Patterns in grass silicification: response to grazing history and defoliation

Summary Morphologically distinct populations of a North American perennial grass, Agropyron smithii, collected from a heavily grazed prairie dog (Cynomys ludovicianus) colony (PDC) and a grazing exclosure (EX), were grown in an environmental chamber to determine whether: (1) leaf silicon (Si) concentrations are greater in plant populations which differentiated under heavy grazing pressure, and (2) leaf silicification is inducible by defoliation. Mean shoot Si concentration of nondefoliated plants was greater in the PDC population (2.2%) than the EX population (1.9%) over the 18 wk experiment, largely as a result of differences in Si concentrations in leaf blades. However, leaf Si concentration was lower in defoliated plants of each population than in nondefoliated plants, indicating that leaf silicification was not an inducible herbivore defense mechanism in A. smithii. The higher leaf Si concentrations from the heavily grazed population may be associated with grazingrelated environmental stresses such as a warmer, drier microclimate or with morphological characteristics related to grazing tolerance or avoidance.     

Intra-specific responses of a dominant C4 grass to altered precipitation patterns

The mechanisms by which global change alters the genotypic structure of populations by selection remain unclear. Key to this understanding is elucidating genotype–phenotype relationships under different environmental conditions as genotypes could differ in their plasticity or in their tolerance to changing environmental conditions. We have previously observed selection of certain genotypes of the dominant C₄ grass Andropogon gerardii L. within the on-going Rainfall Manipulation Plots (RaMPs) experiment at Konza Prairie Biological Station in Kansas. The RaMPs experiment has been experimentally imposing ambient and more variable (altered) precipitation patterns since 1998. Here, we studied phenotypic differences among six genotypes to gain insight into what drove the pattern of selection previously observed and assess potential genotype × environmental interactions. In 2008 and 2009 we sampled individuals of genotypes in the RaMPs and within unmanipulated reference plots located adjacent to the RaMPs experiment. For each individual, we measured both leaf-level (specific leaf area, stomatal conductance) and whole-plant growth (height, biomass) traits. We consistently detected differences among genotypes in the reference plots. Additionally, when focusing on two genotypes found in the altered and ambient RaMPs we observed no genotype × environment interactions. Overall, we found in an intact population of A. gerardii there exists phenotypic variability among genotypes, but no genotype × environment interactions. Thus our results demonstrate that differences in plasticity of genotypes do no explain the pattern of selection we observed.     

Plasticity and overcompensation in grass responses to herbivory

Several hypotheses predict defoliation-induced increases in individual plant fitness. In this paper we examine three such hypotheses: the Herbivore Optimization Hypothesis (HOH); the Continuum of Responses Hypothesis (CRH); and the Growth Rate Model (GRM). All three have in common predictions based on responses of defoliated individuals with the objective of explaining community and higher level phenomena. The latter two extend theory by specifying conditions for overcompensatory responses. They differ in whether overcompensation is sensitive to conditions external (CRH) or internal (GRM) to the plant. We tested these hypotheses with field experiments in a grassland system in which two native, perennial grass species replace each other along a short topographic/resource gradient. We detected positive, neutral, and negative changes in plant mass in response to partial defoliation. Patterns of responses to the edaphic and competitive environment combinations were unique to each species and neither the CRH nor the GRM were able to consistently predict responses in these grasses. Predictions of the HOH were fully supported only by the species naturally limited to lower-resource environments: overcompensation occurred in natural environments and it occurred at herbivory levels these plants experience naturally. Thus, the overcompensatory response can be important for the maintenance of local plant population distributions. However, new mechanistic theory is needed to account for the trend common to both species: overcompensatory responses to herbivory were greater in the edaphic environment in which each species was naturally most abundant.     

Temperature‐mediated responses of carbon fluxes to precipitation variabilities in an alpine meadow ecosystem on the Tibetan Plateau

Effects of climate warming and changing precipitation on ecosystem carbon fluxes have been intensively studied. However, how they co‐regulate carbon fluxes is still elusive for some understudied ecosystems. To fill the gap, we examined net ecosystem productivity (NEP), gross ecosystem productivity (GEP,) and ecosystem respiration (ER) responses to multilevel of temperature increments (control, warming 1, warming 2, warming 3, warming 4) in three contrasting hydrological growing seasons in a typical semiarid alpine meadow. We found that carbon fluxes responded to precipitation variations more strongly in low‐level warming treatments than in high‐level ones. The distinct responses were attributable to different soil water conditions and community composition under low‐level and high‐level warming during the three growing seasons. In addition, carbon fluxes were much more sensitive to decreased than to increased precipitation in low‐level warming treatments, but not in high‐level ones. At a regional scale, this negative asymmetry was further corroborated. This study reveals that future precipitation changes, particularly decreased precipitation would induce significant change in carbon fluxes, and the effect magnitude is regulated by climate warming size.     

Root branching responses to phosphate and nitrate

Plant roots favour colonization of nutrient-rich zones in soil. Molecular genetic evidences demonstrate that roots sense and respond to local and global concentrations of inorganic phosphate and nitrate, in a fashion that depends on the shoot nutrient status. Recent investigations in Arabidopsis highlighted the role of the root tip in phosphate sensing and attributed to already known proteins (multicopper oxidases and nitrate transporters) new and unexpected functions in the root growth response to phosphate or nitrate.     

Differential and reversible responses of common fen meadow species to abandonment

Abstract. We studied the effects of abandonment on two common fen plant species. In mown and a chronosequence of abandoned fen meadows spanning 35 yr, we measured fitness traits of the sedge Carex davalliana and the forb Succisa pratensis. Cessation of mowing had little effect on fitness traits and seed production of C. davalliana, but seedling density decreased more than threefold. Population density of S. pratensis decreased with increasing community biomass, but was not affected by the cessation of mowing. However, flowering frequency increased threefold and seed production was 20% higher in fallow meadows. Consequently, seedling density of S. pratensis increased nearly threefold after abandonment. However, these changes were not dependent on the age of the fallow. In a common garden and germination experiment, we found no differences in either species between plants from fallows and mown fen meadows, except for the height of the flowering stalk of S. pratensis. The combined results from the common garden experiment and the field studies indicate that changes in fitness traits observed in fallows were mostly phenotypic and likely to be reversible. If other species react in similar ways, there is a high potential for re‐establishing traditional fen meadow communities from fallows by mowing.     

Plant responses to precipitation in desert ecosystems: integrating functional types,pulses, thresholds,and delays

The two-layer and pulse-reserve hypotheses were developed 30 years ago and continue to serve as the standard for many experiments and modeling studies that examine relationships between primary productivity and rainfall variability in aridlands. The two-layer hypothesis considers two important plant functional types (FTs) and predicts that woody and herbaceous plants are able to co-exist in savannas because they utilize water from different soil layers (or depths). The pulse-reserve model addresses the response of individual plants to precipitation and predicts that there are biologically important rain events that stimulate plant growth and reproduction. These pulses of precipitation may play a key role in long-term plant function and survival (as compared to seasonal or annual rainfall totals as per the two-layer model). In this paper, we re-evaluate these paradigms in terms of their generality, strengths, and limitations. We suggest that while seasonality and resource partitioning (key to the two-layer model) and biologically important precipitation events (key to the pulse-reserve model) are critical to understanding plant responses to precipitation in aridlands, both paradigms have significant limitations. Neither account for plasticity in rooting habits of woody plants, potential delayed responses of plants to rainfall, explicit precipitation thresholds, or vagaries in plant phenology. To address these limitations, we integrate the ideas of precipitation thresholds and plant delays, resource partitioning, and plant FT strategies into a simple threshold-delay model. The model contains six basic parameters that capture the nonlinear nature of plant responses to pulse precipitation. We review the literature within the context of our threshold-delay model to: (i) develop testable hypotheses about how different plant FTs respond to pulses; (ii) identify weaknesses in the current state-of-knowledge; and (iii) suggest future research directions that will provide insight into how the timing, frequency, and magnitude of rainfall in deserts affect plants, plant communities, and ecosystems.     

Bryophyte vegetation in a wooded meadow: relationships with phanerogam diversity and responses to fertilisation

In the Laelatu wooded meadow in Estonia, famous for its phanerogam diversity, the bryophyte community has been investigated in order to compare its flora and diversity relationships with those of the vascular plant community. Ninety-six bryophyte species were found, 13 of them are hepatics; the majority of the bryophytes are epigeic species common to meadows and forests, including many calciphilous species. Vascular plants and bryophytes display opposite responses to fertilisation. For vascular plants, fertilisation increases the coverage and diminishes the number of species, while for bryophytes it diminishes coverage and increases the number of species. The relationship between the number of species in small plots and the total number of species in the area is similar for vascular plants and bryophytes. No significant changes in the bryophyte community in Laelatu wooded meadow has been detected during the last 30 years.     

Root dynamics in barley,lucerne and meadow fescue investigated with a mini-rhizotron technique

Root development, including depth distribution, was followed in pure barley stands (Hordeum distichum, L.) with or without nitrogen fertilization and in barley undersown with lucerne (Medicago sativa L.) or meadow fescue (Festuca pratensis, Huds.). The number of roots per 5 cm depth level down to 1 m was counted frequently during the growing season using mini-rhizotrons, i.e., transparent tubes inserted into the soil. Root biomass at different depths down to 1 m was estimated from soil cores taken one month before harvest. The results from the two methods were compared and root counts in the different treatments were compared with the above-ground growth and production. Nitrogen-fertilized barley in pure stand had the highest biomass both above and below ground. According to the mini-rhizotron observations this treatment also had a deeper and denser root system, until barley harvest, than the other treatments. After barley harvest, roots from the undersown lucerne continued to increase, whereas the number of roots in the undersown meadow fescue remained the same. The root system in barley/meadow fescue did not penetrate into the subsoil, where more than 60% of the number of roots in barley undersown with lucerne were found. In general, the mini-rhizotron results indicated a higher relative abundance of roots in the deeper layers than the root biomass estimated with the soil coring method.     

Nonlinear responses of productivity and diversity of alpine meadow communities to degradation

为了阐释青藏高原高寒草甸退化的关键生态过程, 该研究依托藏北高原草地生态系统研究站(那曲站), 设置不同退化梯度实验, 即对照、轻度退化、中度退化、重度退化和极度退化5个梯度, 探究群落生产力和物种多样性对不同退化强度的响应机制。结果表明: 1)随着退化程度不断加剧, 地上生物量呈现线性或非线性增加趋势, 在重度退化处理下, 地上生物量显著高于对照32.3%, 其中高山嵩草(Kobresia pygmaea)地上生物量呈非线性下降趋势, 而矮火绒草(Leontopodium nanum)地上生物量呈非线性增加趋势; 2)与地上生物量的响应模式相反, 随着退化程度加剧, 地下生物量与总生物量均呈现非线性降低趋势; 3)高寒草甸退化过程中, 物种辛普森指数、丰富度指数、香农多样性指数和均匀度指数均呈现非线性上升趋势。结构等式方程结果表明, 土壤碳含量和体积含水量与地下生物量均呈现显著的正相关关系。土壤碳含量、体积含水量和砾石质量比对地上生物量无显著影响, 土壤碳、氮含量与物种多样性指数呈现显著的负相关关系。研究认为地上生产力的变化不能很好地指示草地的退化程度, 建议今后研究应以可食性牧草和毒杂草等植物功能群的变化来衡量草地退化。     

高寒草甸退化过程中群落生产力和物种多样性的非线性响应机制研究

为了阐释青藏高原高寒草甸退化的关键生态过程, 该研究依托藏北高原草地生态系统研究站(那曲站), 设置不同退化梯度实验, 即对照、轻度退化、中度退化、重度退化和极度退化5个梯度, 探究群落生产力和物种多样性对不同退化强度的响应机制。结果表明: 1)随着退化程度不断加剧, 地上生物量呈现线性或非线性增加趋势, 在重度退化处理下, 地上生物量显著高于对照32.3%, 其中高山嵩草(Kobresia pygmaea)地上生物量呈非线性下降趋势, 而矮火绒草(Leontopodium nanum)地上生物量呈非线性增加趋势; 2)与地上生物量的响应模式相反, 随着退化程度加剧, 地下生物量与总生物量均呈现非线性降低趋势; 3)高寒草甸退化过程中, 物种辛普森指数、丰富度指数、香农多样性指数和均匀度指数均呈现非线性上升趋势。结构等式方程结果表明, 土壤碳含量和体积含水量与地下生物量均呈现显著的正相关关系。土壤碳含量、体积含水量和砾石质量比对地上生物量无显著影响, 土壤碳、氮含量与物种多样性指数呈现显著的负相关关系。研究认为地上生产力的变化不能很好地指示草地的退化程度, 建议今后研究应以可食性牧草和毒杂草等植物功能群的变化来衡量草地退化。     

草甸棕壤水稻田磷酸酶活性及对施肥措施的响应

1　引　　言土壤有机磷是一种重要的土壤磷素资源 .我国大部分土壤中有机磷占土壤全磷的 2 0 %～ 50 % ,但在森林和草原植被下的土壤可占到 50 %～ 80 % [9].土壤磷酸酶活性直接影响到有机磷库的利用 ,即磷酸酶活性是衡量土壤肥力 ,尤其是土壤有效磷水平的一个重要参考指标[15 ].土壤磷酸酶(Phosphatases)是催化含磷有机酯和酐水解的一类酶的总称 ,其活性高低直接影响着土壤中有机磷的分解转化及其生物有效性 .其中 ,磷酸单酯酶 (酸性、中性、碱性磷酸酶 )活性一直是土壤磷酸酶研究的重点[18].由于土壤中有机磷化合物的复杂性 ,除了磷酸单…     

Leaf gas exchange and water status responses of a native and non-native grass to precipitation across contrasting soil surfaces in the Sonoran Desert

Arid and semi-arid ecosystems of the southwestern US are undergoing changes in vegetation composition and are predicted to experience shifts in climate. To understand implications of these current and predicted changes, we conducted a precipitation manipulation experiment on the Santa Rita Experimental Range in southeastern Arizona. The objectives of our study were to determine how soil surface and seasonal timing of rainfall events mediate the dynamics of leaf-level photosynthesis and plant water status of a native and non-native grass species in response to precipitation pulse events. We followed a simulated precipitation event (pulse) that occurred prior to the onset of the North American monsoon (in June) and at the peak of the monsoon (in August) for 2002 and 2003. We measured responses of pre-dawn water potential, photosynthetic rate, and stomatal conductance of native (Heteropogon contortus) and non-native (Eragrostis lehmanniana) C₄ bunchgrasses on sandy and clay-rich soil surfaces. Soil surface did not always amplify differences in plant response to a pulse event. A June pulse event lead to an increase in plant water status and photosynthesis. Whereas the August pulse did not lead to an increase in plant water status and photosynthesis, due to favorable soil moisture conditions facilitating high plant performance during this period. E. lehmanniana did not demonstrate heightened photosynthetic performance over the native species in response to pulses across both soil surfaces. Overall accumulated leaf-level CO₂ response to a pulse event was dependent on antecedent soil moisture during the August pulse event, but not during the June pulse event. This work highlights the need to understand how desert species respond to pulse events across contrasting soil surfaces in water-limited systems that are predicted to experience changes in climate.     

Selenium deficiency alters epithelial cell morphology and responses to influenza

It is unknown whether nutritional deficiencies affect the morphology and function of structural cells, such as epithelial cells, and modify the susceptibility to viral infections. We developed an in vitro system of differentiated human bronchial epithelial cells (BEC) grown either under selenium-adequate (Se+) or selenium-deficient (Se–) conditions, to determine whether selenium deficiency impairs host defense responses at the level of the epithelium. Se– BECs had normal SOD activity, but decreased activity of the selenium-dependent enzyme GPX1. Interestingly, catalase activity was also decreased in Se– BECs. Both Se– and Se+ BECs differentiated into a mucociliary epithelium; however, Se– BEC demonstrated increased mucus production and increased Muc5AC mRNA levels. This effect was also seen in Se+ BEC treated with 3-aminotriazole, an inhibitor of catalase activity, suggesting an association between catalase activity and mucus production. Both Se– and Se+ were infected with influenza A/Bangkok/1/79 and examined 24 h postinfection. Influenza-induced IL-6 production was greater while influenza-induced IP-10 production was lower in Se– BECs. In addition, influenza-induced apoptosis was greater in Se– BEC as compared to the Se+ BECs. These data demonstrate that selenium deficiency has a significant impact on the morphology and influenza-induced host defense responses in human airway epithelial cells.     

Asymmetric responses of primary productivity to precipitation extremes: A synthesis of grassland precipitation manipulation experiments

Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta‐analytical techniques to search for generalities and asymmetries of aboveground NPP (ANPP) and belowground NPP (BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.     

Early silicification of leaves and roots of seedlings of a panicoid grass grown under different conditions: anatomical relations and structural role

• Grasses accumulate high amounts of silica deposits in tissues of all their organs, especially at mature stage. However, when and under which conditions do grass seedlings begin to produce these silica deposits and their relation with anatomy and development is little known. Here we investigated the silicification process in the first leaves and roots of seedlings of Bothriochloa laguroides grown in different substrate and Si treatments.
• The distribution and content of silica deposits in the organs of the seedlings grown under different conditions were analyzed through staining techniques and SEM‐EDAX analyses.
• Leaf silica deposits were accumulated 3–4 days after the first leaf emergence, also under low silica solution (0.17–0.2 mM). Their location was mainly restricted to short costal cells from basal sectors, and scarcely in trichomes and xylem at tips. Silica content in leaves increased with the age of the seedlings. Roots presented dome‐shaped silica aggregates, between 4–12 μm of diameter, located in the inner tangential wall of endodermal cells and similar to those produced at maturity.
• Silicification begins early in the first photosynthetic leaf, and silica distribution is opposite to that found in mature plants, mainly restricted to basal sectors, probably acting as a reinforcing element. The fast incorporation of solid amorphous silica in leaves and roots, may be useful for farm applications in species that are Si‐fertilized.

     

Root responses to nutrient patches in grassland and forest

Differences between growth forms in root responses to experimentally created heterogeneity have been documented in many greenhouse and plot studies, but not in natural vegetation. Here we examined the response of roots to experimental nutrient patches in undisturbed grassland and forest at the northern edge of the North American Great Plains. Forest vegetation increases the spatial heterogeneity of soil resources, and we tested for differences between forest and grassland roots in response to patches. Ten minirhizotrons (clear tubes, 5 cm diameter, 180 cm long) were installed in both grassland and forest 3 years before the experiment. Minirhizotrons ran horizontally 10 cm beneath the soil surface. Patches of available nitrogen (N) were created over the tubes, using three concentrations (0, 3, 15 g N m⁻² yr⁻¹) and two patch sizes (1␣and 10 cm²). Root images were collected beneath patches over the course of a growing season. Root length was significantly greater in grassland than forest at the start and end of the growing season, but did not respond to N patches. Root production was also significantly greater in grassland than forest, and was significantly greater (about 20%) in high-N patches than in unfertilized patches. This increase, however, did not differ between vegetation types. Turnover did not vary with any treatment, and patch size had no effect on any response variable. Overall, differences caused by experimental patches were much smaller than differences between habitats, and did not vary between habitats. Realistic levels of experimentally imposed hetereogeneity in established vegetation may not be much greater than background levels, and field vegetation has extant root systems which respond to patches via uptake instead of growth. Both mechanisms should contribute to less root proliferation in field experiments than in greenhouse experiments.