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Durant E Mcarthur - One of the best experts on this subject based on the ideXlab platform.
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studies of a new hybrid taxon in the Artemisia tridentata asteraceae anthemideae complex
Western North American Naturalist, 2013Co-Authors: Heather D Garrison, Leila M Shultz, Durant E McarthurAbstract:Members of the Artemisia tridentata complex (ASTERACEAE: Anthemideae: Artemisia subgen. Triden- tatae) have adapted to changing environmental conditions through geographic migration, introgression, and hybridiza- tion. These processes have resulted in morphologic and genetic variation. A presumed hybrid ("Bonneville" big sage- brush) of the complex occurs in the moister ranges of A. t. ssp. wyomingensis and can be found growing with shrub species commonly associated with A. t. ssp. vaseyana. These populations appear to be preferred habitat for sage-grouse and are more heavily grazed by ungulates than the parental populations. We determined ploidy levels and conducted a detailed morphological analysis to determine if "Bonneville" is a hybrid entity. Sixteen populations (12 in Oneida Co., ID, and 4 in Rich Co., UT) were selected for the study, representing the putative hybrid (Taxon B) and the putative parents— A. t. ssp. vaseyana (2n = 18), A. t. ssp. wyomingensis (2n = 36), and A. t. ssp. tridentata (2n = 36). Each population con- sisted of 25 randomly selected individuals for a total of 400 samples. Our analysis showed 3 populations with morpho- logical and chemical characteristics indicating introgression of A. t. ssp. wyomingensis with populations containing A. t. ssp. vaseyana. Based on these results, we designate the Bonneville sagebrush with formal hybrid status of nothotaxon: Artemisia tridentata ssp. ×bonnevillensis H. Garrison, L. Shultz, and E.D. McArthur (pro subsp.), 2n = 36. RESUMEN.—Los arbustos que forman parte del complejo Artemisia tridentata (ASTERACEAE: Anthemideae: Artemisia subgenero tridentatae) se han adaptado a las condiciones ambientales cambiantes a traves de la migracion geografica, la introgresion y la hibridacion, lo cual ha dado como resultado variacion morfologica y genetica. Un supuesto hibrido (Artemisia "Bonneville") del complejo habita en sitios humedos de A. t. ssp. wyomingensis y se puede encontrar creciendo entre las especies de arbustos que habitualmente se asocian con A. t. ssp. vaseyana. Estas pobla- ciones parecen ser el habitat preferido del gallo de salvia y donde pastan mas los ungulados, en comparacion con las poblaciones parentales. Determinamos niveles de ploidia y realizamos un analisis morfologico exhaustivo para determi- nar si "Bonneville" es una entidad hibrida. Se seleccionaron dieciseis poblaciones para realizar el estudio, que represen- tan el hibrido potencial (Taxon B) y los parentales potenciales: A. t. ssp. vaseyana (2n = 18), A. t. ssp. wyomingensis (2n = 36), A. t. ssp. tridentata (2n = 36): doce en el condado de Oneida, Idaho y cuatro en el condado de Rich, Utah. Cada poblacion consistio de veinticinco individuos seleccionados al azar con un total de cuatrocientas muestras. Nuestro analisis mostro tres poblaciones con caracteristicas morfologicas y quimicas que indican introgresion de A. t. ssp. wyomingensis con poblaciones de A. t. ssp. vaseyana. Con base en estos estudios, asignamos a la Artemisia Bonneville la condicion de hibrido formal de un nototaxon, como Artemisia tridentata ssp. ×bonnevillensis H. Garrison, L. Shultz, y E.D. McArthur (pro subsp.), 2n = 36.
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stress induced metabolic differences between populations and subspecies of Artemisia tridentata sagebrush from a single hillside
Thermochimica Acta, 2002Co-Authors: Bruce N Smith, Durant E Mcarthur, Thomas A Monaco, Clayton Jones, Robert A Holmes, Lee D Hansen, Carl D FreemanAbstract:Abstract Big sagebrush or Artemisia tridentata Nutt. ssp. vaseyana grows at slightly higher, cooler, and drier sites than does A. tridentata ssp. tridentata . The two sagebrush subspecies and natural hybrids between them are found along an elevational gradient in Salt Creek Canyon, near Nephi, UT, USA, where the parent populations are separated by 85 m in elevation and 1.1 km along the transect. In 1993, three gardens were established with seedlings from five populations from different elevations planted in each garden. Physiological measurements of carbon isotope ratios, chlorophyll fluorescence, and respiratory heat and CO 2 production show adaptation to the site of origin. When transplanted to foreign sites, stress was noted. Sagebrush has persistent leaves that are metabolically active all year. Seasonal changes in temperature promote metabolic responses in sagebrush that differ with population and garden.
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narrow hybrid zone between two subspecies of big sagebrush Artemisia tridentata asteraceae vi respiration and water potential
Botany, 1998Co-Authors: Durant E Mcarthur, Carl D Freeman, John H Graham, Han Wang, Stewart C Sanderson, Thomas A Monaco, Bruce N SmithAbstract:Respiration and stem water potential (ψ) were examined in parental and hybrid big sagebrush (Artemisia tridentata Nutt. ssp. tridentata and Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle) grown in common gardens within each parental zone and in the hybrid zone. Plants were sampled in July, August, and September, i.e., immediately before, during, and after flowering. The habitats differed from one another, and there were significant garden by date interactions. The source populations also differed significantly from one another. There were no significant garden by source interactions. Hybrid plants had intermediate respiration rates and did not suffer greater water stress than parental plants. Hybrid plants showed no apparent decrease in adaptation in the important physiological traits respiration and water stress in the hybrid zone.
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narrow hybrid zone between two subspecies of big sagebrush Artemisia tridentata asteraceae iv reciprocal transplant experiments
Evolution, 1997Co-Authors: Han Wang, Durant E Mcarthur, John H Graham, Stewart C Sanderson, Carl D FreemanAbstract:Does endogenous or exogenous selection stabilize the big sagebrush (Artemisia tridentata) hybrid zone? After two years of study, our reciprocal transplant experiments showed significant genotype by environment interactions for a number of fitness components, including germination, growth, and reproduction. Hybrids were the most fit within the hybrid garden. In the parental gardens, the native parental taxon was more fit than either the alien parental or hybrids. These results are consistent with the bounded hybrid superiority model, which assumes exogenous selection, but are clearly at odds with the dynamic equilibrium model, which assumes endogenous selection and universal hybrid unfitness.
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narrow hybrid zone between two subspecies of big sagebrush Artemisia tridentata asteraceae ii selection gradients and hybrid fitness
American Journal of Botany, 1995Co-Authors: John H Graham, Carl D Freeman, Durant E McarthurAbstract:The dynamic equilibrium hypothesis proposes that hybrid zones are stabilized by a balance between dispersal and selection against hybrids. A key prediction of this hypothesis is that hybrids should have lower fitness than either parental taxon, regardless of habitat. Hybrid big sagebrush (Artemisia tridentata ssp. tridentata x ssp. vaseyana) in two narrow hybrid zones do show greatly decreased recruitment. Hybrids in one zone also show increased browsing by grasshoppers, while those in the other zone have increased numbers of aphids. Overall herbivore loads, however, are not greater on the hybrids than on the parental subspecies. Browsing by mule deer is greatest on ssp. vaseyana in both hybrid zones. Incidence of galls is also greatest on ssp. vaseyana in one hybrid zone. Moreover, browsing by Artemisia weevils is greatest on ssp. tridentata in one hybrid zone. Hybrids produce more flowers than either ssp. tridentata or ssp. vaseyana, while seed production rates of hybrids do not differ from those of the parental taxa. Finally, hybrid seeds germinate as well as those of ssp. tridentata and
Martyn M Caldwell - One of the best experts on this subject based on the ideXlab platform.
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shifts in depth of water extraction and photosynthetic capacity inferred from stable isotope proxies across an ecotone of juniperus osteosperma utah juniper and Artemisia tridentata big sagebrush
Journal of Ecology, 2005Co-Authors: Joshua A Leffler, Martyn M CaldwellAbstract:Summary 1 In western North America, juniper trees (Juniperus spp.) are apparently encroaching into numerous communities including sagebrush-dominated (Artemisia tridentata) valleys, where, as density of juniper increases, the density and condition of sagebrush decline but juniper condition appears unaffected. 2 We examined stable isotope proxies of plant gas exchange and relative depth of soil water extraction of Juniperus osteosperma and Artemisia tridentata as their relative densities changed across a transition zone in northern Utah, USA. Measurement of 13C and 18O of foliage allowed separation of the contributions of stomatal and biochemical factors to differences in mean intercellular CO2 concentration, while deuterium composition of stem water served as an indicator of the relative depth of water extraction. 3 Leaf δ13C of juniper and, to a lesser extent, of sagebrush increased with decreasing density of juniper. Foliage 18O did not vary significantly with juniper density, indicating that this was primarily caused by an increase in photosynthetic capacity rather than decreased supply of CO2. Increased foliage nitrogen concentration of both species with decreasing juniper density is also consistent with increased photosynthetic capacity. 4 The much greater mean age of juniper trees in the high juniper density plots may explain their lower photosynthetic capacity. 5 Averaged across densities, juniper always extracted water deeper in the profile than did sagebrush, but in June and August, only the high juniper density plots showed an effect. Sagebrush is either restricted from using water at greater depth in the presence of juniper at high density or was in such poor condition that deep-water sources were not necessary to maintain their reduced function. 6 Ecotones can serve as useful, spatially constrained stages for examining resource partitioning and relationships among plant species. We observed spatial and temporal variation in resource use and partitioning of limited resources that will enable better design of manipulative experiments to explicitly examine competition.
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water conservation in Artemisia tridentata through redistribution of precipitation
Oecologia, 2004Co-Authors: Ronald J Ryel, A J Leffler, Michael S Peek, Carolyn Y Ivans, Martyn M CaldwellAbstract:Water conservation is important for plants that maintain physiologically active foliage during prolonged periods of drought. A variety of mechanisms for water conservation exist including stomatal regulation, foliage loss, above- and below-ground allocation patterns, size of xylem vessels and leaf pubescence. Using the results of a field and simulation study with Artemisia tridentata in the Great Basin, USA, we propose an additional mechanism of water conservation that can be used by plants in arid and semi-arid environments following pulses of water availability. Precipitation redistributed more uniformly in the soil column by roots (hydraulic redistribution of water downward) slows the rate at which this water can subsequently be taken up by plants, thus prolonging water availability during periods of drought. By spreading out water more uniformly in the soil column at lower water potentials following precipitation events, water use is reduced due to lower soil conductivity. The greater remaining soil water and more uniform distribution result in higher plant predawn water potentials and transpiration rates later in the drought period. Simulation results indicate that plants can benefit during drought periods from water storage following both summer rain events (small summer pulses) and overwinter recharge (large spring pulse). This mechanism of water conservation may aid in sustaining active foliage, maintaining root-soil hydraulic connectivity, and increasing survival probability of plants which remain physiologically active during periods of drought.
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rapid soil moisture recharge to depth by roots in a stand of Artemisia tridentata
Ecology, 2003Co-Authors: Ronald J Ryel, A J Leffler, Martyn M Caldwell, C K YoderAbstract:The temporal patterns of soil water potential in a stand of Artemisia tridentata in central Utah, USA, were monitored during the summer, which included small periodic rainfall events, and over the winter, when most of the soil recharge occurs in this environment. The pattern of recharge, when compared to an area cleared of aboveground vegetation, strongly indicated that the downward movement of water to 1.5 m was primarily conducted via roots by the process known as hydraulic redistribution. Rainwater was moved rapidly downward shortly after the rain event and continued over a period of a few days. For rainwater reaching a 0.3–1.5 m depth, the portion redistributed by roots was estimated to range from 100% for small rainfall events (<8 mm) to 74% for a 36-mm event. Simulations with a model of soil water movement that compared situations with and without hydraulic redistribution by roots, indicated that during the fall–spring recharge period, 67% of all water moved downward below 0.1 m was via roots, while...
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root responses and nitrogen acquisition by Artemisia tridentata and agropyron desertorum following small summer rainfall events
Oecologia, 2003Co-Authors: Ronald J Ryel, Carolyn Y Ivans, Joshua A Leffler, Usha Spaulding, John M Stark, Martyn M CaldwellAbstract:Resources in the Great Basin of western North America often occur in pulses, and plant species must rapidly respond to temporary increases in water and nutrients during the growing season. A field study was conducted to evaluate belowground responses of Artemisia tridentata and Agropyron desertorum, common Great Basin shrub and grass species, respectively, to simulated 5-mm (typical summer rain) and 15-mm (large summer rain) summer rainfall events. The simulated rainfall was labeled with K15NO3 so that timing of plant nitrogen uptake could be monitored. In addition, soil NH4+ and NO3− concentrations and physiological uptake capacities for NO3− and NH4+were determined before and after the rainfall events. Root growth in the top 15 cm of soil was monitored using a minirhizotron system. Surprisingly, there was no difference in the amount of labeled N acquired in response to the two rainfall amounts by either species during the 7-day sample period. However, there were differences between species in the timing of labeled N uptake. The N label was detected in aboveground tissue of Agropyron within 1 h of the simulated rainfall events, but not until 24 h after the rainfall in Artemisia. For both Agropyron and Artemisia, root uptake capacity was similarly affected by the 5-mm and 15-mm rainfall. There was, however, a greater increase in uptake capacity for NH4+ than for NO3–, and the 15-mm event resulted in a longer response. No root growth occurred in either species in response to either rainfall event during this 8-day period. The results of this study indicate that these species are capable of utilizing nitrogen pulses following even small summer rainfall events during the most stressful period of the summer and further emphasize the importance of small precipitation events in arid systems.
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nitrate and phosphate uptake by agropyron desertorum and Artemisia tridentata from soil patches with balanced and unbalanced nitrate and phosphate supply
New Phytologist, 1998Co-Authors: Muyi Cui, Martyn M CaldwellAbstract:To test the ability of plants to integrate small-scale imbalances in soil nitrate and phosphate patches, plant growth and acquisition of nitrate and phosphate were measured for the perennial grass Agropyron desertorum (Fisch. ex Link) Schult. and the shrub Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle in soil where the principal supply of nitrate and phosphate came from two enriched patches. The soil was calcareous loamy-skeletal Typic Haploxerolls. These patches were applied in two treatments: either nitrate and phosphate were applied in both patches (balanced treatment) or one patch contained only nitrate and the other only phosphate (unbalanced treatment). The same total quantity of nutrients was applied in both treatments and these included 15N and 32P tracers. The plants were in large pots in open field conditions. There were no significant differences in total biomass production and nitrogen concentration between the two treatments, indicating that both species had the physiological ability to integrate soil nutrient resources. Artemisia was able to acquire more phosphate in the unbalanced treatment, probably due to the high local solution phosphate concentration. Generally Artemisia acquired more N and P than did Agropyron.
Michael E Loik - One of the best experts on this subject based on the ideXlab platform.
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water relations and photosynthesis along an elevation gradient for Artemisia tridentata during an historic drought
Oecologia, 2016Co-Authors: Charlotte C Reed, Michael E LoikAbstract:Quantifying the variation in plant-water relations and photosynthesis over environmental gradients and during unique events can provide a better understanding of vegetation patterns in a future climate. We evaluated the hypotheses that photosynthesis and plant water potential would correspond to gradients in precipitation and soil moisture during a lengthy drought, and that experimental water additions would increase photosynthesis for the widespread evergreen shrub Artemisia tridentata ssp. vaseyana. We quantified abiotic conditions and physiological characteristics for control and watered plants at 2135, 2315, and 2835 m near Mammoth Lakes, CA, USA, at the ecotone of the Sierra Nevada and Great Basin ecoregions. Snowfall, total precipitation, and soil moisture increased with elevation, but air temperature and soil N content did not. Plant water potential (Ψ), stomatal conductance (g s), maximum photosynthetic rate (A max), carboxylation rate (V cmax), and electron transport rate (J max) all significantly increased with elevations. Addition of water increased Ψ, g s, J max, and A max only at the lowest elevation; g s contributed about 30 % of the constraints on photosynthesis at the lowest elevation and 23 % at the other two elevations. The physiology of this foundational shrub species was quite resilient to this 1-in-1200 year drought. However, plant water potential and photosynthesis corresponded to differences in soil moisture across the gradient. Soil re-wetting in early summer increased water potential and photosynthesis at the lowest elevation. Effects on water relations and photosynthesis of this widespread, cold desert shrub species may be disproportionate at lower elevations as drought length increases in a future climate.
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sensitivity of water relations and photosynthesis to summer precipitation pulses for Artemisia tridentata and purshia tridentata
Plant Ecology, 2007Co-Authors: Michael E LoikAbstract:For much of the western USA, precipitation occurs in pulses, the nature of which determine soil water potential and plant physiological performance. This research utilized three experiments to examine the sensitivity of photosynthesis and water relations for two widespread Great Basin Desert shrub species, Artemisia tridentata (which has both deep and shallow roots) and Purshia tridentata (which reportedly has only deep roots), to (1) variation in pulse magnitude size, (2) the kinetics of responses to pulses, and (3) the relationship between pulse-size and antecedent soil water content. At the study site in the southwestern Great Basin Desert, USA, summer rainfall exhibits a greater frequency of larger-sized events, and longer inter-pulse intervals, compared to annual patterns. Compared to pre-watering values, stem water potential initially increased by about 2.00 MPa for A. tridentata and 1.00 MPa for P. tridentata following watering to simulate an 11.5 mm rainfall pulse. For the same water addition, stomatal conductance increased by 0.3 mol m−2 s−1 and photosynthetic CO2 assimilation increased 8-fold for A. tridentata and 6-fold for P. tridentata. Water potential and photosynthetic gas exchange were maximal for both species 2–3 days following a pulse addition. In comparison to P. tridentata, the increase in photosynthesis for A. tridentata was more pronounced for plants treated incrementally with several small pulses compared to plants treated with one pulse of an equivalent total volume. The results indicate that both species can respond to a range of summer rainfall pulse magnitudes within about 2 days, with A. tridentata generally exhibiting larger responses in comparison to the co-dominant shrub species P. tridentata, which at this study site does indeed have shallow roots. In a future climate, the timing and magnitude of summer rainfall pulses will determine the extent to which these two species undergo changes in water status and photosynthetic carbon uptake, with implications for their fitness.
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combined drought and episodic freezing effects on seedlings of low and high elevation subspecies of sagebrush Artemisia tridentata
Physiologia Plantarum, 2007Co-Authors: Susan C Lambrecht, Anne K Shattuck, Michael E LoikAbstract:Big sagebrush (Artemisia tridentata) is a dominant shrub throughout much of the arid western United States. Several recognized subspecies differ in physiology, morphology and in their distribution in relation to soil water availability. While several studies have compared mature individuals of these subspecies, there is little information on seedling physiological tolerance to physical stresses. Understanding seedling physiology is essential for predicting how species may respond to changes in temperature and precipitation regimes. Our objective was to examine the drought and freezing tolerance of seedlings of two A. tridentata subspecies: ssp. tridentata, which is found in low-elevation dry sites, and ssp. vaseyana, found in higher, moister sites. We examined growth, gas exchange and quantum yield of chlorophyll a fluorescence from photosystem II (PSII) for seedlings grown in a greenhouse and exposed to two different levels of moisture availability in combination with a simulated growing season freezing event. We found that ssp. tridentata possessed several drought-tolerant characteristics, such as rapid growth rates, thick leaves and low stomatal conductance. Both subspecies shared similar physiological tolerance to the low-moisture treatment but experienced relatively more stress under the freezing treatment. Subspecies vaseyana was more sensitive to freezing when grown with low soil moisture, resulting in reduced stomatal conductance and PSII quantum yield. In contrast, the low-soil moisture treatment did not increase the susceptibility of ssp. tridentata to freezing. These results demonstrate that drought tolerance may be an important trait for seedlings of A. tridentata, but stress because of freezing damage of seedlings may limit the distribution of the species.
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microclimate freezing tolerance and cold acclimation along an elevation gradient for seedlings of the great basin desert shrub Artemisia tridentata
Journal of Arid Environments, 2003Co-Authors: Michael E Loik, Sean P RedarAbstract:Abstract Vegetation, microclimate, seedling frequency, freezing tolerance, and cold acclimation were compared for seedlings of Artemisia tridentata collected from 1775, 2175, and 2575 m elevation in the eastern Sierra Nevada, California. Data were used to test the hypothesis that ecotypic differences in stress physiology are important for seedling survival along gradients from desert to montane ecosystems. The vegetation canopy cover and A. tridentata seedling frequency were greatest at 2575 m, compared to 1775 and 2175 m. Snow cover ameliorated temperatures near the soil surface for part of the winter and depth varied across elevations. Freezing tolerance was compared for seedlings maintained in growth chambers at day/night air temperatures of 25°C/15°C. The temperature at which electrolyte leakage and Photosystem II function (FV/FM) from leaves were half-maximum was approximately −13·5°C for leaves of seedlings from all three elevations. Shifting day/night air temperatures from 25°C/15°C to 15°C/5°C initiated about 1·5° of acclimation by plants from all three altitudes, with seedlings from the highest elevation exhibiting the greatest acclimation change. Measurements of ambient air and canopy temperatures at the three elevations indicated that wintertime average low temperatures were consistent with the measured degree of freezing tolerance. At small spatial scales used in this study, pollen and seed dispersal between study sites may have precluded resolution of ecotypic differences. Patterns of freezing tolerance and cold acclimation may depend on a combination of mesoclimate and microclimate temperatures, canopy cover, snow depth, and snow melt patterns.
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high temperature tolerance of Artemisia tridentata and potentilla gracilis under a climate change manipulation
Oecologia, 1996Co-Authors: Michael E Loik, John HarteAbstract:Leaf tolerance to high temperatures, as determined by electrolyte leakage and chlorophyll a fluorescence, was compared for Artemisia tridentata (Asteraceae), a widespread shrub of the Great Basin, Colorado Plateau, and western slope of the Rocky Mountains, and Potentilla gracilis (Rosaceae), a herbaceous forb common to high-elevation meadows of the western United States. Species-specific and treatment-specific differences in leaf temperature, high-temperature tolerance and chlorophyll a fluorescence from photosystem II were compared, to test the hypothesis that plants at ecosystem borders will exhibit species-specific responses to climate change. Measurements were made for plants exposed to a climate change warming manipulation on a major ecosystem border at the Rocky Mountain Biological Laboratory, Colorado, United States, in July and August 1995. In July, daily maximal leaf temperatures were significantly higher for P. gracilis than for A. tridentata. Leaf temperatures were slightly lower in August than July for leaves of both species, on control and heated plots, despite the fact that daily maximum air temperatures were not significantly different for the two months. High-temperature tolerance was determined for leaves treated for 1 h at temperatures ranging from 15°C to 65°C. LT50 was approximately 46°C for both species on control plots, but was 43°C for leaves of both species from heated plots, contrary to the predictions of the hypothesis. No shift in LT50 (acclimation) was apparent between July and August. Changes in chlorophyll a fluorescence from photosystem II (F V /F M ) were used to characterize the photosynthetic response to high temperatures. For both A. tridentata and P. gracilis in July, F V /F M was about 0.7, but decreased for temperatures above 40°C. The results suggest that plant responses to global warming at ecosystem borders may be influenced by factors other than leaf-level physiological tolerance to elevated temperatures.
Carl D Freeman - One of the best experts on this subject based on the ideXlab platform.
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stress induced metabolic differences between populations and subspecies of Artemisia tridentata sagebrush from a single hillside
Thermochimica Acta, 2002Co-Authors: Bruce N Smith, Durant E Mcarthur, Thomas A Monaco, Clayton Jones, Robert A Holmes, Lee D Hansen, Carl D FreemanAbstract:Abstract Big sagebrush or Artemisia tridentata Nutt. ssp. vaseyana grows at slightly higher, cooler, and drier sites than does A. tridentata ssp. tridentata . The two sagebrush subspecies and natural hybrids between them are found along an elevational gradient in Salt Creek Canyon, near Nephi, UT, USA, where the parent populations are separated by 85 m in elevation and 1.1 km along the transect. In 1993, three gardens were established with seedlings from five populations from different elevations planted in each garden. Physiological measurements of carbon isotope ratios, chlorophyll fluorescence, and respiratory heat and CO 2 production show adaptation to the site of origin. When transplanted to foreign sites, stress was noted. Sagebrush has persistent leaves that are metabolically active all year. Seasonal changes in temperature promote metabolic responses in sagebrush that differ with population and garden.
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characterization of a narrow hybrid zone between two subspecies of big sagebrush Artemisia tridentata asteraceae vii community and demographic analyses
Evolutionary Ecology Research, 1999Co-Authors: Carl D Freeman, Han Wang, Stewart C Sanderson, And Durant E McarthurAbstract:We examined the floristic and vegetative composition of the communities inhabited by basin big sagebrush (Artemisia tridentata ssp. tridentata) and mountain big sagebrush (A. t. ssp. vaseyana) and their hybrids in Clear Creek Canyon near Richfield, Utah. We also examined the demographic structure of the sagebrush populations in their native habitats. The species composition, proportions of annual plants, perennial forbs and grasses, shrubs, rock, litter, bare ground and total vegetative cover differ among the parental sites and hybrid zone. Canonical correspondence analysis and ordination showed that the two big sagebrush subspecies and their hybrids are each associated with different groups of species, and occupy edaphically distinct habitats. Thus, the hybrid zone occurs at an ecotone. Moreover, the hybrid zone is not in a more disturbed habitat than the parental zones. Indeed, annuals comprise a significantly smaller fraction of the total vegetation in the hybrid zone than in either parental zone. Similarly, introduced species represent a significantly greater percentage of the plant cover in the parental areas than in the hybrid zone. We did not observe a population density trough as predicted by the dynamic equilibrium model. Our findings are consistent with the predictions of the bounded hybrid superiority model, which postulates that hybrids occupy unique habitats.
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narrow hybrid zone between two subspecies of big sagebrush Artemisia tridentata asteraceae vi respiration and water potential
Botany, 1998Co-Authors: Durant E Mcarthur, Carl D Freeman, John H Graham, Han Wang, Stewart C Sanderson, Thomas A Monaco, Bruce N SmithAbstract:Respiration and stem water potential (ψ) were examined in parental and hybrid big sagebrush (Artemisia tridentata Nutt. ssp. tridentata and Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle) grown in common gardens within each parental zone and in the hybrid zone. Plants were sampled in July, August, and September, i.e., immediately before, during, and after flowering. The habitats differed from one another, and there were significant garden by date interactions. The source populations also differed significantly from one another. There were no significant garden by source interactions. Hybrid plants had intermediate respiration rates and did not suffer greater water stress than parental plants. Hybrid plants showed no apparent decrease in adaptation in the important physiological traits respiration and water stress in the hybrid zone.
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narrow hybrid zone between two subspecies of big sagebrush Artemisia tridentata asteraceae iv reciprocal transplant experiments
Evolution, 1997Co-Authors: Han Wang, Durant E Mcarthur, John H Graham, Stewart C Sanderson, Carl D FreemanAbstract:Does endogenous or exogenous selection stabilize the big sagebrush (Artemisia tridentata) hybrid zone? After two years of study, our reciprocal transplant experiments showed significant genotype by environment interactions for a number of fitness components, including germination, growth, and reproduction. Hybrids were the most fit within the hybrid garden. In the parental gardens, the native parental taxon was more fit than either the alien parental or hybrids. These results are consistent with the bounded hybrid superiority model, which assumes exogenous selection, but are clearly at odds with the dynamic equilibrium model, which assumes endogenous selection and universal hybrid unfitness.
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narrow hybrid zone between two subspecies of big sagebrush Artemisia tridentata asteraceae ii selection gradients and hybrid fitness
American Journal of Botany, 1995Co-Authors: John H Graham, Carl D Freeman, Durant E McarthurAbstract:The dynamic equilibrium hypothesis proposes that hybrid zones are stabilized by a balance between dispersal and selection against hybrids. A key prediction of this hypothesis is that hybrids should have lower fitness than either parental taxon, regardless of habitat. Hybrid big sagebrush (Artemisia tridentata ssp. tridentata x ssp. vaseyana) in two narrow hybrid zones do show greatly decreased recruitment. Hybrids in one zone also show increased browsing by grasshoppers, while those in the other zone have increased numbers of aphids. Overall herbivore loads, however, are not greater on the hybrids than on the parental subspecies. Browsing by mule deer is greatest on ssp. vaseyana in both hybrid zones. Incidence of galls is also greatest on ssp. vaseyana in one hybrid zone. Moreover, browsing by Artemisia weevils is greatest on ssp. tridentata in one hybrid zone. Hybrids produce more flowers than either ssp. tridentata or ssp. vaseyana, while seed production rates of hybrids do not differ from those of the parental taxa. Finally, hybrid seeds germinate as well as those of ssp. tridentata and
R D Evans - One of the best experts on this subject based on the ideXlab platform.
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growth photosynthesis and resource investment for vegetative and reproductive modules of Artemisia tridentata
Ecology, 1993Co-Authors: R D Evans, Alan R BlackAbstract:Growth of vegetative and reproductive structures in Artemisia tridentata is temporally separated during the growing season; vegetative growth occurs during spring and early summer when soil moisture is most abundant, while reproductive growth occurs during summer and fall when soil moisture may be limiting. Vegetative and reproductive structures may therefore exhibit contrasting efficiencies of resource acquisition and in- vestment resulting from temporal differences in resource availability during their devel- opment. We examined the effect of water stress on growth, photosynthesis, and resource investment for vegetative and reproductive modules of Artemisia tridentata by applying supplemental water. No differences were observed in vegetative biomass between shrubs in the two watering treatments. Growth of vegetative structures occurred in the spring when water was not limiting, and shrubs in both watering treatments exerted little stomatal control over water loss. Conversely, reproductive growth occurred through the summer when water was limiting, and supplemental watering increased reproductive growth. Shrubs conserved water during summer by abscising leaves and lowering stomatal conductance of both vegetative and reproductive modules in response to decreases in xylem pressure potential and increases in evaporative demand. Leaf abscission can occur without decreas- ing the amount of carbon available for reproductive growth because inflorescences are capable of positive photosynthetic rates comparable to vegetative leaves. Water stress did not alter tissue construction costs or carbon and nitrogen contents for either vegetative or reproductive modules. Resource limitations were, however, reflected in the efficiency of water use during tissue construction; floral leaves and floral heads of shrubs not receiving supplemental water were produced with higher water-use efficiency. Conservative use of water during production of vegetative modules would offer no advantage to A. tridentata because neighboring species are also most active at this time. Reproductive growth in A. tridentata occurs during summer when neighboring species are largely dormant, and so efficient use of water may allow development of reproductive structures to continue through- out the summer even with limited supplies of water.
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osmotic relations of the drought tolerant shrub Artemisia tridentata in response to water stress
Plant Cell and Environment, 1992Co-Authors: R D Evans, R A Black, Wayne Loescher, R J FellowsAbstract:Turgor maintenance, solute content and recovery from water stress were examined in the drought-tolerant shrub Artemisia tridentata. Predawn water potentials of shrubs receiving supplemental water remained above −2 MPa throughout summer, while predawn water potentials of untreated shrubs decreased to −5 MPa. Osmotic potentials decreased in conjunction with water potentials maintaining turgor pressures above 0 MPa. The decreases in osmotic potentials were not the result of osmotic adjustment (i.e. solute accumulation). Leaf solute contents decreased during drought, but leaf water volumes decreased more than 75% from spring to summer, thereby passively concentrating solutes within the leaves. The maintenance of positive turgor pressures despite decreases in leaf water volumes is consistent with other studies of species with elastic cell walls. Inorganic ion, organic acid, and carbohydrate contents of leaves declined during drought. The only solutes accumulating in leaves of A. tridentata with water stress were proline and a cyclitol, both considered compatible solutes. Total and osmotic potentials recovered rapidly following rewatering of shrubs; solute contents did not change except for a decrease in proline. Maintaining turgor through the passive concentration of solutes may be advantageous compared to synthesis of new solutes for osmotic adjustment in arid environments.
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rehydration induced changes in pressure volume relationships of Artemisia tridentata nutt ssp tridentata
Plant Cell and Environment, 1990Co-Authors: R D Evans, R A Black, S O LinkAbstract:Abstract. Pressure-volume measurements were made on Artemisia tridentata Nutt. ssp. tridentata samples rehydrated for 0, 1.5, 3, 6 or 24 h. Increasing rehydration time caused a significant increase in osmotic potential at turgor loss, cell elasiticity, and the relative water content at turgor loss, and a significant decrease in pressure potential at saturation. Osmotic potential at saturation was changed significantly by rehydration, but no consistent trend was observed. The symplastic water fraction did not differ significantly among treatments. The increase in the osmotic potential at turgor loss did not correspond with decreasing cell elasticity or synthesis of solutes. Instead, the leaf solute content remained constant suggesting a redistribution of solutes between the symplast and apoplast. Using non-rehydrated samples for pressure-volume analysis introduced errors in estimates of the symplastic water fraction, osmotic potential at full turgor, and the relative water content at turgor loss. These errors are due to uncertainties in the determination of saturated weights.