Shrub Artemisia Tridentata Research Articles

Restoration temporarily supports the resilience of sagebrush‐steppe ecosystems subjected to repeated fires

AbstractMany ecosystems are experiencing increased fire frequencies and species invasions that can erode their resilience and cause a shift to alternative states. In the sagebrush‐steppe, a semi‐arid shrubland ecosystem in North America, restoration treatments are often implemented following wildfire to enhance their resilience to invasion. However, little is known about the long‐term effectiveness of these treatments. We investigated whether repeated restoration efforts provide greater resilience in sagebrush‐steppe communities initially dominated by species with different post‐fire regeneration traits and subjected to compounding wildfires and invasion byBromus tectorumover 25 years.We studied 37 permanent transects (Columbia Basin, Washington, USA) in which species abundance was recorded multiple times from 1992 to 2017. We quantified community change and its relationship with fire, restoration, and moisture availability. Resilience was evaluated by quantifying community resistance and stability indices.The greatest change occurred in communities where the obligate seeding shrubArtemisia tridentatawas initially common. Repeated fires led to the extirpation of this shrub and eventual dominance ofB. tectorum. Herbicide applications temporarily suppressedB. tectorumpost‐fire. Seeding treatments and above average precipitation initially increased native cover. Although communities where resprouting species were common showed the least change, repeated fires did lead to a gradual but substantial decline (86%) in resprouting shrubs.Synthesis and applications. Our findings show that repeated restoration efforts, together with elevated precipitation, can support native species re‐establishment in systems experiencing altered disturbance regimes and species invasions. Our unique long‐term dataset demonstrates, however, that many such interventions have short‐lived effects due to the strong ‘unhelpful resilience’ of highly invaded systems. This implicitly suggests that many such systems have experienced fundamental shifts in ecosystem state. The likelihood of this occurring is strongly associated with the dominant species post‐fire regeneration traits. We predict that community composition and resilience will continue to degrade in the sagebrush‐steppe unless management prioritizes fire suppression and an adaptive restoration approach that considers resource availability.

A realized facilitation cascade mediated by biological soil crusts in a sagebrush steppe community

Biological soil crusts can have strong effects on vascular plant communities which have been inferred from short-term germination and early establishment responses. However, biocrusts are often assumed to function as an “organizing principle” in communities because their effects can “cascade” to interactions among crust-associated plant species. We conducted surveys and experiments to explore these cascades and found that biocrusts were positively associated with large patches (> 10 m diameter) of a dominant shrub Artemisia tridentata. At the smaller scale of individual shrubs and the open matrices between shrubs, biocrusts were negatively associated with Artemisia. Juveniles of Artemisia were found only in biocrusts in intershrub spaces and never under shrubs or in soil without biocrusts. In two-year field experiments, biocrusts increased the growth of Festuca and the photosynthetic rates of Artemisia. Festuca planted under Artemisia were also at least twice as large as those planted in open sites without crusts or where Artemisia were removed. Thus, biocrusts can facilitate vascular plants over long time periods and can contribute to a “realized” cascade with nested negative and positive interactions for a range of species, but unusual among documented cascades in that it includes only autotrophs.

Herbivory Amplifies Adverse Effects of Drought on Seedling Recruitment in a Keystone Species of Western North American Rangelands.

Biotic interactions can affect a plant’s ability to withstand drought. Such an effect may impact the restoration of the imperiled western North American sagebrush steppe, where seedlings are exposed to summer drought. This study investigated the impact of herbivory on seedlings’ drought tolerance for a keystone species in this steppe, the shrub Artemisia tridentata. Herbivory effects were investigated in two field experiments where seedlings were without tree protectors or within plastic or metal-mesh tree protectors. Treatment effects were statistically evaluated on herbivory, survival, leaf water potential, and inflorescence development. Herbivory occurrence was 80% higher in seedlings without protectors. This damage occurred in early spring and was likely caused by ground squirrels. Most plants recovered, but herbivory was associated with higher mortality during the summer when seedlings experienced water potentials between −2.5 and −7 MPa. However, there were no differences in water potential between treatments, suggesting that the browsed plants were less tolerant of the low water potentials experienced. Twenty months after outplanting, the survival of plants without protectors was 40 to 60% lower than those with protectors. The percentage of live plants developing inflorescences was approximately threefold higher in plants with protectors. Overall, spring herbivory amplified susceptibility to drought and delayed reproductive development.

Invasive and native grasses exert negative plant-soil feedbacks on the woody shrub Artemisia tridentata.

Displacement of diverse native plant communities by low-diversity invasive communities is a global problem. In the western United States, the displacement of sagebrush-dominated communities by cheatgrass has increased since the 1920s. Restoration outcomes are poor, potentially due to soil alteration by cheatgrass. We explored the poorly understood role of plant-soil feedbacks in the dominance of cheatgrass in a greenhouse study where uninvaded sagebrush soils were conditioned with either cheatgrass, a native bunchgrass or sagebrush. Sagebrush seedlings were grown in the soils that remained following the removal of conditioning plants. We expected cheatgrass to strongly suppress sagebrush due to a change in belowground microbial communities, conspecifics to have neutral effects and the native bunchgrass to have intermediate effects as it coevolved with sagebrush but belongs to a different functional group. We assessed the effects of conditioning on sagebrush growth, tissue nutrients, and carbon allocation. We also characterized the abundance, diversity and community composition of root microbial associates. Cheatgrass strongly suppressed sagebrush growth at high and low conditioning densities, the native bunchgrass showed suppression at high conditioning densities only and conspecific effects were neutral. Tissue nutrients, amount of root colonization by soil fungi or root microbial community composition were not associated with these plant-soil feedbacks. Although we did not identify the precise mechanism, our results provide key evidence that rapid soil alteration by cheatgrass results in negative plant-soil feedbacks on sagebrush growth. These feedbacks likely contribute to cheatgrass dominance and the poor success of sagebrush restoration.

Intraspecific variation in surface water uptake in a perennial desert shrub

Abstract Despite broad recognition that water is a major limiting factor in arid ecosystems, we lack an empirical understanding of how this resource is shared and distributed among neighbouring plants. Intraspecific variability can further contribute to this variation via divergent life‐history traits, including root architecture. We investigated these questions in the shrub Artemisia tridentata and hypothesized that the ability to access and utilize surface water varies among subspecies and cytotypes. We used an isotope tracer to quantify below‐ground zone of influence in A. tridentata, and tested whether spatial neighbourhood characteristics can alter plant water uptake. We introduced deuterium‐enriched water to the soil in plant interspaces in a common garden experiment and measured deuterium composition of plant stems. We then applied spatially explicit models to test for differential water uptake by A. tridentata, including intermingled populations of three subspecies and two ploidy levels. The results suggest that lateral root functioning in A. tridentata is associated with intraspecific identity and ploidy level. Subspecies adapted to habitats with deep soils generally had a smaller horizontal reach, and polyploid cytotypes were associated with greater water uptake compared to their diploid variants. We also found that plant crown volume was a weak predictor of water uptake, and that neighbourhood crowding had no discernable effect on water uptake. Intraspecific variation in lateral root functioning can lead to differential patterns of resource acquisition, an essential process in arid ecosystems in the contexts of changing climate and seasonal patterns of precipitation. Altogether, we found that lateral root development and activity are more strongly related to genetic variability within A. tridentata than to plant size. Our study highlights how intraspecific variation in life strategies is linked to mechanisms of resource acquisition. A free Plain Language Summary can be found within the Supporting Information of this article.

Effects of a Companion Plant on the Formation of Mycorrhizal Propagules in Artemisia tridentata Seedlings

Inoculation of seedlings with arbuscular mycorrhizal fungi (AMF) can increase their establishment after outplanting. The success of this practice depends partly on the extent of root colonization and abundance of AMF propagules in the outplanted seedlings. We conducted a greenhouse experiment to investigate the effects of a companion plant, the native grass Poa secunda J Presl (Sandberg bluegrass), on the formation of spores and vesicles, AMF colonization, and AMF taxa present in the roots of the shrub Artemisia tridentata Nutt (big sagebrush). These effects were tested at two phosphorus (P) fertilization levels, 5 μM and 250 μM. Neither coplanting nor differences in P had an effect on spore density in the potting mix. In contrast, coplanting increased vesicular colonization of A. tridentata from 5% to 18%, but only at low P. Differences in P also affected vesicular colonization of P. secunda, which was 10% and 30% at high and low P, respectively. Arbuscular colonization of A. tridentata was not affected by the treatments and ranged between 12% and 20%. In P. secunda, arbuscular colonization was lower but increased from high to low P. Coplanted seedlings exposed to low P also had the highest levels of total AMF colonization, 70% for A. tridentata and 63% for P. secunda. On the basis of partial sequences of the 28S ribosomal RNA gene, coplanting did not affect the AMF taxa, which were within the Glomeraceae. In some taxa within this family, root fragments containing vesicles are the main propagules. Particularly in this situation, increases in vesicle density caused by coplanting and low P are likely to facilitate mycorrhization of A. tridentata after outplanting, resulting in higher levels of colonization than those naturally occurring in the soil. Such outcomes are critical for assessing the extent to which A. tridentata establishment is limited by insufficient AMF colonization.

Disturbance type influences plant community resilience and resistance to Bromus tectorum invasion in the sagebrush steppe

AbstractQuestionHow does type of disturbance alter plant community composition when an invasive species with high intrinsic population growth rate is present? The sagebrush steppe is a cold semi‐arid steppe dominated by the native shrub Artemisia tridentata Nutt., native bunchgrasses, and has been invaded by the non‐native winter annual Bromus tectorum L.LocationSagebrush steppe, Montana, USA.MethodsWe assessed the effect of fire and soil disturbance, due to bulldozing to create a firebreak, on the resilience of plant communities and their resistance to invasion by B. tectorum. Plant species richness and species composition were monitored for 3 years at two sites post‐fire and firebreak construction.ResultsBurned plant communities were resilient and had similar native grass cover and native species richness compared with the unburned sites after 3 years. Soil disturbance from firebreak construction resulted in species composition that was distinct and had lower native grass cover. Type of disturbance also affected the community's resistance to B. tectorum. Bromus tectorum cover was similar in burned and unburned areas, but increased up to three times and remained high where soil disturbance occurred, suggesting a shift to an alternative state.ConclusionIn this northern portion of the sagebrush steppe, communities with native plant cover were resilient to fire but not soil disturbance, which facilitated B. tectorum increase and a transition to an alternative state. In areas of high native plant cover, management tactics should avoid soil disturbance.

Water relations and photosynthesis along an elevation gradient for Artemisia tridentata during an historic drought

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.

Responses of high-altitude graminoids and soil fungi to 20 years of experimental warming.

High-elevation ecosystems are expected to be particularly sensitive to climate warming because cold temperatures constrain biological processes. Deeper understanding of the consequences of climate change will come from studies that consider not only the direct effects of temperature on individual species, but also the indirect effects of altered species interactions. Here we show that 20 years of experimental warming has changed the species composition of graminoid (grass and sedge) assemblages in a subalpine meadow of the Rocky Mountains, USA, by increasing the frequency of sedges and reducing the frequency of grasses. Because sedges typically have weak interactions with mycorrhizal fungi relative to grasses, lowered abundances of arbuscular mycorrhizal (AM) fungi or other root-inhabiting fungi could underlie warming-induced shifts in plant species composition. However, warming increased root colonization by AM fungi for two grass species, possibly because AM fungi can enhance plant water uptake when soils are dried by experimental warming. Warming had no effect on AM fungal colonization of three other graminoids. Increased AM fungal colonization of the dominant shrub Artemisia tridentata provided further grounds for rejecting the hypothesis that reduced AM fungi caused the shift from grasses to sedges. Non-AM fungi (including dark septate endophytes) also showed general increases with warming. Our results demonstrate that lumping grasses and sedges when characterizing plant community responses can mask significant shifts in the responses of primary producers, and their symbiotic fungi, to climate change.

Burial increases seed longevity of two Artemisia tridentata (Asteraceae) subspecies

Seed longevity and persistence in soil seed banks may be especially important for population persistence in ecosystems where opportunities for seedling establishment and disturbance are unpredictable. The fire regime, an important driver of population dynamics in sagebrush steppe ecosystems, has been altered by exotic annual grass invasion. Soil seed banks may play an active role in postfire recovery of the foundation shrub Artemisia tridentata, yet conditions under which seeds persist are largely unknown. We investigated seed longevity of two Artemisia tridentata subspecies in situ by retrieving seed bags that were placed at varying depths over a 2 yr period. We also sampled naturally dispersed seeds in litter and soil immediately after seed dispersal and before flowering in subsequent seasons to estimate seed persistence. After 24 mo, seeds buried at least 3 cm below the soil surface retained 30-40% viability whereas viability of seeds on the surface and under litter declined to 0 and < 11%, respectively. The density of naturally dispersed seeds in the seed bank was highly heterogeneous both spatially and temporally, and attrition varied significantly by region. Our study suggests that Artemisia tridentata has the potential to form a short-term soil seed bank that persists longer than has been commonly assumed, and that burial is necessary for seed longevity. Use of seeding techniques that promote burial of some seeds to aid in formation of a soil seed bank may increase restoration potential.

Enhanced photosynthesis and flower production in a sagebrush morphotype associated with animal burrows

Two morphotypes of the evergreen shrub Artemisia tridentata Nutt. ssp. wyomingensis occur in the Shirley Basin of central Wyoming (USA), one of which was associated exclusively with Mima-like mounds generated by animal burrowing activity. Measured on a particularly dry year according to a 34-year precipitation record, plants growing on mounds (M) versus inter-mound locations (IM)were taller with greater leaf biomass and leaf area per unit ground area, and had over 90% of all inflorescences. As a result, the landscape consists of a patchy distribution of reproductive islands (~ 20-40 m-2 in size) separated by a mean distance of ~ 30 m. In addition, greater photosynthesis per unit leaf area occurred for M plants when ephemeral leaves dominated total leaf area in spring and early summer, as well as during short time periods (< 3 days) following sporadic rainfall events in summer when only perennial leaves were present. As a result, estimated total annual carbon gain was 41% greater for M plants from May to mid-June, but was not significantly different from IM plants for the remainder of the season, resulting in a total summer carbon gain that was 14% greater in M plants. Stomatal and nonstomatal conductances to CO2 uptake were also greater for the ephemeral leaves of M plants, along with lower internal CO2 concentrations (193 ± 4 μl l-1 vs. 209 ± 8 μl-1, respectively). M plants also maintained higher xylem water potentials throughout most of the growth season (−1.1 ± 0.1 SD MPa in May, declining to −4.4 ± 0.3 SD MPa in August), along with higher water use efficiencies (photosynthesis/transpiration). M and IM soils did not differ significantly in total organic or nitrate contents, although leaf nitrogen content was higher in M plants when photosynthesis was also greater. Photosynthesis in M plants also responded more positively to afternoon showers greater than about 7 mm compared to IM plants. Thus, improved water and nutrient relations was associated with enhanced photosynthetic carbon gain in M plants, enabling greater flower production. Moreover, morphotypic plasticity coupled with the effects of animal burrows may have substantially increased sexual reproductive success in A. t. wyomingensis.

In situ photosynthetic freezing tolerance for plants exposed to a global warming manipulation in the Rocky Mountains, Colorado, USA

Summary • This research tested the hypothesis that experimental infrared warming will reduce photosynthesis for the evergreen shrub Artemisia tridentata and the subalpine, herbaceous Erythronium grandiflorum exposed to an in situ experimental freezing event during the spring snowmelt period. • Photosynthetic tolerance of freezing was measured for plants growing under infrared (IR) warming at 3050 m in the Rocky Mountains, Colorado, USA. In situ freezing was imposed using cold nitrogen gas (from a pressurized container of liquid nitrogen) passed through a heat exchanger placed on top of stems and leaves. • Plant water potential, photosynthetic CO2 assimilation, and stomatal conductance to water vapor were higher for both species on IR-warmed compared with control plots. For A. tridentata, IR warming caused enhanced tolerance of in situ freezing temperatures. There was no difference in freezing tolerance for E. grandiflorum on control vs IR plots. • These results suggest that some species will not be negatively affected by freezing, whereas others may exhibit enhanced tolerance of subzero air temperatures, under a future warmer climate in which snowmelt occurs earlier in the year.

Inorganic N turnover and availability in annual- and perennial-dominated soils in a northern Utah shrub-steppe ecosystem

The exotic annual grass Bromus tectorum has replaced thousands of hectares of native peren- nial vegetation in semi-arid ecosystems of the western United States. Inorganic N availability and pro- duction were compared in soil from monodominant patches of Bromus tectorum, the perennial bunch- grass Elymus elymoides, and the shrub Artemisia tridentata, in Curlew Valley, a salt-desert shrub site in Northern Utah. Bromus-dominated soil had greater %N in the top 10 cm than Artemisia or Elymus- dominated soils. As determined by spring isotope-dilution assays, gross mineralization and nitrification rates were higher in Bromus-dominated than Artemisia-dominated soils, but gross rates of NH4 and NO3 consumption were also higher. Litterbags had greater mass loss and N mineralization when buried in Bromus stands than in Artemisia stands, indicating the soil environment under the annual grass pro- motes decomposition. As determined by nitrification potential assays, nitrifier populations were higher under Bromus than under Artemisia and Elymus. Soil inorganic N concentrations were similar among vegetation types in the spring, but NO3 accumulated under Bromus once it had senesced. An in situ net mineralization assay conducted in autumn indicated that germinating Bromus seedlings are a strong sink for soil NO3 , and that net nitrification is inherently low in soils under Artemisia and Elymus. Results of the study suggest that differences in plant uptake and the soil environment promote greater inorganic N availability under Bromus than under perennial species at the site.

Overlapping resource use in three Great Basin species: implications for community invasibility and vegetation dynamics

Summary In the Great Basin of the western United States of America, the invasive annual grass Bromus tectorum has extensively replaced native shrub and bunchgrass communities, but the native bunchgrass Elymus elymoides has been reported to suppress Bromus. Curlew Valley, a site in Northern Utah, provides a model community to test the effects of particular species on invasion by examining competitive relationships among Elymus, Bromus and the native shrub Artemisia tridentata. The site contains Bromus/Elymus, Elymus/Artemisia and monodominant Elymus stands. Transect data indicate that Elymus suppresses Bromus disproportionately relative to its above‐ground cover. Artemisia seedlings recruit in Elymus stands but rarely in the presence of Bromus. This relationship might be explained by competition between the two grasses involving a different resource or occurring in a different season to that between each grass and Artemisia. Time reflectometry data collected in monodominant patches indicated that in spring, soil moisture use by Bromus is rapid, whereas depletion under Elymus and Artemisia is more moderate. Artemisia seedlings may therefore encounter a similar moisture environment in monodominant or mixed perennial stands. However, efficient autumn soil moisture use by Elymus may help suppress Bromus. In competition plots, target Artemisia grown with Bromus were stunted relative to those grown with Elymus, despite equivalent above‐ground biomass of the two grasses. Competition for nitrogen in spring and autumn, assessed with 15N tracer, appears to be secondary to moisture availability in determining competitive outcomes. Elymus physiology and function appear to play an important role in determining the composition of communities in Curlew Valley, by maintaining zones free of Bromus where Artemisia can recruit.

Effects of perennial neighbors and nitrogen pulses on growth and nitrogen uptake by Bromus tectorum

An experiment was conducted to determine if growth and biomass responsesof the annual grass Bromus tectorum are affected by themagnitude and timing of nitrogen (N) pulses and if these responses areinfluenced by different perennial neighbor species. Nitrogen(NH4:NO3) was applied in three pulse treatments of varyinginterpulse length (3-d, 9-d, or 21-d between N additions). The total amount of Nadded was the same among treatments; hence, both the frequency and magnitude ofN pulses varied (i.e., the longer the interpulse period,the greater the amount of N added for a single pulse).Bromus showed little response to the different N-pulsetreatments. The only characteristic that varied among pulse treatments wasspecific leaf area (SLA), which was significantly greater whenBromus was grown under the 21-d N pulse than when grownunder the 3-d or 9-d N pulses. Bromus height, leaf andtiller numbers, leaf area and aboveground biomass were not affected by theN-pulse treatments nor were tissue-N contents and concentrations. However,Bromus production and tissue-N were significantly differentwhen Bromus was grown with different perennial neighborspecies. Tiller production, aboveground biomass, and seed numbers ofBromus were lowest when the perennial neighbor was thetussock grass Agropyron desertorum, intermediate when theneighbor was the evergreen shrub Artemisia tridentata, andgreatest when the neighbor was the deciduous shrub Chrysothamnusnauseosus. N contents of Bromus leaves were alsolowest when the neighbor was Agropyron. In contrast, root Nuptake capacities were greatest for Agropyron-Bromus rootmixes and lowest for Chrysothamnus-Bromus root mixes. Theseresults suggest that perennial neighbors affect growth, seed production, and Nuptake of Bromus to a greater extent than the timing andmagnitude of N pulses.

Photosynthetic responses to a climate‐warming manipulation for contrasting meadow species in the Rocky Mountains, Colorado, USA

Abstract1. Microclimate was measured and photosynthetic responses to a climate warming manipulation were compared for the evergreen shrub Artemisia tridentata and the herbaceous forb Erigeron speciosus in the Rocky Mountains, Colorado, USA.2. Soil was warmer and drier under infra‐red heaters compared with control plots.3. Midday xylem pressure potential did not differ for A. tridentata on heated vs control plots but was lower for E. speciosus on heated plots compared with controls. Leaf temperatures did not vary for the two species on heated or control plots.4. There were no significant treatment or species differences in the diurnal patterns of CO2 assimilation or stomatal conductance to water vapour. Also, there were no differences in responses to leaf temperature.5. The quantum yield for CO2 assimilation over a range of PPFD was lower for plants on heated plots. There was a marked difference between species in the pattern of stomatal conductance to water vapour over a range of PPFD, but no differences as a result of the heating treatment.6. The quantum efficiency of PSII electron transport was significantly affected by heating. Non‐radiative energy dissipation was greater for A. tridentata compared with E. speciosus. There was recovery of FV/FM for A. tridentata but not for E. speciosus.7. Heating appears to affect plants via changes in soil water content rather than by increasing leaf temperature. The deciduous species E. speciosus appears to undergo some permanent closure of PSII on heated plots, in contrast to the evergreen shrub A. tridentata. Such differences may help explain the increase in above‐ground biomass accumulation in response to heating for shrubs, compared with the decrease observed for deciduous herbaceous species.

Nitrate and phosphate uptake by Agropyron desertorum and Artemisia tridentata from soil patches with balanced and unbalanced nitrate and phosphate supply

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.

Local reduction of mycorrhizal arbuscule frequency in enriched soil microsites

Increased nutrient availability reduces vesicular–arbuscular mycorrhizal (VAM) associations with plants, but whether increased nutrients in small volumes of soil affect local VAM colonization is not known. In a field experiment we investigated VAM colonization at different times following fertilization of small soil patches. Soil volumes of ~ 1000 cm3 were treated with a nutrient solution (enriched patch) or distilled water (control patch) on opposite sides of individual plants of the tussock grass Agropyron desertorum and the shrub Artemisia tridentata. Agropyron had significantly lower (p = 0.03) arbuscular infection in the locally enriched patches compared to control patches (32 and 40%, respectively). This reduced arbuscule frequency was apparent at the first sampling (3 days following treatment application) and remained lower in each subsequent sampling (as much as 30% lower than the control patches). Artemisia revealed a similar pattern in arbuscule frequency but was not statistically significant. Our results suggest that a plant can locally reduce VAM development, since arbuscule frequency specifically was locally reduced even though vesicle and overall infection was not. Since mycorrhizal infection does not increase, we conclude that increased plant root proliferation and uptake capacity are likely to be more important for the exploitation of temporary nutrient pulses or patches than is increased mycorrhizal activity. Key words: arbuscule, nutrient exploitation, phosphorus, reduced development, regulation of colonization, soil heterogeneity, vesicular–arbuscular mycorrhizae.

Osmotic relations of the drought‐tolerant shrub Artemisia tridentata in response to water stress

ABSTRACTTurgor 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.

The timing and degree of root proliferation in fertile-soil microsites for three cold-desert perennials

Root proliferation in nutrient-rich soil patches is an important mechanism facilitating nutrient capture by plants. Although the phenomenon of root proliferation is well documented, the specific timing of this proliferation has not been investigated. We studied the timing and degree of root proliferation for three perennial species common to the Great Basin region of North America: a shrub, Artemisia tridentata, a native tussock grass, Agropyron spicatum, and an introduced tussock grass, Agropyron desertorum. One day after we applied nutrient solution to small soil patches, the mean relative growth rate of Agropyron desertorum roots in these soil patches was two to four times greater than for roots of the same plants in soil patches reated with distilled water. Most of the increased root growth came from thin, laterally branching roots within the patches. This rapid and striking root proliferation by Agropyron desertorum occurred in response to N-P-K enrichment as well as to P or N enrichment alone. A less competitive bunchgrass, Agrophyron spicatum, showed no tendency to proliferate roots in enriched soil patches during these two-week experiments. The shrub Artemisia tridentata proliferated roots within one day of initial solution injection in the N-enrichment experiment, but root proliferation of this species was more gradual and less consistent in the N-P-K and P-enrichment experiments, respectively. The ability of Agropyron desertorum to proliferate roots rapidly may partly explain both its general competitive success and its superior ability to exploit soil nutrients compared to Agropyron spicatum in Great Basin rangelands of North America.