Comparison of Herbicides for Reducing Annual Grass Emergence in Two Great Basin Soils

Reducing seed germination and seedling emergence of downy brome (Bromus tectorum L.) improves the success of revegetating degraded shrubland ecosystems. While pre-emergence herbicides can potentially reduce these two processes, their impact on germination and emergence of downy brome and revegetation species in semiarid ecosystems is poorly understood and has not been comprehensively studied in soils with potentially contrasting herbicide bioavailability (i.e., residual plant activity). We designed a greenhouse experiment to evaluate the effects two pre-emergence acetolactate synthase–inhibiting herbicides (rimsulfuron and imazapic) on germination and emergence of downy brome and two revegetation grass species (crested wheatgrass [Agropyron cristatum {L.} Gaertn.] and bottlebrush squirreltail [Elymus elymoides {Raf.} Swezey]) that were grown in representative soils from salt desert and sagebrush shrublands. Pre-emergence herbicides significantly (P &spilt; 0.05) reduced seedling emergence and biomass production of downy brome and crested wheatgrass and increased mortality more so in sagebrush compared to salt desert soil, suggesting that these common Great Basin soils fundamentally differ in herbicide bioavailability. Also, germination and emergence of the two highly responsive species (crested wheatgrass and downy brome) were clearly more impacted by rimsulfuron than imazapic. We discuss these results in terms of how the specific soil physiochemical properties influence herbicide adsorption and leaching. Our results shed new light on the relative performance of these two promising herbicides and the importance of considering soil properties when applying pre-emergence herbicides to reduce germination and emergence of invasive annual grasses and create suitable seedbed conditions for revegetation.     

Fungal Facilitation in Rangelands: Do Arbuscular Mycorrhizal Fungi Mediate Resilience and Resistance in Sagebrush Steppe?

Arbuscular mycorrhizal fungi (AMF) may exert profound influences on ecosystem resilience and invasion resistance in rangelands. Maintenance of plant community structure through ecological feedback mechanisms such as facilitation of nutrient cycling and uptake by host plants, physical and chemical contributions to soil structural stability, and mediation of plant competition suggest AMF may be important facilitators in stressful arid environments. Plant-AMF interactions could influence succession by increasing native plant community resilience to drought, grazing, and fire and resistance to exotic plant invasion. However, invasive exotic plants may benefit from associations with, as well as alter, native AMF communities. Furthermore, questions remain on the role of AMF in stressful environments, specifically the mycorrhizal dependency of sagebrush (Artemisia spp.) steppe plant species. Here, we review scientific literature relevant to AMF in rangelands, with specific focus on impacts of land management, disturbance, and invasion on AMF communities in sagebrush steppe. We highlight the nature of AMF ecology as it relates to rangelands and discuss the methods used to measure mycorrhizal responsiveness. Our review found compelling evidence that AMF mediation of resilience to disturbance and resistance to invasion varies with plant and fungal community composition, including plant mycorrhizal host status, plant functional guild, and physiological adaptations to disturbance in both plants and fungi. We conclude by outlining a framework to advance knowledge of AMF in rangeland invasion ecology. Understanding the role of AMF in semiarid sagebrush steppe ecosystems will likely require multiple study approaches due to the highly variable nature of plant-AMF interactions, the complex mechanisms of resilience conference, and the unknown thresholds for responses to environmental stressors. This may require shifting away from the plant biomass paradigm of assessing mycorrhizal benefits in order to obtain a more holistic view of plant dependency on AMF, or lack thereof, in sagebrush steppe and other semiarid ecosystems.     

Native Plant Growth and Seedling Establishment in Soils Influenced by Bromus Tectorum

The invasion of 40 million hectares of the American West by cheatgrass (Bromus tectorum L.) has caused widespread modifications in the vegetation of semi-arid ecosystems and increased the frequency of fires. In addition to well-understood mechanisms by which cheatgrass gains competitive advantage, it has been implicated in reducing arbuscular mycorrhizal fungi (AMF) abundance and taxa diversity. We evaluated this possibility at a high elevation site in a two-pronged approach. To test whether cheatgrass changed native AMF communities in ways that affected subsequent native plant growth, we grew cheatgrass and native plants in native soils and then planted native plants into these soils in a greenhouse experiment. We found that cheatgrass-influenced soils did not inhibit native plant growth or AMF sporulation or colonization. To test whether soils in cheatgrass-dominated areas inhibited establishment and growth of native plants, cheatgrass was removed and six seeding combinations were applied. We found that 14.02 ±  seedlings · m⁻² established and perennial native plant cover increased fourfold over the three years of this study. Glyphosate reduced cheatgrass cover to less than 5% in the year it was applied but did not facilitate native plant establishment or growth compared with no glyphosate. We conclude that cheatgrass influence on the soil community does not appear to contribute to its invasion success in these high elevation soils. It appears that once cheatgrass is controlled on sites with sufficient native plant abundance, there may be few lingering effects to inhibit the natural reestablishment of native plant communities.     

生物炭、磷及AMF对Cd胁迫下柳枝稷生长及土壤性质的影响

为探讨添加磷、生物炭和接种丛枝菌根真菌对柳枝稷镉（Cd）耐性的作用，采用盆栽法研究土壤Cd浓度为20 mg·kg^-1时，不同处理［对照（CK）、4.5%生物炭（biochar，B）、60 mg·kg^-1磷（phosphorus，P₆₀）、4.5%生物炭+60 mg·kg^-1磷（B+P₆₀）］结合丛枝菌根真菌（AMF）对柳枝稷生长状况、矿质离子（P、Se）和Cd含量、土壤pH值、速效磷和酸性磷酸酶活性及土壤不同形态Cd含量的影响。结果表明：磷添加可显著提高柳枝稷根系侵染率，P₆₀处理下根系侵染率达到了56.9%。与CK相比，B、P₆₀、B+P₆₀处理对柳枝稷植株高度、叶片SPAD值、生物量无显著影响，各处理结合AMF的植株高度、叶片SPAD值、生物量均显著升高，但接菌后各处理间差异不显著。除了P₆₀处理下地上部Se和Cd含量及B+P₆₀处理下地上部Cd含量高于CK外， B、P₆₀、B+P₆₀处理下P、Se、Cd与CK无显著差异；接种AMF后CK和B处理下地上部Se和Cd含量均高于未接菌处理，且B+AM处理地上部Cd含量显著高于其他处理；但接菌后P₆₀处理地上部Se和Cd含量均低于未接菌处理。此外，无论是否接种AMF，B和B+P₆₀处理根系Cd含量均显著高于CK和P₆₀，土壤速效磷含量高于CK，土壤酸性提取态Cd含量低于CK和P₆₀。接种AMF后CK、B和B+P₆₀处理的土壤残渣态Cd含量高于CK和对应未接菌处理，但接菌P₆₀处理的土壤残渣态Cd含量低于未接菌处理。由此可见，B或B+P₆₀处理提高了根系Cd和土壤速效磷含量，降低了土壤酸性提取态Cd含量；二者结合AMF提高了柳枝稷生物量和地上部Se和Cd含量及土壤残渣态Cd含量。因此，AMF结合生物炭或生物炭/磷添加提高了柳枝稷生物量、Cd吸收量，降低了土壤中重金属的生物活性，可以在重度Cd污染土壤中应用。     

增温对蒿类荒漠草地可萌发土壤种子库及其多样性的影响

为探讨草地土壤种子库种子萌发对全球气候变暖的适应机制,本研究采用常规方法对模拟温度增加[对照（CK）、增加2℃（T2）、增加4℃（T4）、增加6℃（T6）]下中度退化蒿类荒漠草地可萌发土壤种子库数量及其多样性进行了观测。结果表明：0~5,5~10 cm土层土壤种子库萌发的物种数及其幼苗总密度随温度增加呈下降趋势,且T6处理0~5（5~10）cm萌发物种数及萌发密度依次较对照降低57.1%（33.3%）,70.4%（75.0%）。随温度增加,0~5,5~10 cm土层蒿类荒漠可萌发土壤种子库物种组成与地上植被物种组成相似性总体呈下降趋势,且T6处理相似性最低。温度增加后蒿类荒漠土壤萌发物种的Shannon-Wiener指数、Pielou指数、Patrick指数总体呈下降趋势,且0~5 cm土层较5~10 cm土层下降明显。综上,增温抑制了土壤种子库种子的萌发,可能不利于退化蒿类荒漠草地的恢复。     

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.     

Assessing Greater Sage-Grouse Breeding Habitat With Aerial and Ground Imagery

Anthropogenic disturbances, wildfires, and weedy-plant invasions have destroyed and fragmented many sagebrush (Artemisia L. spp.) habitats. Sagebrush-dependent species like greater sage-grouse (Centrocercus urophasianus) are vulnerable to these changes, making habitat monitoring essential to effective management. Conventional ground inventory methods are time consuming (expensive) and have lower data collection potentials than remote sensing. Our study evaluated the feasibility of ground (0.3-mm ground surface distance [GSD]) and aerial imagery (primarily, 1-mm GSD) to assess ground cover for big sagebrush (Artemisia tridentata Nutt.) and other vegetation functional groups important in sage-grouse breeding habitat (lekking, nesting, and brood rearing). We surveyed ∼ 526 km² of the upper Powder River watershed in Natrona County, Wyoming, USA, a region dominated by Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle & Young) communities interspersed with narrow riparian corridors. Our study area was used year-round by sage-grouse and included 16 leks. In June 2010, we acquired aerial images (1-mm resolution) for 3 228 systematic sampling locations; additional images were acquired as rapid-succession bursts where aerial transects crossed riparian areas and for 39 riparian and 39 upland ground locations (0.3-mm resolution) within 3.2-km of leks. We used SamplePoint software to quantify cover for plant taxa and functional groups using all ground images and a systematic sampling of aerial images. Canopy cover of sage-grouse food forbs—as averaged across aerial and ground imagery around all leks—was 1.8% and 7.8% in riparian and 0.5% and 4.0% in upland areas, respectively. Big sagebrush cover was 8.7% from upland aerial images and 9.4% from upland ground images. Aerial and ground imagery provided similar values for bare ground and shrubs in riparian and upland areas, whereas ground imagery provided finer-scale herbaceous-cover data that complemented the aerial imagery. These and other image-derived archival data provide a practical basis for landscape-scale management and are a cost-effective means for monitoring extensive sagebrush habitats.     

Recovery of Big Sagebrush Following Fire in Southwest Montana

Fire plays a large role in structuring sagebrush ecosystems; however, we have little knowledge of how vegetation changes with time as succession proceeds from immediate postfire to mature stands. We sampled at 38 sites in southwest Montana dominated by 3 subspecies of big sagebrush (Artemisia tridentata Nutt.). At each site we subjectively located 1 sample plot representing the burned area and an unburned macroplot in similar, adjacent, unburned vegetation. Canopy cover of sagebrush was estimated, and plants were counted in 10 microplots. Age and height of randomly chosen sagebrush plants in each size class were determined from 5 microplots. Average postfire time to full recovery of mountain big sagebrush (ssp. vasseyana [Rydb.] Beetle) canopy cover was 32 years, shorter for basin (ssp. tridentata) and much longer for Wyoming (ssp. wyomingensis Beetle & Young) big sagebrush. Height recovered at similar rates. There was no difference in canopy cover or height recovery between prescribed fires and wildfires in stands of mountain big sagebrush. We found no relationship between mountain big sagebrush canopy cover recovery and annual precipitation, heat load, or soil texture. Nearly all unburned sagebrush macroplots were uneven-aged, indicating that recruitment was not limited to immediate postfire conditions in any of the subspecies. Average canopy cover of three-tip sagebrush (A. tripartita Rydb.) did not increase following fire, and many three-tip sagebrush plants established from seed instead of sprouting. Our results suggest that the majority of presettlement mountain big sagebrush stands would have been in early to midseral condition in southwest Montana assuming a mean fire interval of 25 years. Only long fire-return intervals will allow stands dominated by Wyoming big sagebrush to remain on the landscape in our study area. We speculate that effects of site-specific factors conducive to sagebrush recovery are small compared to stochastic effects such as fire.     

蒿类荒漠草地土壤种子库特征及其萌发植物多样性对降水增加的响应

为揭示土壤种子库种子萌发对降水增加的适应规律,采用人工模拟降水方法,对不同降水量[对照(CK)、增加5% (W₅)、增加10% (W₁₀)、增加15% (W₁₅)、增加20% (W₂₀)、增加25% (W₂₅)]下中度退化伊犁绢蒿荒漠土壤可萌发种子库种子萌发数量及其多样性进行了调测。结果表明:0~5 cm土层土壤种子库萌发物种数及其萌发总密度随降水量增加呈上升趋势,且W₂₀、W₂₅下萌发密度显著高于其他处理(P<0.05),萌发物种数较对照(7种)依次增加2.6、2.0倍;5~10 cm土层也呈现相同的变化趋势。无论降水增加与否,0~10 cm土层伊犁绢蒿、木地肤、毛梗顶冰花均有萌发,且降水增加显著促进伊犁绢蒿、木地肤、串珠老鹳草、毛梗顶冰花的萌发。随降水量增加,0~5 cm土层伊犁绢蒿可萌发土壤种子库与地上植被Sorensen相似性总体呈先增后降趋势,且降水增加≥20%处理相似性高于降水增加≤15%处理。降水增加后伊犁绢蒿荒漠土壤种子萌发物种的Shannon-Wiener指数、Pielou指数、Patrick指数总体呈增加趋势,且0~5 cm土层较5~10 cm土层增加明显。土壤种子库可萌发种子密度具明显的空间异质性,且受空间取样位置及降水的交互影响。综上所述,降水增加促进了伊犁绢蒿荒漠草地土壤种子库可萌发植物种数、种子萌发密度、植物多样性指数的增加,导致其与地上植被的相似性增加,有利于退化蒿类荒漠草地的恢复。     

Postfire Invasion Potential of Rush Skeletonweed (Chondrilla Juncea)

North American sagebrush steppe communities have been transformed by the introduction of invasive annual grasses and subsequent increase in fire size and frequency. We examined the effects of wildfires and environmental conditions on the ability of rush skeletonweed (Chondrilla juncea L.), a perennial Eurasian composite, to invade degraded sagebrush steppe communities, largely dominated by cheatgrass (Bromus tectorum L.). Recruitment of rush skeletonweed from seed and root buds was investigated on 11 burned and unburned plot pairs on Idaho's Snake River Plain following summer 2003 wildfires. Emergence from soil seedbanks was similar on burned and unburned plots in 2003 and 2004 (P = 0.37). Soils from recently burned plots (P = 0.05) and sterilized field soil (P &spilt; 0.01) supported greater emergence than did unburned field soils when rush skeletonweed seeds were mixed into the soils in the laboratory. These decreases may indicate susceptibility of this exotic invasive to soil pathogens present in field soils. Seeds in bags placed on field soil in late October 2003 reached peak germination by mid-January 2004 during a wet period; 1% remained viable by August 2004. Seedling emergence from sown plots or the native seedbank and establishment of new rosettes from root sprouts in 2003–2005 indicate that seed germination of rush skeletonweed on the Snake River Plain may be facultative, occurring in fall or spring if soil moisture is adequate, although many germinants may not survive. Stand development results primarily from root sprouting. Establishment from seed is episodic but provides for dispersal, with increasing fire frequency and size expanding the areas of disturbance available for new invasions.     

放牧及AM真菌对垂穗披碱草生长和白粉病抗性的影响

于兰州大学玛曲高山草原站设3个不同放牧强度样地,采集样地的土壤和优势植物垂穗披碱草(Elymus nutans)种子,在温室建立盆栽试验,研究轻度、中度和重度放牧条件下,AM真菌对垂穗披碱草生长和白粉病(Blumeria graminis)发生的影响。结果表明:中度和重度放牧处理下接种AM真菌土(AM)侵染率显著高于轻度放牧土壤(P0.05),重度放牧条件下自然土(NS)和AM处理的侵染率比轻度和中度放牧分别高32.48%、2.39%和69.2%、21.17%;轻度、中度和重度放牧条件下AM处理的发病率比SS处理分别低22.1%,57.26%和20.46%,AM真菌有效抑制垂穗披碱草白粉病的发生,降低发病率,且对抗病相关酶活性产生显著影响,随放牧强度的增强AM真菌处理的SOD酶和PPO酶活性增高,POD酶活性在中度放牧强度下最高;中度放牧土壤的植物根系全磷含量最高;AM真菌不同程度提高植物根系全磷含量、总生物量、根冠比、叶绿素含量及光合速率。因此,在玛曲高山草原,适当增加放牧强度可提高AM真菌侵染率。AM真菌通过促进植物生长,调节抗病相关酶活性,有效降低了垂穗披碱草白粉病的发生。     

短命植物生长特性及叶绿素荧光特性对不同土壤类型的响应

本研究通过对莫索湾地区2种土壤类型(风沙土和灰漠土)中的优势短命植物生物量、养分含量、叶绿素荧光和土壤因子进行测定，阐明短命植物对不同土壤类型的响应及土壤与植物之间的关系。结果表明，2种土壤的pH值、速效钾(Soil available potassium，SAK)含量等均有极显著差异(P<0.01)；灰漠土中植物总生物量(Total biomass，TB)、全氮(Plant total nitrogen，PTN)含量、N∶P等显著高于风沙土中植物(P<0.05)；灰漠土中植物PSⅡ实际光化学效率(Actual photochemical efficiency of PSⅡ，Φ_PSⅡ)、相对电子传递速率(Apparent electron transfer rate，rETR)极显著高于风沙土中植物(P<0.01)；土壤与植物各指标存在一定的相关关系，如土壤SAK与植物TB、最大荧光(Maximum fluorescence，F_m)呈极显著负相关关系，与PTN呈极显著正相关关系。综上，不同土壤对短命植物的生长有显著影响，灰漠土为短命植物生长提供了更有利的条件。     

滴灌AM真菌孢子水溶液对苜蓿的促生长效应研究

以紫花苜蓿作为研究对象,通过采用Sufer软件对滴灌AM真菌孢子前后土壤的水、盐分和丛枝菌根真菌(AMF)孢子进行等值线图的绘制,分析滴灌前后的运移状态;在苜蓿的生长期进行定期滴灌菌肥AMF孢子,在50 d后测定苜蓿植株菌根侵染率、根瘤数、产孢性能和生物量等指标,分析不同AMF的滴灌效果,对苜蓿进行关于滴灌AMF孢子水溶液的可行性研究。结果表明,1)AMF孢子在水溶液中的数量随着时间的延长而不断降低,滴灌前土壤含水量呈现出表层低而深层高的总体特点。土层的电导率分布范围较均匀。AM真菌孢子主要集中在土层10~25 cm处。滴灌后,水分大部分集中在距离滴头30 cm左右的土层,含水量较滴灌前升高。在滴头附近盐分向四周扩散,在30~45 cm处形成盐分高值区。孢子主要集中在距滴头0~25 cm左右,对于远距离的湿润区,其孢子数有一定的下降趋势。滴灌不同AM菌种孢子水溶液及滴灌距离对苜蓿生长的影响具有一定的差异性。近距离滴灌的植株地上生物量和株高显著高于远距离滴灌的植株(P<0.05);2)滴灌菌种根内球囊霉(Gi)的苜蓿植株干物质(地上、地下)显著高于对照6.59%和13.29%(P<0.05)。菌种摩西球囊霉(Gm)、内球囊霉(Gi)和幼套球囊霉(Ge)处理的地下干重显著高于对照处理9.05%,13.29%和9.96%(P<0.05),地表球囊霉(Gv)处理的地下干重与对照无显著性差异(P>0.05)。Gi处理苜蓿的分枝数显著高于对照处理19.73%(P<0.05),其他菌种间无显著性差异但都显著高于对照处理(P<0.05)。Gi和Ge处理的苜蓿植株的根瘤数、孢子数和侵染率显著高于对照组(P<0.05)。滴灌菌种间菌根侵染率和根瘤数无显著性差异(P>0.05)。滴灌距离对植株地下部干重、株高、孢子数有显著性影响(P<0.05)。滴灌距离和滴灌菌种的互作除了对菌根侵染率、根瘤数和根长具有显著地影响外(P<0.05),对其余的各指标都没有显著性影响(P>0.05)。综合分析Gi菌种的滴灌应用对苜蓿的效果较好。     

Consequences of Treating Wyoming Big Sagebrush to Enhance Wildlife Habitats

Sagebrush (Artemisia L.) taxa historically functioned as the keystone species on 1 090 000 km² of rangeland across the western United States, and Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle and Young) is or was dominant on a substantial amount of this landscape. Wyoming big sagebrush provides habitat for numerous wildlife species. Nevertheless, Wyoming big sagebrush communities are commonly manipulated to decrease shrub cover and density and increase the productivity and diversity of herbaceous plants. We examined relationships between management-directed changes in Wyoming big sagebrush and greater sage-grouse (Centrocercus urophasianus), elk (Cervus elaphus), pronghorn (Antilocapra americana), and mule deer (Odocoileus hemionus), species commonly associated with these ecosystems. We focused on herbicide applications, mechanical treatments, and prescribed burning, because they are commonly applied to large areas in big sagebrush communities, often with the goal to improve wildlife habitats. Specifically, our objective was to identify treatments that either enhance or imperil sagebrush habitats for these wildlife species. The preponderance of literature indicates that habitat management programs that emphasize treating Wyoming big sagebrush are not supported with respect to positive responses by sage-grouse habitats or populations. There is less empirical information on ungulate habitat response to Wyoming big sagebrush treatments, but the value of sagebrush as cover and food to these species is clearly documented. A few studies suggest small-scale treatments (≤ 60-m width) in mountain big sagebrush (Artemisia tridentata ssp. vaseyana &lsqb;Rydb.] Beetle) may create attractive foraging conditions for brooding sage-grouse, but these may have little relevance to Wyoming big sagebrush. Recommendations or management programs that emphasize treatments to reduce Wyoming big sagebrush could lead to declines of wildlife species. More research is needed to evaluate the response of sagebrush wildlife habitats and populations to treatments, and until that time, managers should refrain from applying them in Wyoming big sagebrush communities.     

干旱荒漠区土壤种子库研究进展

荒漠生态系统是地球上最脆弱的生态系统。近年来,荒漠生态系统保护和恢复的研究已受到广泛关注。土壤种子库作为种子源,在植被恢复过程中起着至关重要的作用。本研究系统回顾了近20年来国内外对干旱荒漠区土壤种子库的研究进展,从土壤种子库的基本特征、时空分布、与地上植被的关系、研究方法以及影响因素等方面进行综述并提出研究展望。     

丛枝菌根真菌接种对白车轴草耐盐性的影响

丛枝菌根真菌(AMF)可与植物共生形成复杂的菌丝网络,影响植物生长及抗逆能力。目前关于AMF对白车轴草耐盐性的影响尚存争议,本研究采用盆栽试验,研究在盐胁迫条件下(NaCl浓度为150 mmol·L^-1),接种AMF对拉丁诺白车轴草耐盐性的影响。结果表明,在盐胁迫条件下,与对照相比,白车轴草的生长与生理指标均受到抑制。盐胁迫下接种AMF后,白车轴草株高、干重、PSⅡ最大光能转换效率和相对含水量均有增加,丙二醛(MDA)含量以及相对电导率有所降低,渗透调节物质均有提高,其中可溶性糖(SS)和游离脯氨酸(Pro)含量分别提高了32.03%和9.42%。说明盐胁迫抑制白车轴草的生长,接种AMF增强了白车轴草抗逆适应能力,促进白车轴草的生长,增加渗透调节物质含量,提高白车轴草耐盐胁迫的能力。     

放牧对冷蒿繁殖特性的影响

重度放牧对冷蒿(Artemisia frigida)的有性生殖具有严重的阻碍作用,严重影响其有性生殖;冷蒿生殖枝分化率、生殖枝花序数、种子产量、种子成熟度和发芽率之间在同一牧压下表现并不一致,在不同牧压下存在一定的平衡关系;放牧强度起调节作用是冷蒿对放牧的适应机制.     

Natural Regeneration Processes in Big Sagebrush (Artemisia tridentata)

Big sagebrush, Artemisia tridentata Nuttall (Asteraceae), is the dominant plant species of large portions of semiarid western North America. However, much of historical big sagebrush vegetation has been removed or modified. Thus, regeneration is recognized as an important component for land management. Limited knowledge about key regeneration processes, however, represents an obstacle to identifying successful management practices and to gaining greater insight into the consequences of increasing disturbance frequency and global change. Therefore, our objective is to synthesize knowledge about natural big sagebrush regeneration. We identified and characterized the controls of big sagebrush seed production, germination, and establishment. The largest knowledge gaps and associated research needs include quiescence and dormancy of embryos and seedlings; variation in seed production and germination percentages; wet-thermal time model of germination; responses to frost events (including freezing/thawing of soils), CO₂ concentration, and nutrients in combination with water availability; suitability of microsite vs. site conditions; competitive ability as well as seedling growth responses; and differences among subspecies and ecoregions. Potential impacts of climate change on big sagebrush regeneration could include that temperature increases may not have a large direct influence on regeneration due to the broad temperature optimum for regeneration, whereas indirect effects could include selection for populations with less stringent seed dormancy. Drier conditions will have direct negative effects on germination and seedling survival and could also lead to lighter seeds, which lowers germination success further. The short seed dispersal distance of big sagebrush may limit its tracking of suitable climate; whereas, the low competitive ability of big sagebrush seedlings may limit successful competition with species that track climate. An improved understanding of the ecology of big sagebrush regeneration should benefit resource management activities and increase the ability of land managers to anticipate global change impacts.     

Cheatgrass Invasion in Salt Desert Shrublands: Benefits of Postfire Reclamation

In 1998, fires burned more than 11 330 ha of rangeland on Dugway Proving Ground in Utah's west desert. Postfire revegetation was implemented in 2 affected salt desert shrub communities (greasewood; Sarcobatus vermiculatus Hook. and black sagebrush/shadscale; Artemisia nova A. Nels; Atriplex confertifolia Torr. & Frem.) to deter cheatgrass (Bromus tectorum L.) encroachment. We monitored cheatgrass densities for 3 years after the fire in burned drill seeded, burned not-seeded, and unburned plots to assess the rate of invasion and determine the impact on cheatgrass of drill seeding perennial species. Cheatgrass invaded quickly in both shrub sites following the fires. In the greasewood site, drill seeded species germinated but did not establish. This was likely due to a combination of soil salinity and extremely dry weather conditions during the second year of the study. Drill seeded species in the black sagebrush site germinated and established well, resulting in the establishment of 16.5 perennial grasses · m^-2 and 1 356 shrubs · ha^-1. Cheatgrass densities were consistently lower in drill seeded versus not-seeded plots, although these were not always statistically different when Bonferroni comparisons were considered. The initial decrease in cheatgrass densities in drill seeded plots may have resulted from soil disturbance coupled with extremely low precipitation rather than competitive effects. Nevertheless, as seeded species mature and increase their competitive ability, we predict long-term suppression of cheatgrass in the absence of further disturbance.     

Short-term Effects of Burning Wyoming Big Sagebrush Steppe in Southeast Oregon

Wyoming big sagebrush (Artemisia tridentata subsp. wyomingensis [Beetle & A. Young] S.L. Welsh) plant communities of the Intermountain West have been greatly reduced from their historic range as a result of wildfire, agronomic practices, brush control treatments, and weed invasions. The impact of prescribed fall burning Wyoming big sagebrush has not been well quantified. Treatments were sagebrush removed with burning (burned) and sagebrush present (control). Treatments were applied to 0.4-ha plots at 6 sites. Biomass production, vegetation cover, perennial herbaceous vegetation diversity, soil water content, soil inorganic nitrogen (NO-3, NH+4), total soil nitrogen (N), total soil carbon (C), and soil organic matter (OM) were compared between treatments in the first 2 years postburn. In 2003 and 2004, total (shrub and herbaceous) aboveground annual biomass production was 2.3 and 1.2 times greater, respectively, in the control compared to the burned treatment. In the upper 15 cm of the soil profile, inorganic N concentrations were greater in the burned than control treatment, while soil water, at least in the spring, was greater in the control than burned treatment. Regardless, greater herbaceous aboveground annual production and cover in the burned treatment indicated that resources were more available to herbaceous vegetation in the burned than the control treatment. Exotic annual grasses did not increase with the burn treatment. Our results suggest in some instances that late seral Wyoming big sagebrush plant communities can be prescribed fall burned to increase livestock forage or alter wildlife habitat without exotic annual grass invasion in the first 2 years postburn. However, long-term evaluation at multiple sites across a larger area is needed to better quantify the effects of prescribed fall burning on these communities. Thus, caution is advised because of the value of Wyoming big sagebrush plant communities to wildlife and the threat of invasive plants.