models by an average of 47%. Restoration “success” was positively associated with an increase in the number of collaborators, the number of information sources used, and the relative priority of plant-related goals. Worse outcomes were associated with an increase in the number of roles the manager held, monitoring frequency, and with higher manager education level. These results point to the indirect impacts of the human element, and specifically supports recommendations to include multiple partners and set specific goals. To our knowledge, this is the first time the importance of human characteristics as drivers of restoration outcomes has been quantified.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Invasion by the non-native tree Tamarix has led to implementation of restoration projects aimed at maintaining the ecological integrity of many riparian communities in the southwestern United States. ...These restoration efforts may include Tamarix removal, manipulation of hydrologic regimes, and active revegetation of native species. The goal of this study was to determine which site characteristics are correlated with restoration success, defined in terms of reductions of undesirable species such as Tamarix and establishment of desirable, native species. To accomplish this, vegetative and environmental data were collected at 28 sites in the southwestern United States where active revegetation was completed after Tamarix removal. These data were incorporated into regression tree models with predictor variables that included number of years since removal (1-18 years) and multiple management, climate, soils, and hydrological variables to determine success of Tamarix control, revegetation success, and general plant community responses. Our results suggest that there are easily measurable site characteristics that are associated with greater native cover and richness, planting success, and Tamarix control. Close proximity to perennial water, sufficient precipitation, recent flooding, and good drainage as well as coarser soil texture, and lower soil pH all favored native species. Overall, those site characteristics associated with native species success were the same as those related to lower Tamarix cover. These quantitative models are intended to assist researchers and land managers to design more effective riparian restoration efforts in this critical arid lands ecosystem.
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Resource availability is often characterized by mean annual amounts, while ignoring the spatial variation within habitats and the temporal variation within a year. Yet, temporal and spatial variation ...may be especially important for identifying the source of stress in low productivity environments such as deserts where resources are often pulsed and resource renewal events are separated by long periods of low resource availability. Therefore, the degree of stress will be determined in part by the length of time between recharge events. Here, we investigated the effect of timing and total amount of water application on two congeneric pairs, each with a population from a low (desert) and a high (Mediterranean) productivity habitat. As expected, highest survival and greatest growth were found at low or intermediate recharge intervals, and the magnitude of response to increases in total seasonal amounts was greater for Mediterranean species than desert species. The species that had greater survival switched in the hierarchy under high total water depending on interval length. These results demonstrate that temporal variation in resource availability can be as important as annual total amounts for plant performance and that response to temporal dynamics can vary between species. This has implications for community-level processes, as competitive hierarchies may switch based on resource dynamics rather than only total availability.
Changes in historical disturbance regimes have been shown to facilitate non-native plant invasions, but reinstatement of disturbance can be successful only if native colonizers are able to outcompete ...colonizing invasives. Reintroduction of flooding in the southwestern United States is being promoted as a means of reestablishing Populus deltoides subsp. wislizenii, but flooding can also promote establishment of an introduced, invasive species, Tamarix ramosissima. We investigated competition between Populus and Tamarix at the seedling stage to aid in characterizing the process by which Tamarix may invade and to determine the potential ability of Populus to establish itself with competitive pressure from Tamarix. We planted seedlings of Tamarix and Populus in five ratios at three densities for a total of 15 treatments. The growth response of each species was measured in terms of height, above-ground biomass, and tissue concentrations of nitrogen and phosphorous. These measurements across treatments were modeled as three-dimensional response surfaces. For both species, Populus density was more important than Tamarix density for determining growth response. Both species were negatively affected by increasing numbers of Populus seedlings. Due to the larger size of the native Populus, we predict that its superior competitive ability can lead to its dominance when conditions allow native establishment. Our results suggest that even in the presence of an invader that positively responds to disturbance, reestablishment of historical flooding regimes and post-flood hydrology can restore this ecosystem by promoting its dominant plant species.
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Throughout the world, the condition of many riparian ecosystems has declined due to numerous factors, including encroachment of non-native species. In the western United States, millions of dollars ...are spent annually to control invasions of Tamarix spp., introduced small trees or shrubs from Eurasia that have colonized bottomland ecosystems along many rivers. Resource managers seek to control Tamarix in attempts to meet various objectives, such as increasing water yield and improving wildlife habitat. Often, riparian restoration is an implicit goal, but there has been little emphasis on a process or principles to effectively plan restoration activities, and many Tamarix removal projects are unsuccessful at restoring native vegetation. We propose and summarize the key steps in a planning process aimed at developing effective restoration projects in Tamarix-dominated areas. We discuss in greater detail the biotic and abiotic factors central to the evaluation of potential restoration sites and summarize information about plant communities likely to replace Tamarix under various conditions. Although many projects begin with implementation, which includes the actual removal of Tamarix, we stress the importance of pre-project planning that includes: (1) clearly identifying project goals; (2) developing realistic project objectives based on a detailed evaluation of site conditions; (3) prioritizing and selecting Tamarix control sites with the best chance of ecological recovery; and (4) developing a detailed tactical plan before Tamarix is removed. After removal, monitoring and maintenance as part of an adaptive management approach are crucial for evaluating project success and determining the most effective methods for restoring these challenging sites.
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Soil seed banks serve as reservoirs of seeds for subsequent regeneration of plants particularly in areas prone to disturbance, such as intensive grazed areas along rivers. The potential of soil seed ...bank (SSB) for vegetation restoration was assessed in overgrazed riparian areas in Gamasyab river, Iran. Therefore, SSB composition, density and diversity and, its relationship with aboveground vegetation (AGV) were investigated along the river. Fifteen transects were set perpendicular to the river and along each transect, 6 plots (1 m × 1 m) were established, in total 90 plots. In each plot, soil was collected for SSB experiments and AGV was sampled during the growing season. The similarity between SSB and AGV was estimated by the Jaccard similarity index and non-metric dimensional scaling (NMDS). Spatial beta diversity was calculated by the mean Bray-Curtis index for the SSB and vegetation compositions, separately. A total of 136, 108 and 134 species were identified in the upper (0–5 cm) and lower (5–10 cm) soil layers and, AGV, respectively. Annual and perennial grasses were dominant in SSB composition at 0–5 cm depth while the frequency of perennial grasses was highest in SSB at deeper depth and vegetation composition. The beta diversity of AGV was significantly higher than for SSB. Similarity between SSB and vegetation was low (7.92%–22.40%), nevertheless, most species in AGV were found frequently in the SSB. So, we argued that restoration of degraded vegetation in our riparian could rely on SSB. It is expected that after grazing exclosure, vegetation would restore mostly with perennial grasses through SSB and vegetative growth.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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