The invasion of nonnative plants is considered one of the main threats to the structure and function of North American ecosystems. Moreover, they can alter ecosystem processes and reduce biodiversity. In arid and semi...The invasion of nonnative plants is considered one of the main threats to the structure and function of North American ecosystems. Moreover, they can alter ecosystem processes and reduce biodiversity. In arid and semi-arid region of North America, the species of European annual grass Bromus tectorum L. is an outstanding example of these problems, which not only increase the fire density and change the fire regime, but replace native communities. Therefore, there are amount of researches on B. tectorum, including resource acquisition, water use efficiency and growth. Whereas the relevant research on the morphology of diaspore is scare. Diaspores have a fundamental role in seed germination and seedling establishment. Besides, as an important link between different generations, diaspores have a vital significance on individual reproduction and population extension. Hence, dias- pores under selection for studying have an important implication. This study compares differences in seed mor- phology for Bromus tectorum collected from the United States, Kazakhstan, and Xinjiang of China. The following indices of B. tectorum diaspores were analyzed： size, thickness of covering layers, and micromorphological char- acteristics of the base, middle and transition area of diaspores as well as of the awn. Micromorphology of the lemma and the cross-section of the diaspore were observed by scanning electron microscopy. Results showed that thick- ness of the lemma and the palea of diaspores from B. tectorum-infested grasslands in the United States were re- duced （P〈0.05）, likely because of environmental influences. This reduction facilitated the germination of diaspores and lowered the resistance of B. tectorum to adverse environmental conditions. The length of the awn also in- creased significantly （P〈0.05）, which helped in dispersal and anchoring of diaspores. Therefore, B. tectorum adapted ecologically to its new environment in the United States by strengthening its establishment ability. However, the defense capability of B. tectorum decreased. These results fit the evolution of increased competitive ability hy- pothesis （EICA） of invasive species. Analysis of various cells on the lemma revealed that prickle densities and col- lapsed, long epidermal cells were easily influenced by environmental factors such as temperature and moisture because of the physiologic function of these structures on silicon accumulation. However, the form and the position of silica cells, which were not greatly influenced by environmental factors, might be genetically controlled. Studying these structures at the microscopic level helps define the relationship between the diaspore and its environment. This study has a reference value for future studies on B. tectorum.展开更多
This paper aimed to study the change in MDA content and antioxidant enzyme activity of roots in wintering period under single sowing of alfalfa and mixed sowing of alfalfa and Bromus inermis Leyss.By the single sowing...This paper aimed to study the change in MDA content and antioxidant enzyme activity of roots in wintering period under single sowing of alfalfa and mixed sowing of alfalfa and Bromus inermis Leyss.By the single sowing of 4 alfalfa varieties with different cold resistance and mixed sowing of alfalfa varieties and Bromus inermis Leyss,the changes in MDA content,SOD,POD and CAT activity of alfalfa roots throughout the wintering period were measured.Results indicated that after single sowing of alfalfa and mixed sowing of alfalfa and Bromus inermis Leyss,the MDA content of roots showed an up-down-up trend with temperature; the CAT activity increased with the decreasing temperature but decreased when the temperature rose in the spring of the following year; the SOD and POD activity showed an increasing trend with sharp decline in temperature,and decreased when the temperature continued to decline,but increased with alfalfa reviving in the following year.The enzymic activity of the same variety under mixed sowing was higher than under single sowing.The membership function was used for comprehensive evaluation of cold resistance,and the cold resistance under different treatments was in the order of Wega7F + Bromus inermis Leyss >Wega7F > Xunlu + Bromus inermis Leyss > Xunlu > Aohan + Bromus inermis Leyss > Aohan > WL319HQ + Bromus inermis Leyss > WL319HQ.This was of great significance to the study on cold resistant alfalfa breeding and cultivation in cold areas of northern China.展开更多
Responses of caryopsis germination, seedling emergence, and development of Agropyron cristatum （L.） Gaertn. （Gramineae） and Bromus inermis Leyss. （Gramineae）, two dominant perennial grasses in the Otindag Sandla...Responses of caryopsis germination, seedling emergence, and development of Agropyron cristatum （L.） Gaertn. （Gramineae） and Bromus inermis Leyss. （Gramineae）, two dominant perennial grasses in the Otindag Sandland of China, to different sand water content （SWC; 1%, 2%, 3%, 4%, 6%, 8%, 12%, 16%, and 20%） were studied comparatively. The results showed that the germination responses of the two grasses to SWC were similar （i.e. caryopses could not germinate when the SWC was below 3%; at SWC ranging from 3% to 12%, the higher the SWC, the higher the germination percentage; and at a SWC of 12%-20%, germination reached similarly high percentages）. At a sand burial depth of 0.5 cm, the threshold of SWC for seedling emergence was 6% forA. cristatum and 8% forB. inermis; at 12%-20% SWC, the seedling emergence of both species reached similarly high percentages. The seedling growth responses of these two species to SWC gradients were different. For A. cristatum, the biomass of seedlings increased with SWC from 6% to 12%, and decreased with SWC from 12% to 20%. For B. inermis, the biomass of seedlings always increased with SWC from 8% to 20%. The results also showed that the seedlings of both species allocated more biomass to the roots with decreases in SWC. The SWC changes from April to October in natural microhabitats of both species suggested that the SWC may play an important role in caryopsis germination, seedling emergence, and the growth characteristics of the two grasses. The responses of caryopsis germination, seedling emergence, and the growth characteristics of these two species to SWC may determine their distribution pattems in the Otindag Sandland.展开更多
Planting grass and legume mixtures on improved grasslands has the potential advantage of realizing both higher yields and lower environmental pollution by optimizing the balance between applied N fertilizer and the na...Planting grass and legume mixtures on improved grasslands has the potential advantage of realizing both higher yields and lower environmental pollution by optimizing the balance between applied N fertilizer and the natural process of legume biological nitrogen fixation. However, the optimal level of N fertilization for grass-legume mixtures, to obtain the highest yield, quality, and contribution of N2 fixation, varies with species. A greenhouse pot experiment was conducted to study the temporal dynamics of N2 fixation of alfalfa （Medicago sativa L.） grown alone and in mixture with smooth bromegrass （Bromus inermis Leyss.） in response to the addition of fertilizer N. Three levels of N （0, 75, and 150 kg ha-1） were examined using 15N-labeled urea to evaluate N2 fixation via the 15N isotope dilution method. Treatments were designated NO （0.001 g per pot）, N75 （1.07 g per pot） and N150 （2.14 g per pot）. Alfalfa grown alone did not benefit from the addition of fertilizer N; dry matter was not significantly increased. In contrast, dry weight and N content of smooth bromegrass grown alone was increased significantly by N application. When grown as a mixture, smooth bromegrass biomass was increased significantly by N application, resulted in a decrease in alfalfa biomass. In addition, individual alfalfa plant dry weight （shoots＋roots） was significantly lower in the mixture than when grown alone at all N levels. Smooth bromegrass shoot and root dry weight were significantly higher when grown with alfalfa than when grown alone, regardless of N application level. When grown alone, alfalfa＇s N2 fixation was reduced with N fertilization （R2=0.9376,P=0.0057）. When grown in a mixture with smooth bromegrass, with 75 kg ha-1 of N fertilizer, the percentage of atmospheric N2 fixation contribution to total N in alfalfa （%Ndfa） had a maximum of 84.07 and 83.05% in the 2nd and 3rd harvests, respectively. Total 3-harvest %Ndfa was higher when alfalfa was grown in a mixture than when grown alone （shoots： ｜t｜=3.39, P=0.0096; root： ｜t｜=3.57, P=0.0073）. We believe this was due to smooth bromegrass being better able to absorb available soil N （due to its fibrous root system）, resulting inlower soil N availability and allowing alfalfa to develop an effective N2 fixing symbiosis prior to the 1st harvest. Once soil N levels were depleted, alfalfa was able to fix N2, resulting in the majority of its tissue N being derived from biological nitrogen fixation （BNF） in the 2nd and 3rd harvests. When grown in a mixture, with added N, alfalfa established an effective symbiosis earlier than when grown alone; in monoculture BNF did not contribute a significant portion of plant N in the N75 and N150 treatments, whereas in the mixture, BNF contributed 17.90 and 16.28% for these treatments respectively. Alfalfa has a higher BNF efficiency when grown in a mixture, initiating BNF earlier, and having higher N2 fixation due to less inhibition by soil-available N. For the greatest N-use-efficiency and sustainable production, grass-legume mixtures are recommended for imDrovino orasslands, usino a moderate amount of N fertilizer （75 kq N ha-l） to provide optimum benefits.展开更多
Exotic plant invasion is a growing concern in the conservation and management of indigenous arid land ecosystems. By creating areas of ameliorated microclimates and fertile soil below their canopies, perennial plants ...Exotic plant invasion is a growing concern in the conservation and management of indigenous arid land ecosystems. By creating areas of ameliorated microclimates and fertile soil below their canopies, perennial plants might influence exotic annual plant invasions. We conducted a quantitative literature review of studies that compared exotic annual plant abundance among native perennial plant species and interspace （open areas） microsites in North America＇s Mojave Desert, where exotic plant invasion has corresponded with increasing extent of wildfire and broad-scale ecosystem transformation. Ten studies compared exotic annual plant abundance between inter- spaces and below a total of 36 native perennial species. These studies revealed that： （1） With few exceptions, most native perennial species supported a greater abundance of exotic annuals than interspaces, indicating overall facilitation of exotic species by native perennials. （2） Exotic species abundance varied by orders of magnitude among native perennial species, with some perennial species harboring amounts of exotics similar to interspaces. （3） Distributions of dominant exotic species varied, where Bromus rubens displayed a greater affinity for below-perennial microsites than did Schismus spp. and Erodium cicutarium that often were most abundant in interspaces. Results suggest that the degree of facilitation of exotic plants warrants consideration when selecting native perennial species for revegetation and restoration projects.展开更多
Natural scientists have long recognized that regions with similar climate tend to have similar vegetation.Preliminary observations suggest that shrub steppe communities of the western US and western China may be two s...Natural scientists have long recognized that regions with similar climate tend to have similar vegetation.Preliminary observations suggest that shrub steppe communities of the western US and western China may be two such regions with similar annual precipitation,temperature,land use,and vegetation.These cold dry shrub steppes have traditionally been grazed.Despite these similarities,patterns of species dominance are different.Annual species that are rare in China become dominant when introduced to the United States.The objective of this study was to investigate how climate,land use and community structure may explain these patterns of species dominance.Community structure and grazing intensity were measured at 5 sites in each region.This information was combined with a broader review of the literature describing the history of grazing in both basins.Climate was analyzed based on a spatially-gridded,interpolated weather time series （monthly records） and climatological summary （1961-1990 mean conditions） data set from the Climate Research Unit.We found that differences in summer precipitation and winter minimum temperature,land use intensity,and shrub size may all contribute to the dominance of annual species in the Great Basin,particularly Bromus tectorum.In particular,previous work indicates that summer precipitation and winter temperature drive the distribution of Bromus tectorum in the Great Basin.As a result,sites with wet summers and cold springs,similar to the Chinese sites,would not be expected to be dominated by Bromus tectorum.A history of more intense grazing of the Chinese sites,as described in the literature,also is likely to decrease fire frequency,and decreases litter and shrub dominance,all of which have been demonstrated to be important in Bromus tectorum establishment and ultimate dominance.Further research is necessary to determine if other annuals that follow the same pattern of scarcity in the Junggar Basin and dominance in the Great Basin are responding to the same influences.展开更多
Meadow bromegrass (Bromus riparius Rehmann) is an important forage crop in Canada and the Intermountain West, but it has not been extensively evaluated in the northern Great Plains (NGP). Our objectives were to 1) eva...Meadow bromegrass (Bromus riparius Rehmann) is an important forage crop in Canada and the Intermountain West, but it has not been extensively evaluated in the northern Great Plains (NGP). Our objectives were to 1) evaluate meadow, smooth, and hybrid bromegrasses for forage production under intensive management in eastern South Dakota, and 2) evaluate the effect of selection for vigor in multiple environments on forage production of meadow bromegrass. Thirteen populations (7 meadow bromegrass, 5 hybrid bromegrass, and 1 smooth bromegrass) were evaluated for 4 yrs (2005-2007, 2009) at Brookings, SD. Biomass at anthesis was greater for smooth (6.75 Mg ha-1) than for meadow or hybrid brome (5.4 Mg ha-1) in 2005, but production at anthesis during 2006 and 2007 was similar for meadow and smooth bromegrass. Regrowth harvested during July and October 2005 and November 2006 was greater for meadow than smooth or hybrid bromegrass. Forage production at anthesis in 2009, after rest (i.e., no cutting) and fertilization in 2008, was 4.2 Mg ha-1 for meadow bromegrass compared with 3.3 Mg ha-1 and 2.6 Mg ha-1 for hybrid and smooth bromegrass, respectively. Smooth and hybrid bromegrasses had more leaves·tiller-1 than meadow bromegrass. After 6 yrs, meadow bromegrass had higher tiller density and greater potential for tolerating multiple harvests during a growing season than did smooth or hybrid bromegrass. Selection for vigor in multiple environments in North America resulted in experimental populations of meadow bromegrass with superior forage yield compared with ‘Fleet’ on marginal crop land in the northern Great Plains.展开更多
The changes of Ca2＋ levels in young leaf cells of bromegrass under different controlled chilling temperatures were inves- tigated by an antimonite precipitation cytochemical method. The main results were as follows：...The changes of Ca2＋ levels in young leaf cells of bromegrass under different controlled chilling temperatures were inves- tigated by an antimonite precipitation cytochemical method. The main results were as follows： under 25/20℃ （day/night） tempera- ture and 14 h photoperiod, electron-dense Ca2＋ antimonite precipitates, indicators of Ca2＋ localization, were mainly localized in the vacuoles, cell walls and intercellular spaces; few Ca2＋ deposits were observed in the cytosol and nuclei. After a 3℃ chilling treatment for 3 h, many Ca2＋ precipitates appeared in the cytosol and nuclei, indicating that Ca2＋ influx had occurred in the cytosol and nuclei. When the 3℃ treatment was prolonged to 8 h, more Ca2＋ deposits appeared in the nuclei and cytosol, but the amount of Ca2＋ deposits in both the cytosol and nuclei decreased markedly after a 24 h treatment and most Ca2＋ deposits were returned to the vacuoles and intercellular spaces after an 8 d treatment. When bromegrass was exposed to 7℃ for 3 h, the Ca2＋ distribution in the cells had no visible changes, compared with that of the 25/20℃ grown control plants. However, when the chilling treatment of 7℃ was prolonged to 8 h, a Ca2＋ influx occurred, where many Ca2＋ deposits were observed in the nuclei and cytosol. More Ca2＋ deposits appeared in the nuclei and cytosol after a 24 h treatment, but the amount of Ca2＋ deposits in the cytosol and nuclei was reduced markedly after an 8 d treatment. After a 14 d treatment, the remaining low level of Ca2＋ was recovered in both the cytosol and nuclei and the Ca2＋ deposits were again located in the vacuoles and the intercellular spaces. The dynamics of subcellular Ca2＋ localization in young leaf cells of bromegrass during a 12℃ chilling treatment were similar to those of the 7℃ treatment. Besides, the results showed that the frost tolerance ofbromegrass exposed to 3℃ for 8 d increased by 6℃, for 7℃ and 8 d by 4℃ and for 12℃ and 14 d by 3℃, compared with the controls. Finally, the relationship between different Ca2＋ dynamics and induced frost tolerance was also explored.展开更多
Squirreltail (Elymus elymoides [Raf] Swezey) can grow in cheatgrass (Bromus tectorum L.) stands, and has reduced the biomass production of cheatgrass in its immediate vicinity. A field experiment was conducted to dete...Squirreltail (Elymus elymoides [Raf] Swezey) can grow in cheatgrass (Bromus tectorum L.) stands, and has reduced the biomass production of cheatgrass in its immediate vicinity. A field experiment was conducted to determine: 1) if competition for nitrogen resources occurs between seedlings of squirreltail and seedlings of cheatgrass, under low, medium, and high nitrogen levels, and;2) if competition for nitrogen resources in the seedling stage is a mechanism that allows squirreltail to establish in cheatgrass stands. Five accessions of squirreltail were each seeded with a single accession of cheatgrass in a cultivated field near Ephraim, Utah in the fall of 1995 and again in 1996. A completely randomized strip-plot design with 3 replications was used including the 5 joint seedings as well as pure seedings of each accession of both species. Three nitrogen levels representing high, medium (control), and low were applied. The study was replicated over 2 years in different areas of the same field. Harvests of above-ground biomass of squirreltail and cheatgrass within each treatment were conducted in July of 1996 and 1997. Using biomass production as a measure of efficient nitrogen use, cheatgrass competed for and used nitrogen resources more efficiently than squirreltail when nitrogen was not limiting. All squirreltail accessions were able to compete for and use nitrogen more efficiently than cheatgrass when there was low availability of nitrogen. Some accessions of squirreltail competed for nitrogen resources more efficiently than others both in the control and at the reduced nitrogen level. White Rocks and Sublette squirreltail accessions were the best competitors with cheatgrass at the low nitrogen level reducing the cheatgrasss biomass by as much as 75% and 67% respectively. An intermediate competitor with cheatgrass was the Gunnison accession. The Washakie and Pueblo accessions were poor competitors with cheatgrass at low nitrogen levels.展开更多
Our objective was to evaluate the combining ability among cultivars of forage species, commonly sown in temperate regions of Argentina using a short rotation system. Three genetically diverse cultivars of prairie brom...Our objective was to evaluate the combining ability among cultivars of forage species, commonly sown in temperate regions of Argentina using a short rotation system. Three genetically diverse cultivars of prairie brome grass (Bromus catharticus, cv Copetona, cv ?andú and cv Tango), a white clover cultivar (Trifolium repens, cv Lucero) and a red clover cultivar (T. pratense, cv Tropero) were evaluated. A randomized complete block design experiment was established in 2005. The treatments included five monocultures and ten binary mixtures. The experiment was harvested 6 times over an 18-month period. Cumulative dry matter yield (kg?ha–1) was calculated as the sum of the six individual harvests. Diallel analysis provided estimates of the general combining ability (GCA) and specific combining ability (SCA). Red clover in monoculture and mixtures produced the highest yields, with significant positive GCA effects (P P P P < 0.01). Given the short-term nature of this study, we must limit our inference to short-term pastures (<2 years). Under these conditions, red clover had the best combination with prairie brome grass.展开更多
Introduction:Communities change over time due to disturbances,variations in climate,and species invasions.Biological soil crust communities are important because they contribute to erosion control and nutrient cycling...Introduction:Communities change over time due to disturbances,variations in climate,and species invasions.Biological soil crust communities are important because they contribute to erosion control and nutrient cycling.Crust types may respond differently to changes in environmental conditions:single-celled organisms and bryophytes quickly recover after a disturbance,while lichens are slow growing and dominate favorable sites.Community change in crusts has seldom been assessed using repeated measures.For this study,we hypothesized that changes in crust composition were related to disturbance,topographic position,and invasive vegetation.Methods:We monitored permanent plots in the Columbia Basin in 1999 and 2010 and compared changes in crust composition,cover,richness,and turnover with predictor variables of herbivore exclosure,elevation,heat load index,time since fire,presence of an invasive grass,and change in cover of the invasive grass.Results:Bryophytes were cosmopolitan with high cover.Dominant lichens did not change dramatically.Indicator taxa differed by monitoring year.Bryophyte and total crust cover declined,and there was lower turnover outside of herbivore exclosures.Lichen cover did not change significantly.Plots that burned recently had high turnover.Increase in taxon richness was correlated with presence of an invasive grass in 1999.Change in cover of the invasive grass was positively related to proportional loss and negatively related to gain.Conclusions:Composition and turnover metrics differed significantly over 11 years,though cover was more stable between years.This study can be a baseline for assessing change in crust composition due to anthropogenic influences.展开更多
基金supported by the International Science and Technology Cooperation Program of China (2010DFA92720-06)with partial support to RSN from the US National Science Foundation (1047575) and the Nevada Agricultural Experiment Station
文摘The invasion of nonnative plants is considered one of the main threats to the structure and function of North American ecosystems. Moreover, they can alter ecosystem processes and reduce biodiversity. In arid and semi-arid region of North America, the species of European annual grass Bromus tectorum L. is an outstanding example of these problems, which not only increase the fire density and change the fire regime, but replace native communities. Therefore, there are amount of researches on B. tectorum, including resource acquisition, water use efficiency and growth. Whereas the relevant research on the morphology of diaspore is scare. Diaspores have a fundamental role in seed germination and seedling establishment. Besides, as an important link between different generations, diaspores have a vital significance on individual reproduction and population extension. Hence, dias- pores under selection for studying have an important implication. This study compares differences in seed mor- phology for Bromus tectorum collected from the United States, Kazakhstan, and Xinjiang of China. The following indices of B. tectorum diaspores were analyzed： size, thickness of covering layers, and micromorphological char- acteristics of the base, middle and transition area of diaspores as well as of the awn. Micromorphology of the lemma and the cross-section of the diaspore were observed by scanning electron microscopy. Results showed that thick- ness of the lemma and the palea of diaspores from B. tectorum-infested grasslands in the United States were re- duced （P〈0.05）, likely because of environmental influences. This reduction facilitated the germination of diaspores and lowered the resistance of B. tectorum to adverse environmental conditions. The length of the awn also in- creased significantly （P〈0.05）, which helped in dispersal and anchoring of diaspores. Therefore, B. tectorum adapted ecologically to its new environment in the United States by strengthening its establishment ability. However, the defense capability of B. tectorum decreased. These results fit the evolution of increased competitive ability hy- pothesis （EICA） of invasive species. Analysis of various cells on the lemma revealed that prickle densities and col- lapsed, long epidermal cells were easily influenced by environmental factors such as temperature and moisture because of the physiologic function of these structures on silicon accumulation. However, the form and the position of silica cells, which were not greatly influenced by environmental factors, might be genetically controlled. Studying these structures at the microscopic level helps define the relationship between the diaspore and its environment. This study has a reference value for future studies on B. tectorum.
基金Supported by Special Postdoctoral Funding in Heilongjiang Province(LBHTZ06020)
文摘This paper aimed to study the change in MDA content and antioxidant enzyme activity of roots in wintering period under single sowing of alfalfa and mixed sowing of alfalfa and Bromus inermis Leyss.By the single sowing of 4 alfalfa varieties with different cold resistance and mixed sowing of alfalfa varieties and Bromus inermis Leyss,the changes in MDA content,SOD,POD and CAT activity of alfalfa roots throughout the wintering period were measured.Results indicated that after single sowing of alfalfa and mixed sowing of alfalfa and Bromus inermis Leyss,the MDA content of roots showed an up-down-up trend with temperature; the CAT activity increased with the decreasing temperature but decreased when the temperature rose in the spring of the following year; the SOD and POD activity showed an increasing trend with sharp decline in temperature,and decreased when the temperature continued to decline,but increased with alfalfa reviving in the following year.The enzymic activity of the same variety under mixed sowing was higher than under single sowing.The membership function was used for comprehensive evaluation of cold resistance,and the cold resistance under different treatments was in the order of Wega7F + Bromus inermis Leyss >Wega7F > Xunlu + Bromus inermis Leyss > Xunlu > Aohan + Bromus inermis Leyss > Aohan > WL319HQ + Bromus inermis Leyss > WL319HQ.This was of great significance to the study on cold resistant alfalfa breeding and cultivation in cold areas of northern China.
基金国家重点基础研究发展计划(973计划)，the National Natural Science Foundation of China，the Knowledge Innovation Program of the Chinese Academy of Sciences
文摘Responses of caryopsis germination, seedling emergence, and development of Agropyron cristatum （L.） Gaertn. （Gramineae） and Bromus inermis Leyss. （Gramineae）, two dominant perennial grasses in the Otindag Sandland of China, to different sand water content （SWC; 1%, 2%, 3%, 4%, 6%, 8%, 12%, 16%, and 20%） were studied comparatively. The results showed that the germination responses of the two grasses to SWC were similar （i.e. caryopses could not germinate when the SWC was below 3%; at SWC ranging from 3% to 12%, the higher the SWC, the higher the germination percentage; and at a SWC of 12%-20%, germination reached similarly high percentages）. At a sand burial depth of 0.5 cm, the threshold of SWC for seedling emergence was 6% forA. cristatum and 8% forB. inermis; at 12%-20% SWC, the seedling emergence of both species reached similarly high percentages. The seedling growth responses of these two species to SWC gradients were different. For A. cristatum, the biomass of seedlings increased with SWC from 6% to 12%, and decreased with SWC from 12% to 20%. For B. inermis, the biomass of seedlings always increased with SWC from 8% to 20%. The results also showed that the seedlings of both species allocated more biomass to the roots with decreases in SWC. The SWC changes from April to October in natural microhabitats of both species suggested that the SWC may play an important role in caryopsis germination, seedling emergence, and the growth characteristics of the two grasses. The responses of caryopsis germination, seedling emergence, and the growth characteristics of these two species to SWC may determine their distribution pattems in the Otindag Sandland.
基金supported by the China Forage and Grass Research System (CARS-35)the National Key Technology R&D Program of China (2011BAD17B01)
文摘Planting grass and legume mixtures on improved grasslands has the potential advantage of realizing both higher yields and lower environmental pollution by optimizing the balance between applied N fertilizer and the natural process of legume biological nitrogen fixation. However, the optimal level of N fertilization for grass-legume mixtures, to obtain the highest yield, quality, and contribution of N2 fixation, varies with species. A greenhouse pot experiment was conducted to study the temporal dynamics of N2 fixation of alfalfa （Medicago sativa L.） grown alone and in mixture with smooth bromegrass （Bromus inermis Leyss.） in response to the addition of fertilizer N. Three levels of N （0, 75, and 150 kg ha-1） were examined using 15N-labeled urea to evaluate N2 fixation via the 15N isotope dilution method. Treatments were designated NO （0.001 g per pot）, N75 （1.07 g per pot） and N150 （2.14 g per pot）. Alfalfa grown alone did not benefit from the addition of fertilizer N; dry matter was not significantly increased. In contrast, dry weight and N content of smooth bromegrass grown alone was increased significantly by N application. When grown as a mixture, smooth bromegrass biomass was increased significantly by N application, resulted in a decrease in alfalfa biomass. In addition, individual alfalfa plant dry weight （shoots＋roots） was significantly lower in the mixture than when grown alone at all N levels. Smooth bromegrass shoot and root dry weight were significantly higher when grown with alfalfa than when grown alone, regardless of N application level. When grown alone, alfalfa＇s N2 fixation was reduced with N fertilization （R2=0.9376,P=0.0057）. When grown in a mixture with smooth bromegrass, with 75 kg ha-1 of N fertilizer, the percentage of atmospheric N2 fixation contribution to total N in alfalfa （%Ndfa） had a maximum of 84.07 and 83.05% in the 2nd and 3rd harvests, respectively. Total 3-harvest %Ndfa was higher when alfalfa was grown in a mixture than when grown alone （shoots： ｜t｜=3.39, P=0.0096; root： ｜t｜=3.57, P=0.0073）. We believe this was due to smooth bromegrass being better able to absorb available soil N （due to its fibrous root system）, resulting inlower soil N availability and allowing alfalfa to develop an effective N2 fixing symbiosis prior to the 1st harvest. Once soil N levels were depleted, alfalfa was able to fix N2, resulting in the majority of its tissue N being derived from biological nitrogen fixation （BNF） in the 2nd and 3rd harvests. When grown in a mixture, with added N, alfalfa established an effective symbiosis earlier than when grown alone; in monoculture BNF did not contribute a significant portion of plant N in the N75 and N150 treatments, whereas in the mixture, BNF contributed 17.90 and 16.28% for these treatments respectively. Alfalfa has a higher BNF efficiency when grown in a mixture, initiating BNF earlier, and having higher N2 fixation due to less inhibition by soil-available N. For the greatest N-use-efficiency and sustainable production, grass-legume mixtures are recommended for imDrovino orasslands, usino a moderate amount of N fertilizer （75 kq N ha-l） to provide optimum benefits.
基金facilitated by funding provided by cooperative agreements between the National Park Service (Lake Mead National Recreation Area,in particular Alice NEWTON) and the University of Nevada Las Vegas (UNLV)
文摘Exotic plant invasion is a growing concern in the conservation and management of indigenous arid land ecosystems. By creating areas of ameliorated microclimates and fertile soil below their canopies, perennial plants might influence exotic annual plant invasions. We conducted a quantitative literature review of studies that compared exotic annual plant abundance among native perennial plant species and interspace （open areas） microsites in North America＇s Mojave Desert, where exotic plant invasion has corresponded with increasing extent of wildfire and broad-scale ecosystem transformation. Ten studies compared exotic annual plant abundance between inter- spaces and below a total of 36 native perennial species. These studies revealed that： （1） With few exceptions, most native perennial species supported a greater abundance of exotic annuals than interspaces, indicating overall facilitation of exotic species by native perennials. （2） Exotic species abundance varied by orders of magnitude among native perennial species, with some perennial species harboring amounts of exotics similar to interspaces. （3） Distributions of dominant exotic species varied, where Bromus rubens displayed a greater affinity for below-perennial microsites than did Schismus spp. and Erodium cicutarium that often were most abundant in interspaces. Results suggest that the degree of facilitation of exotic plants warrants consideration when selecting native perennial species for revegetation and restoration projects.
基金funded by the US National Science Foundation(1047575)with additional support from the Nevada Agricultural Experiment Station and National Science Foundation Cooperative Agreement(EPS-0814372)
文摘Natural scientists have long recognized that regions with similar climate tend to have similar vegetation.Preliminary observations suggest that shrub steppe communities of the western US and western China may be two such regions with similar annual precipitation,temperature,land use,and vegetation.These cold dry shrub steppes have traditionally been grazed.Despite these similarities,patterns of species dominance are different.Annual species that are rare in China become dominant when introduced to the United States.The objective of this study was to investigate how climate,land use and community structure may explain these patterns of species dominance.Community structure and grazing intensity were measured at 5 sites in each region.This information was combined with a broader review of the literature describing the history of grazing in both basins.Climate was analyzed based on a spatially-gridded,interpolated weather time series （monthly records） and climatological summary （1961-1990 mean conditions） data set from the Climate Research Unit.We found that differences in summer precipitation and winter minimum temperature,land use intensity,and shrub size may all contribute to the dominance of annual species in the Great Basin,particularly Bromus tectorum.In particular,previous work indicates that summer precipitation and winter temperature drive the distribution of Bromus tectorum in the Great Basin.As a result,sites with wet summers and cold springs,similar to the Chinese sites,would not be expected to be dominated by Bromus tectorum.A history of more intense grazing of the Chinese sites,as described in the literature,also is likely to decrease fire frequency,and decreases litter and shrub dominance,all of which have been demonstrated to be important in Bromus tectorum establishment and ultimate dominance.Further research is necessary to determine if other annuals that follow the same pattern of scarcity in the Junggar Basin and dominance in the Great Basin are responding to the same influences.
文摘Meadow bromegrass (Bromus riparius Rehmann) is an important forage crop in Canada and the Intermountain West, but it has not been extensively evaluated in the northern Great Plains (NGP). Our objectives were to 1) evaluate meadow, smooth, and hybrid bromegrasses for forage production under intensive management in eastern South Dakota, and 2) evaluate the effect of selection for vigor in multiple environments on forage production of meadow bromegrass. Thirteen populations (7 meadow bromegrass, 5 hybrid bromegrass, and 1 smooth bromegrass) were evaluated for 4 yrs (2005-2007, 2009) at Brookings, SD. Biomass at anthesis was greater for smooth (6.75 Mg ha-1) than for meadow or hybrid brome (5.4 Mg ha-1) in 2005, but production at anthesis during 2006 and 2007 was similar for meadow and smooth bromegrass. Regrowth harvested during July and October 2005 and November 2006 was greater for meadow than smooth or hybrid bromegrass. Forage production at anthesis in 2009, after rest (i.e., no cutting) and fertilization in 2008, was 4.2 Mg ha-1 for meadow bromegrass compared with 3.3 Mg ha-1 and 2.6 Mg ha-1 for hybrid and smooth bromegrass, respectively. Smooth and hybrid bromegrasses had more leaves·tiller-1 than meadow bromegrass. After 6 yrs, meadow bromegrass had higher tiller density and greater potential for tolerating multiple harvests during a growing season than did smooth or hybrid bromegrass. Selection for vigor in multiple environments in North America resulted in experimental populations of meadow bromegrass with superior forage yield compared with ‘Fleet’ on marginal crop land in the northern Great Plains.
基金supported by the National Natural Science Foundation of China (Grant No. 60575018).
文摘The changes of Ca2＋ levels in young leaf cells of bromegrass under different controlled chilling temperatures were inves- tigated by an antimonite precipitation cytochemical method. The main results were as follows： under 25/20℃ （day/night） tempera- ture and 14 h photoperiod, electron-dense Ca2＋ antimonite precipitates, indicators of Ca2＋ localization, were mainly localized in the vacuoles, cell walls and intercellular spaces; few Ca2＋ deposits were observed in the cytosol and nuclei. After a 3℃ chilling treatment for 3 h, many Ca2＋ precipitates appeared in the cytosol and nuclei, indicating that Ca2＋ influx had occurred in the cytosol and nuclei. When the 3℃ treatment was prolonged to 8 h, more Ca2＋ deposits appeared in the nuclei and cytosol, but the amount of Ca2＋ deposits in both the cytosol and nuclei decreased markedly after a 24 h treatment and most Ca2＋ deposits were returned to the vacuoles and intercellular spaces after an 8 d treatment. When bromegrass was exposed to 7℃ for 3 h, the Ca2＋ distribution in the cells had no visible changes, compared with that of the 25/20℃ grown control plants. However, when the chilling treatment of 7℃ was prolonged to 8 h, a Ca2＋ influx occurred, where many Ca2＋ deposits were observed in the nuclei and cytosol. More Ca2＋ deposits appeared in the nuclei and cytosol after a 24 h treatment, but the amount of Ca2＋ deposits in the cytosol and nuclei was reduced markedly after an 8 d treatment. After a 14 d treatment, the remaining low level of Ca2＋ was recovered in both the cytosol and nuclei and the Ca2＋ deposits were again located in the vacuoles and the intercellular spaces. The dynamics of subcellular Ca2＋ localization in young leaf cells of bromegrass during a 12℃ chilling treatment were similar to those of the 7℃ treatment. Besides, the results showed that the frost tolerance ofbromegrass exposed to 3℃ for 8 d increased by 6℃, for 7℃ and 8 d by 4℃ and for 12℃ and 14 d by 3℃, compared with the controls. Finally, the relationship between different Ca2＋ dynamics and induced frost tolerance was also explored.
文摘Squirreltail (Elymus elymoides [Raf] Swezey) can grow in cheatgrass (Bromus tectorum L.) stands, and has reduced the biomass production of cheatgrass in its immediate vicinity. A field experiment was conducted to determine: 1) if competition for nitrogen resources occurs between seedlings of squirreltail and seedlings of cheatgrass, under low, medium, and high nitrogen levels, and;2) if competition for nitrogen resources in the seedling stage is a mechanism that allows squirreltail to establish in cheatgrass stands. Five accessions of squirreltail were each seeded with a single accession of cheatgrass in a cultivated field near Ephraim, Utah in the fall of 1995 and again in 1996. A completely randomized strip-plot design with 3 replications was used including the 5 joint seedings as well as pure seedings of each accession of both species. Three nitrogen levels representing high, medium (control), and low were applied. The study was replicated over 2 years in different areas of the same field. Harvests of above-ground biomass of squirreltail and cheatgrass within each treatment were conducted in July of 1996 and 1997. Using biomass production as a measure of efficient nitrogen use, cheatgrass competed for and used nitrogen resources more efficiently than squirreltail when nitrogen was not limiting. All squirreltail accessions were able to compete for and use nitrogen more efficiently than cheatgrass when there was low availability of nitrogen. Some accessions of squirreltail competed for nitrogen resources more efficiently than others both in the control and at the reduced nitrogen level. White Rocks and Sublette squirreltail accessions were the best competitors with cheatgrass at the low nitrogen level reducing the cheatgrasss biomass by as much as 75% and 67% respectively. An intermediate competitor with cheatgrass was the Gunnison accession. The Washakie and Pueblo accessions were poor competitors with cheatgrass at low nitrogen levels.
基金Commercial cultivars seed used in this research were kindly provided by GAPP semillas S.A.and Gentos S.A,Argentina.Anonymous suggestions are acknowledged.
文摘Our objective was to evaluate the combining ability among cultivars of forage species, commonly sown in temperate regions of Argentina using a short rotation system. Three genetically diverse cultivars of prairie brome grass (Bromus catharticus, cv Copetona, cv ?andú and cv Tango), a white clover cultivar (Trifolium repens, cv Lucero) and a red clover cultivar (T. pratense, cv Tropero) were evaluated. A randomized complete block design experiment was established in 2005. The treatments included five monocultures and ten binary mixtures. The experiment was harvested 6 times over an 18-month period. Cumulative dry matter yield (kg?ha–1) was calculated as the sum of the six individual harvests. Diallel analysis provided estimates of the general combining ability (GCA) and specific combining ability (SCA). Red clover in monoculture and mixtures produced the highest yields, with significant positive GCA effects (P P P P < 0.01). Given the short-term nature of this study, we must limit our inference to short-term pastures (<2 years). Under these conditions, red clover had the best combination with prairie brome grass.
基金Information on site history and assistance with site logistics were provided by Barbara Benner and the Bureau of Land Management.Jessica DaBell assisted with field work,Jean Caldwell with GIS analysis,and Loveday Conquest with statistical analysis.Funding for the 2010 re-measurement of these plots was provided by the Joint Fire Science Program under Project JFSP 08-1-5-20.
文摘Introduction:Communities change over time due to disturbances,variations in climate,and species invasions.Biological soil crust communities are important because they contribute to erosion control and nutrient cycling.Crust types may respond differently to changes in environmental conditions:single-celled organisms and bryophytes quickly recover after a disturbance,while lichens are slow growing and dominate favorable sites.Community change in crusts has seldom been assessed using repeated measures.For this study,we hypothesized that changes in crust composition were related to disturbance,topographic position,and invasive vegetation.Methods:We monitored permanent plots in the Columbia Basin in 1999 and 2010 and compared changes in crust composition,cover,richness,and turnover with predictor variables of herbivore exclosure,elevation,heat load index,time since fire,presence of an invasive grass,and change in cover of the invasive grass.Results:Bryophytes were cosmopolitan with high cover.Dominant lichens did not change dramatically.Indicator taxa differed by monitoring year.Bryophyte and total crust cover declined,and there was lower turnover outside of herbivore exclosures.Lichen cover did not change significantly.Plots that burned recently had high turnover.Increase in taxon richness was correlated with presence of an invasive grass in 1999.Change in cover of the invasive grass was positively related to proportional loss and negatively related to gain.Conclusions:Composition and turnover metrics differed significantly over 11 years,though cover was more stable between years.This study can be a baseline for assessing change in crust composition due to anthropogenic influences.
