Arbuscular Mycorrhizal Assemblages in Native Plant Roots Change in the Presence of Invasive Exotic Grasses

Plant invasions have the potential to significantly alter soil microbial communities, given their often considerable aboveground effects. We examined how plant invasions altered the arbuscular mycorrhizal fungi of native plant roots in a grassland site in California and one in Utah. In the California site, we used experimentally created plant communities composed of exotic (Avena barbata, Bromus hordeaceus) and native (Nassella pulchra, Lupinus bicolor) monocultures and mixtures. In the Utah semi-arid grassland, we took advantage of invasion by Bromus tectorum into long-term plots dominated by either of two native grasses, Hilaria jamesii or Stipa hymenoides. Arbuscular mycorrhizal fungi colonizing roots were characterized with PCR amplification of the ITS region, cloning, and sequencing. We saw a significant effect of the presence of exotic grasses on the diversity of mycorrhizal fungi colonizing native plant roots. In the three native grasses, richness of mycorrhizal fungi decreased; in the native forb at the California site, the number of fungal RFLP patterns increased in the presence of exotics. The exotic grasses also caused the composition of the mycorrhizal community in native roots to shift dramatically both in California, with turnover of Glomus spp., and Utah, with replacement of Glomus spp. by apparently non-mycorrhizal fungi. Invading plants may be able to influence the network of mycorrhizal fungi in soil that is available to natives through either earlier root activity or differential carbon provision compared to natives. Alteration of the soil microbial community by plant invasion can provide a mechanism for both successful invasion and the resulting effects of invaders on the ecosystem.     

C<Subscript>4</Subscript> plants in the deserts of China: occurrence of C<Subscript>4</Subscript> photosynthesis and its morphological functional types

C₄ photosynthetic pathway and morphological functional types were determined for 104 species in 45 genera and 10 families from the deserts of China. 67 C₄ species (64.4 %) were found in Dicotyledoneae (e.g. Chenopodiaceae, Polygonaceae, and Amaranthaceae), the other 37 species were in Monocotyledoneae (e.g. Gramineae, Cyperaceae, and Commelinaceae). 36.5 % of the Chenopodiaceae species (predominantly members of the genera Anabasis, Atriplex, Kochia, Salsola, and Suaeda) identified in the desert regions were found with C₄ photosynthesis, which was about 48 % of the total C₄ species. Many C₄ species (58.7 %) were annuals (e.g. Amaranthus, Atriplex, Digitaria, Eragrostis, Kochia, and Salsola) and experienced long-term droughts, high temperature, and high irradiance. Relatively more shrub C₄ species (28 species of 104) were found in Chenopodiaceae (e.g. Anabasis, Camphorosma, Haloxylon, and Salsola) and Polygonaceae (e.g. Calligonum) in the desert regions. Most of shrub C₄ species with small leaf area were no more than 1 m in height and distributed in sandy soils. Composition of relatively more annual species, shrubs, and Chenopodiaceae C₄ species was the primary characteristic for the C₄ species occurrence in deserts, and this was remarkably related with the arid environmental conditions.     

科尔沁沙地3种草本植物根系生长动态

利用微管对狗尾草(Setaria viridis)、黄蒿(Artemisia scoparia)和沙米(Agriophyllum squarrosum)根系生长动态进行了观测。结果表明:1)在观察期内,狗尾草和黄蒿的根系生长表现出先增后减的“抛物线”型,而沙米的根系成指数生长。在7月末至8月初狗尾草、黄蒿和沙米根系具有最高的生产量,其根量密度分别为4 690.91、2 975.76和2 354.55条·m^-2;在8月末,狗尾草和黄蒿根系都表现出不同程度的衰减,沙米根数保持增加。2)狗尾草优先生长表层根系,根系主要生长在0~30 cm的土层内,根长密度最大时为2.23 cm·cm^-2;黄蒿生长前期表层根长密度大,生长后期下层根长密度开始增加;沙米优先发展下层根系,快速生长期后,地下30~50 cm处的根长密度增加高于上层。3)生长前期和快速生长期,0~50 cm狗尾草根长密度最大,黄蒿次之,沙米最小,生长末期,沙米根长密度大于狗尾草。     

Spider community responses to grassland restoration: balancing trade‐offs between abundance and diversity

Spiders (Araneae) play key roles in ecosystems, not only as common and abundant generalist predators, but also as major contributors to biodiversity in many areas. In addition, due to their short generation times and high mobility, spiders respond rapidly to small changes in their environment, potentially making them useful indicators for restoration monitoring. However, few studies have focused on spider responses to grassland restoration in the United States. We compared degraded, native, and restored grassland sites to examine how spider communities and habitat respond to arid grassland restoration. We also examined how responses varied with the age of the restoration project. Spider communities in native sites differed from those in restored and degraded sites in several ways: native sites had fewer spiders and a different community composition than degraded and restored sites. However, native and restored sites had more species than degraded sites. Chronosequence data showed trends for lower abundance, higher species richness, and changing community composition as restoration projects mature. Several habitat variables were closely linked to variation in spider communities including cover of invasive annual grasses, litter, and biological soil crusts. Our data suggest that spider and vegetation responses to grassland restoration efforts can be successful in the long term—with resulting communities becoming more similar to native ones—and that spiders are useful indictors of grassland restoration. Our results also suggest that restoration may involve balancing trade‐offs between ecosystem services, with potential losses in predatory control offset by increases in biodiversity with restoration effort.     

Postwildfire seeding to restore native vegetation and limit exotic annuals: an evaluation in juniper‐dominated sagebrush steppe

Reestablishment of perennial vegetation is often needed after wildfires to limit exotic species and restore ecosystem services. However, there is a growing body of evidence that questions if seeding after wildfires increases perennial vegetation and reduces exotic plants. The concern that seeding may not meet restoration goals is even more prevalent when native perennial vegetation is seeded after fire. We evaluated vegetation cover and density responses to broadcast seeding native perennial grasses and mountain big sagebrush (Artemisia tridentata Nutt. spp. vaseyana [Rydb.] Beetle) after wildfires in the western United States in six juniper (Juniperus occidentalis ssp. occidentalis Hook)‐dominated mountain big sagebrush communities for 3 years postfire. Seeding native perennial species compared to not seeding increased perennial grass and sagebrush cover and density. Perennial grass cover was 4.3 times greater in seeded compared to nonseeded areas. Sagebrush cover averaged 24 and less than 0.1% in seeded and nonseeded areas at the conclusion of the study, respectively. Seeding perennial species reduced exotic annual grass and annual forb cover and density. Exotic annual grass cover was 8.6 times greater in nonseeded compared to seeded areas 3 years postfire. Exotic annual grass cover increased over time in nonseeded areas but decreased in seeded areas by the third‐year postfire. Seeded areas were perennial‐dominated and nonseeded areas were annual‐dominated at the end of the study. Establishing perennial vegetation may be critical after wildfires in juniper‐dominated sagebrush steppe to prevent the development of annual‐dominated communities. Postwildfire seeding increased perennial vegetation and reduced exotic plants and justifies its use.     

Grass Physiognomic Trait Variation in African Herbaceous Biomes

African herbaceous biomes will likely face drastic changes in the near future, due to climate change and pressures from increasing human activities. However, these biomes have been simulated only by dynamic global vegetation models and failing to include the diversity of C₄ grasses has limited the accuracy of these models. Characterizing the floristic and physiognomic diversity of these herbaceous biomes would enhance the parameterization of C₄ grass plant functional types, thereby improving simulations. To this end, we used lowermost and uppermost values of three grass physiognomic traits (culm height, leaf length, and leaf width) available in most floras to identify several grass physiognomic groups that form the grass cover in Senegal. We then checked the capacity of these groups to discriminate herbaceous biomes and mean annual precipitation domains. Specifically, we assessed whether these groups were sufficiently generic and robust to be applied to neighboring (Chad) and distant (South Africa) phytogeographic areas. The proportions of two physiognomic groups, defined by their lowermost limits, delineate steppe from savanna and forest biomes in Senegal, and nama‐karoo, savanna, and grassland biomes in South Africa. Proportions of these two physiognomic groups additionally delineate the mean annual precipitation domains <600 mm and >600 mm in Senegal, Chad, and South Africa, as well as the <250 mm and >1000 mm domains in South Africa. These findings should help to identify and parameterize new C₄ grass plant functional types in vegetation models applied to West and South Africa.     

高寒山区草本植物的保护酶系统及其在低温生长中的作用

研究了高寒山区几种草本植物在低温生长过程中植物叶中抗氧化酶、抗氧化物和膜脂过氧化产物的变化及其作用。结果表明,耐寒性差、生长期短的一年生植物微孔草MDA含量较低,保护酶活力也较低。耐寒性强、生长期长的苜蓿、垂穗披碱草、草地早熟禾、无芒雀4种多年生植物MDA含量较高,保护酶活力和抗氧化物含量也较高。其生长季平均MDA含量分别比微孔草高2．27倍、1．64倍、2．07倍和1．55倍,超氧化物歧化酶（SOD）活力分别比微孔草高16．7％、27．1％、46．2％和11．5％;地氧化氢酶（CAT）分别高51．5％、123．0％、25．3％和9．4％,类胡萝卜麦含量分别高126．5％、142．7％、138．4％和394．4％。当气温低于0℃后,微孔草死亡,完成生活史,以种子越冬,在生理上无明显抗冻特征。而此时多年生植物SOD活力趋于增强,仍能一定生长,至－5℃～10℃低温时地上部才枯死,其细胞具有较强的耐膜脂过氧化能力和抗氧化酶系统,对环境的快速反应可能是其在高寒山区低温生长的重要生理机理。     

Modeling the Impact of Exotic Annual Brome Grasses on soil Organic Carbon Storage in a Northern Mixed-Grass Prairie

Annual brome grasses, Bromus japonicus and B. tectorum, are common invaders of northern mixed-grass prairie, and have been shown to alter the structure and function of prairie ecosystems, including plant biomass production and litter decomposition. To build on previous findings, our objective was to model the impact of annual brome grasses on soil organic carbon storage as a step towards forecasting ecological change. Specifically, we measured differences in carbon storage between patches dominated by annual bromes and perennial grasses, in addition to evaluating key plant functional characteristics that impact carbon storage. Using the CENTURY model, we simulated high- and low-brome vegetation based on differences in functional characteristics, allowing us to extrapolate the findings from the field study across a broader time scale. We sampled a prairie site in 1996 and 1997 to quantify differences between the high- and low-brome cover plots. High-brome plots averaged 40% brome cover, while the low-brome plots averaged 1% brome cover. We found differences in functional attributes for growth characteristics and litter quality, as well as minor differences in edaphic variables between the plots. Based on field measurements, more soil organic carbon was stored under high-brome vegetation than low-brome, but the differences were not statistically significant. Results from model simulations were consistent with field measurements, and suggested that this prairie ecosystem was not significantly impacted by the functional differences between high- and low-brome vegetation for the first 50 years after the brome invasion under historical management and climate. However, the model results also showed that the differences in soil organic carbon storage continue to diverge after 50 years and consequently could be significant in the future.     

Oak canopy effects on the distribution patterns of two annual grasses: the role of competition and soil nutrients

Within the oak woodlands of California there is often a distinct shift in the botanical composition between the open grassland and the herbaceous understory beneath oak canopy. Botanical sampling at two woodland sites indicated that the annual grass Bromus diandrus was dominant under deciduous blue oak canopy, while a congener, Bromus hordeaceus, was dominant in open grassland. We examined the relative importance of congeneric competition and edaphic factors in creating these differences in species distribution in two separate field experiments that manipulated both congeneric and intraspecific competition, as well as soil type. We used the demographic measure of relative reproductive rate as an index of population growth. In general, demographic performance correctly predicted the distribution of the two annual grasses in the field. Our results indicate that reduced abundance of B. hordeaceus under canopy reflects the negative effects of competition with B. diandrus. In contrast, B. diandrus is little affected by competition from B. hordeaceus. The reduced abundance of B. diandrus in open grassland may result, in part, from its inability to adapt as well as B. hordeaceus to lower nutrient availability in soils of the open grassland.     

Comparative studies on desiccation resistance in leaves ofDactylis glomerata L. andFestuca arundinacea Schreb

Festuca arundinacea Schreb. andDactylis glomerata L., which differ in their ecological requirements toward moisture conditions also express different levels of desiccation tolerance.F. arundinacea was less resistant to severe water stress, showing lower ability to avoid and to tolerate desiccation (according to Levitt’s terminology, 1972). Growth stage of the leaves had no influence on the level of water deficits developed under the field conditions. It is concluded that adaptation ofF. arundinacea to growth in well watered environments might be associated with a decrease of desiccation resistance of its tissue.     

C4 photosynthetic modifications in the evolutionary transition from land to water in aquatic grasses

Cladistic analysis supports the conclusion that the Orcuttieae tribe of C₄ grasses reflect evolution from a terrestrial ancestry into seasonal pools. All nine species in the tribe exhibit adaptations to the aquatic environment, evident in the structural characteristics of the juvenile foliage, which persist submerged for 1–3 months prior to metamorphosis to the terrestrial foliage. Aquatic leaves of the least derived or basal genus Neostapfia have few morphological and anatomical characteristics specialized to the aquatic environment and have retained full expression of the C₄ pathway, including Kranz anatomy. Orcuttia species have many derived characteristics and are more specialized to the aquatic environment. These latter species germinate earlier in the season and persist in the submerged stage longer than Neostapfia and evidence from the literature indicates length of submergence is positively correlated with fitness components. Aquatic leaves of Orcuttia species lack Kranz or PCR bundle sheath anatomy, yet ¹⁴C-pulse chase studies indicate >95% malate + aspartate as the initial products of photosynthesis and these products turn over rapidly to phosphorylated sugars, indicating a tight coupling of the C₄ and C₃ cycles. Presence of the C₄ pathway is further supported by enzymological data. Contemporary dogma that Kranz anatomy is a sine qua non for operation of the C₄ pathway is contradicted by the patterns in Orcuttia; however, it is unknown whether the pathway acts as a CO₂ concentrating mechanism in these aquatic plants. Received: 12 June 1997 / Accepted: 10 February 1997     

Competition between Australian native and introduced grasses along a nutrient gradient

Abstract Seven grass species were grown in monocultures and in multispecies mixtures along a gradient of total nutrient levels that ranged from 1/64 to 16× the normal level of nutrient solution. The seven grasses represented three ecological groups: (i) three perennial species native to Australia (Themeda triandra, Poa labillardieri and Danthonia carphoides); (ii) two introduced annuals (Vulpia bromoides and Hordeum leporinum); and (iii) two introduced perennials (Lolium perenne and Dactylis glomerata). We hypothesized that the native grasses would prove less competitive when grown at increased nutrient levels than those introduced from Europe. Results supported the hypothesis. The native species were unable to compete in mixtures even at the lowest nutrient level, where T. triandra was the most productive species in monoculture. Lolium perenne and Dactylis glomerata dominated mixtures at intermediate nutrient levels. The responses of the annual introduced grasses differed in that Vulpia bromoides showed an optimum at intermediate nutrient levels in both monoculture and in mixtures, whereas Hordeum leporinum dominated at the highest nutrient levels in mixture but was suppressed by V. bromoides, L. perenne and D. glomerata at intermediate levels. The results are discussed in terms of predicting species responses in mixtures from their performance in monocultures as well as in terms of previous observations on the sequential changes in botanical composition of south‐eastern Australian grasslands after 150 years of continuous grazing by sheep.     

Reproductive effort and phenology of seed production of savanna grasses with different growth form and life history

The phenology of seed production in natural savanna grasslands was studied in the grass speciesAristida congesta, Cymbopogon plurinodis, Cynodon dactylon, Digitaria eriantha ssp.pentzii, Eragrostis rigidior, Eragrostis superba, Panicum coloratum, Schmidtia pappophoroides, Tragus berteronianus andUrochloa panicoides. Maximum seed production varied according to life history strategy and growth form from 0.03 mg seed g^-1 shoot dry weight in the perennialD. eriantha ssp.pentzii which produces long stolons and 14.8 mg seed g^-1 shoot inE rigidior, which produces short geniculate stolons, to 169.1 mg g^-1 in the annualT. berteronianus. Seed production was in most species divided over several peaks during the season. Peaks of seed production were observed 3 to 7 months after the onset of the growth season depending on the start of the rains and the life history strategy and growth form of the species. Seed production varied from maxima of 180 seeds m^-2 inD. eriantha ssp.pentzii to 47000 seeds m^-2 in annual stands ofT. berteronianus. Except for annual grasslands withU. panicoides, seedling emergence data reported are smaller by at least a factor of 10 than the observed seed production. Among other factors, a low quality of produced seeds, predation by birds and insects and previous grazing by livestock may have contributed to this difference.     

Litter accumulation alters the abiotic environment and drives community successional changes in two fenced grasslands in Inner Mongolia

Fencing is an effective and practical method for restoring degraded grasslands in northern China. However, little is known about the role of excess litter accumulation due to long‐term fencing in regulating abiotic environment and driving changes in community structure and function. We conducted a three‐year field experiment in two fenced grasslands in Inner Mongolia, and monitored light quantity, soil temperature, and soil moisture continuously, and determined community height, community aboveground net primary productivity (ANPP), and the relative dominance of different plant functional groups. Litter accumulation reduced light quantity and soil temperature but increased soil moisture. The regulating effects of litter accumulation on soil temperature and soil moisture fluctuated temporally and gradually weakened over the growing season. Litter accumulation also altered community vertical structure and function by increasing community height and ANPP. The increase in soil moisture increased the relative dominance of rhizome grasses but suppressed bunch grasses, thereby shifting bunch grass grasslands to rhizome grass grasslands. Our findings provide a potential mechanism for community succession in the context of litter accumulation in fenced grasslands and indicate that the vegetation and ecosystem services of degraded grasslands are improved after appropriate fencing.     

八种引种草坪植物的抗寒性比较研究

通过测定引种的八种草坪植物越冬存活率以及叶和越冬器官(根、根茎及分蘖节)质膜透性的变化、可溶性糖积累量、脯氨酸积累量等指标,得出:可溶性糖积累量是比较越冬器官抗寒性的有效生理生化指标,质膜透性的变化是比较叶器官抗寒性的有效生理生化指标;对八种引种草坪植物叶片进行过氧化物同工酶分析,得出过氧化物同工酶区带条数只能在一定程度上反映草坪植物抗寒性的大小,用其作为比较植物抗寒性的参照指标。     

Grazing and neighborhood interactions limit woody encroachment in wet subtropical savannas

Woody encroachment in savannas is a worldwide concern, and there is growing consensus that anthropogenic activities play a central role in changing tree – grass interactions. We evaluated the influence of livestock grazing and neighborhood interactions on seedling emergence and survival of the native tree Acacia caven in wet savannas of northeastern Argentina. We hypothesized that grazing and grass competition act as biotic barriers limiting tree recruitment, but the relative magnitude of such barriers differs according to grass patch type. In two consecutive years (cohort 1 and 2) we sowed seeds and transplanted seedlings of Acacia in two grass patch types (prostrate/palatable and tussock/unpalatable grasses) in both, grazed and ungrazed plots. Each grass patch type was further manipulated to create three levels of grass competition (unclipped control, above-ground biomass removal and total biomass removal).Cattle grazing diminished seedling emergence of both cohorts and seedling survival of cohort 1. The effect of grass competition changed according to grass patch type. Prostrate grass cover enhanced emergence but lowered early survival, while tussock grass cover and also its total biomass removal facilitated early survival. During the second year, a severe drought drastically reduced Acacia recruitment, and it was strong enough to eliminate any grazing effects although the effect of grass competition on seedling establishment remained significant.Our results suggest that grazing and grass competition additively diminished the risk of woody establishment in this wet savanna. However, the stocking rate should be carefully balanced, thus contributing to the maintenance of a competitive grass cover to limit tree recruitment.     

Analysis of adaptive ribosomal gene diversity in wild plant populations from contrasting climatic environments

Plant populations may contain variation that reflects adaptation to local environmental conditions. Clues to adaptive evolution of plants may be found in the genomes of species growing in diverse environments or across steep environmental gradients, and under stress. We have examined populations of wild relatives of barley and rice across diverse environmental gradients. Greater diversity, in a nuclear biotic stress defense gene and in chloroplast genes, was found in the more stressed, hotter and dryer environments. This may reflect the greater heterogeneity of these environments. Adaptation of plants to different abiotic stresses (temperatures and levels of water availability) may also require significant adaptation to the different biotic (pest and disease) pressures in these environments.¹   Plants growing across environmental gradients revealed greater diversity in a defense gene (Isa) in more stressed, hotter and dryer environments.² Chloroplast genome diversity also exhibited a similar variation with environment.³ We now report analysis of nuclear ribosomal genes from the same wild population. Two contrasting environments did not show significant differences in the level of diversity. However the pattern of SNP distribution within the rDNA did vary with greater SNP density in the RNA coding sequences compared with the internal transcribed spacers.