Composition of soil seed banks in southern California coastal sage scrub and adjacent exotic grassland

Soil seed banks are important to many plant communities and are recognized as an important component of management plans. Understanding seed bank composition and density is especially important when communities have been invaded by exotic species and must be managed to promote desirable species. We examined germinable soil seed banks in southern California coastal sage scrub (CSS) that is heavily invaded by exotic grasses and in adjacent exotic grassland. Soils from both communities had similar seed banks, dominated by high densities of exotic grass and forb species. Up to 4,000 exotic grass seeds and at least 400 exotic forb seeds/m² were found in most soils, regardless of aboveground vegetation type. Native forbs averaged 400 seeds/m² in grass-dominated areas and about 800 in shrub-dominated soils. Shrub seed density was <1 and <10 seeds/m² in grass- and shrub-dominated areas, respectively, indicating that the shrub seed bank is not persistent compared to annuals. We also compared pre- and post-burn soil seed banks from one location that burned in October 2003. Late-season burning in both grass- and CSS-dominated areas disproportionately reduced exotic grass seed densities relative to native seed densities. The similarity of the seed banks in adjacent grass and shrub communities suggests that without intervention, areas currently dominated by CSS may become more similar to grass-dominated areas in terms of aboveground vegetation. In such areas, the first growing season following a wildfire is a window of opportunity for increasing native diversity at a time when density of exotic grass seeds is low. At time of research, Robert D. Cox was graduate student.     

Ecosystem responses to changes in plant functional type composition: An example from the Patagonian steppe

Abstract. Grass cover along a grazing intensity gradient in Patagonia decreases, whereas bare soil and shrub cover increases. Our objective was to study the effect of a change in the dominant plant functional type on soil water balance, primary production, herbivore biomass, roughness, and albedo. Using a soil water balance model, we found increases in evaporation and deep drainage, and a decrease in total transpiration along the grazing intensity gradient. Above-ground primary production, estimated from transpiration, decreased along the grazing intensity gradient because shrubs did not fully compensate for the decrease in grass production. Using a statistical model, we calculated herbivore biomass from estimates of above-ground primary production. Estimated herbivore biomass was lowest in the shrub-dominated extreme of the grazing gradient. Roughness increased from the grass-dominated to the shrub-dominated community. Albedo had a maximum at an intermediate position along the gradient. Our results suggest that changes in plant functional type composition, independent of changes in biomass, affect ecosystem functioning and the exchange of energy and material with the atmosphere. Grasses and shrubs proved to be appropriate plant functional types to link structure and function of ecosystems.     

Net carbon exchange and evapotranspiration in postfire and intact sagebrush communities in the Great Basin

Invasion of non-native annuals across the Intermountain West is causing a widespread transition from perennial sagebrush communities to fire-prone annual herbaceous communities and grasslands. To determine how this invasion affects ecosystem function, carbon and water fluxes were quantified in three, paired sagebrush and adjacent postfire communities in the northern Great Basin using a 1-m³ gas exchange chamber. Most of the plant cover in the postfire communities was invasive species including Bromus tectorum L., Agropyron cristatum (L.) Gaertn and Sisymbrium altissimum L. Instantaneous morning net carbon exchange (NCE) and evapotranspiration (ET) in native shrub plots were greater than either intershrub or postfire plots. Native sagebrush communities were net carbon sinks (mean NCE 0.2–4.3 μmol m⁻² s⁻¹) throughout the growing season. The magnitude and seasonal variation of NCE in the postfire communities were controlled by the dominant species and availability of soil moisture. Net C exchange in postfire communities dominated by perennial bunchgrasses was similar to sagebrush. However, communities dominated by annuals (cheatgrass and mustard) had significantly lower NCE than sagebrush and became net sources of carbon to the atmosphere (NCE declined to −0.5 μmol m⁻² s⁻¹) with increased severity of the summer drought. Differences in the patterns of ET led to lower surface soil moisture content and increased soil temperatures during summer in the cheatgrass-dominated community compared to the adjacent sagebrush community. Intensive measurements at one site revealed that temporal and spatial patterns of NCE and ET were correlated most closely with changes in leaf area in each community. By altering the patterns of carbon and water exchange, conversion of native sagebrush to postfire invasive communities may disrupt surface-atmosphere exchange and degrade the carbon storage capacity of these systems.     

Modifying the ‘pulse–reserve’ paradigm for deserts of North America: precipitation pulses,soil water,and plant responses

The ‘pulse–reserve’ conceptual model—arguably one of the most-cited paradigms in aridland ecology—depicts a simple, direct relationship between rainfall, which triggers pulses of plant growth, and reserves of carbon and energy. While the heuristics of ‘pulses’, ‘triggers’ and ‘reserves’ are intuitive and thus appealing, the value of the paradigm is limited, both as a conceptual model of how pulsed water inputs are translated into primary production and as a framework for developing quantitative models. To overcome these limitations, we propose a revision of the pulse–reserve model that emphasizes the following: (1) what explicitly constitutes a biologically significant ‘rainfall pulse’, (2) how do rainfall pulses translate into usable ‘soil moisture pulses’, and (3) how are soil moisture pulses differentially utilized by various plant functional types (FTs) in terms of growth? We explore these questions using the patch arid lands simulation (PALS) model for sites in the Mojave, Sonoran, and Chihuahuan deserts of North America. Our analyses indicate that rainfall variability is best understood in terms of sequences of rainfall events that produce biologically-significant ‘pulses’ of soil moisture recharge, as opposed to individual rain events. In the desert regions investigated, biologically significant pulses of soil moisture occur in either winter (October–March) or summer (July–September), as determined by the period of activity of the plant FTs. Nevertheless, it is difficult to make generalizations regarding specific growth responses to moisture pulses, because of the strong effects of and interactions between precipitation, antecedent soil moisture, and plant FT responses, all of which vary among deserts and seasons. Our results further suggest that, in most soil types and in most seasons, there is little separation of soil water with depth. Thus, coexistence of plant FTs in a single patch as examined in this PALS study is likely to be fostered by factors that promote: (1) separation of water use over time (seasonal differences in growth), (2) relative differences in the utilization of water in the upper soil layers, or (3) separation in the responses of plant FTs as a function of preceding conditions, i.e., the physiological and morphological readiness of the plant for water-uptake and growth. Finally, the high seasonal and annual variability in soil water recharge and plant growth, which result from the complex interactions that occur as a result of rainfall variability, antecedent soil moisture conditions, nutrient availability, and plant FT composition and cover, call into question the use of simplified vegetation models in forecasting potential impacts of climate change in the arid zones in North America.     

Interannual climate variability mediates changes in carbon and nitrogen pools caused by annual grass invasion in a semiarid shrubland

Exotic plant invasions alter ecosystem properties and threaten ecosystem functions globally. Interannual climate variability (ICV) influences both plant community composition (PCC) and soil properties, and interactions between ICV and PCC may influence nitrogen (N) and carbon (C) pools. We asked how ICV and non-native annual grass invasion covary to influence soil and plant N and C in a semiarid shrubland undergoing widespread ecosystem transformation due to invasions and altered fire regimes. We sampled four progressive stages of annual grass invasion at 20 sites across a large (25,000 km²) landscape for plant community composition, plant tissue N and C, and soil total N and C in 2013 and 2016, which followed 2 years of dry and wet conditions, respectively. Multivariate analyses and ANOVAs showed that in invasion stages where native shrub and perennial grass and forb communities were replaced by annual grass-dominated communities, the ecosystem lost more soil N and C in wet years. Path analysis showed that high water availability led to higher herbaceous cover in all invasion stages. In stages with native shrubs and perennial grasses, higher perennial grass cover was associated with increased soil C and N, while in annual-dominated stages, higher annual grass cover was associated with losses of soil C and N. Also, soil total C and C:N ratios were more homogeneous in annual-dominated invasion stages as indicated by within-site standard deviations. Loss of native shrubs and perennial grasses and forbs coupled with annual grass invasion may lead to long-term declines in soil N and C and hamper restoration efforts. Restoration strategies that use innovative techniques and novel species to address increasing temperatures and ICV and emphasize maintaining plant community structure—shrubs, grasses, and forbs—will allow sagebrush ecosystems to maintain C sequestration, soil fertility, and soil heterogeneity.     

Effects of experimental rainfall manipulations on Chihuahuan Desert grassland and shrubland plant communities

Aridland ecosystems are predicted to be responsive to both increases and decreases in precipitation. In addition, chronic droughts may contribute to encroachment of native C₃ shrubs into C₄-dominated grasslands. We conducted a long-term rainfall manipulation experiment in native grassland, shrubland and the grass–shrub ecotone in the northern Chihuahuan Desert, USA. We evaluated the effects of 5 years of experimental drought and 4 years of water addition on plant community structure and dynamics. We assessed the effects of altered rainfall regimes on the abundance of dominant species as well as on species richness and subdominant grasses, forbs and shrubs. Nonmetric multidimensional scaling and MANOVA were used to quantify changes in species composition in response to chronic addition or reduction of rainfall. We found that drought consistently and strongly decreased cover of Bouteloua eriopoda, the dominant C₄ grass in this system, whereas water addition slightly increased cover, with little variation between years. In contrast, neither chronic drought nor increased rainfall had consistent effects on the cover of Larrea tridentata, the dominant C₃ shrub. Species richness declined in shrub-dominated vegetation in response to drought whereas richness increased or was unaffected by water addition or drought in mixed- and grass-dominated vegetation. Cover of subdominant shrubs, grasses and forbs changed significantly over time, primarily in response to interannual rainfall variability more so than to our experimental rainfall treatments. Nevertheless, drought and water addition shifted the species composition of plant communities in all three vegetation types. Overall, we found that B. eriopoda responded strongly to drought and less so to irrigation, whereas L. tridentata showed limited response to either treatment. The strong decline in grass cover and the resistance of shrub cover to rainfall reduction suggest that chronic drought may be a key factor promoting shrub dominance during encroachment into desert grassland.     

Seasonal variation and annual fluctuation in granite outcrop plant communities

Permanent quadrats in granite outcrop plant communities allowed us to monitor seasonal variation and annual fluctuation in community structure. Seasonal species turn-over was significant in communities on shallow soil, but not in communities on deeper soil where seasonal dominance shifts were common. Exceptional meteorological events appeared to mediate phenomena of competitive release in some island communities. A decrease in the abundance of Arenaria uniflora in Lichen-annual island communities, following a spring drought, was correlated with an increase in the abundance of Sedum smallii, a shallower-soil species. Richness in Annual-perennial island communities was higher in spring 1985 than in 1984 or 1986, and this occurred as the dominant species, Senecio tomentosus, temporarily declined in importance following a severe drought in late summer 1984. Significant annual fluctuation in the cover of Viguiera porteri could also be related to variations in the summer precipitation regime. Overall, plant responses to drought were individualistic and depended largely on the timing of these meteorological events in relation to the life-stages and/or the physiological status of the plants.Abbreviations AP = Annual-perennial island communities - HST = Herb-shrub-tree island communities - LA = Lichen-annual island communities - SS = Sedum smallii island communities     

Succession patterns following soil disturbance in a sagebrush steppe community

Summary A study was begun in 1976 to measure succession patterns following soil disturbance within a sagebrush community in northwestern Colorado. The principal hypothesis was that type of disturbance affects the direction of succession, resulting in different plant communities over time. Successional dynamics were studied through 1988. Four types of soil disturbance resulted in 3 early seral communities: one dominated by grasses, one by annuals, and one intermediate. The annual-dominated communities were opportunistic on these sites, lasting 3–5 years and not determining the direction in which succession proceeded following their replacement. Twelve years after disturbance, 3 communities (one grass-dominated, one shrub-dominated, and one intermediate) occupied the site, the characteristics of which were functions of type of initial soil disturbance. For the period of time covered by this study (12 years), degree of disturbance was found to affect the direction of succession, resulting in different plant communities over time. There were, however, successional characteristics toward the end of the study that suggest that over a longer time period, succession might progress to a single community regardless of type of disturbance.     

A new ecological-wind erosion model to simulate the impacts of aeolian transport on dryland vegetation patterns

Drylands cover more than 40% of the land surface of the Earth and are characterized by patchy vegetation and that permits erosion of the surface. Vegetation-aeolian transport is an important feedback in drylands, particularly those undergoing shrub encroachment. Although one side of the feedback, the influence of vegetation loss on aeolian transport, has been well studied, the other, the influence of aeolian transport on existing vegetation, has been never studied in detail. In this study, a new ecological-wind erosion model (ECO-WEMO) that contains an aeolian transport component was created to simulate how aeolian transport impacts vegetation pattern and causes the state change. Two modeling scenarios were investigated: 1) stable grass and shrub communities without/with aeolian transport and 2) unstable shrub and grass communities without/with aeolian transport disturbed by different drought conditions. The first scenario focuses on the simulation of the influence of aeolian transport on vegetation communities and the second scenario focuses on the simulation of the state change of vegetation communities. The results from the first scenario show that: First, the mean biomasses of grass and shrub become consistent in the case of no wind in both shrub-dominated and grass-dominated communities. Second, the mean biomass of shrub becomes higher than the grass in the case of wind in shrub-dominated communities and the mean biomass of grass becomes higher than the shrub in the case of wind in grass-dominated communities. Third, the dust flux of shrub-dominated communities is higher than the grass-dominated communities. Fourth, the net change in surface height in shrub-dominated communities has a considerably higher range than in grass-dominated communities. Fifth, the spatial pattern of shrub-dominated communities is sparser than the spatial pattern of grass in the vegetation communities in the case of wind. The results from the second scenario show that: First, the state change only took place from grass-dominated communities to shrub-dominated communities in the condition of drought. Second, the state change only took place in the case of wind. Third, the state change didn't take place after the slight and moderate droughts but only took place after the drought. Fourth, large vegetation biomass reduction only took place in the case of wind after the severe drought. Our results confirm, in a modeling context, the important role that aeolian transport can play in vegetation dynamics and state change in deserts.     

Effects of variable precipitation on the structure and diversity of a California salt marsh community

Abstract. We measured the effects of annual variation in climate and experimentally augmented rainfall on patterns of distribution and above-ground productivity in annual plant communities at Carpinteria Salt Marsh in central California. In the driest year, Hutchinsia procumbens was codominant throughout much of the upper marsh; however, Hutchinsia was very rare or not present in the wetter years. Conversely, Juncus bufonius was common in the wettest year and absent in the driest year. Elevational distributions of other annual species also differed among years with different total precipitation. In 1989–1990, an exceptionally dry season, supplemental water decreased soil salinity, increased above-ground productivity of annuals, and caused significant changes in spatial patterns and relative density. In the lowest zone, Hutchinsia occurred only in watered plots and supplemental water increased the density of Spergularia marina. At intermediate elevations Lasthenia glabrata occurred only in watered plots and supplemental water increased the density of Spergularia, Hutchinsia, and Parapholis incurva. At upper elevations, Juncus occurred only in watered plots, and Lasthenia was the only species that increased significantly in density with watering. Unlike natural shifts in species abundance, no species declined significantly in cover in any zone in the watered treatment. Although climatic variation has complex affects on annual plant communities, our experiments isolated important affects of total annual rainfall on the structure of annual plant communities that were similar to those that occurred with natural variation in rainfall. We conclude that variation in total annual precipitation promotes dynamic community composition and spatial distributions among years, and thus increases overall species diversity in the salt marsh.     

Exotic plant communities shift water-use timing in a shrub-steppe ecosystem

Semiarid areas in the US have realized extensive and persistent exotic plant invasions. Exotics may succeed in arid regions by extracting soil water at different times or from different depths than native plants, but little data is available to test this hypothesis. Using estimates of root mass, gravimetric soil water, soil-water potential, and stable isotope ratios in soil and plant tissues, we determined water-use patterns of exotic and native plant species in exotic- and native-dominated communities in Washington State, USA. Exotic and native communities both extracted 12 ± 2 cm of water from the top 120 cm of soil during the growing season. Exotic communities, however, shifted the timing of water use by extracting surface (0–15 cm) soil water early in the growing season (i.e., April to May) before native plants were active, and by extracting deep (0–120 cm) soil water late in the growing season (i.e., June to July) after natives had undergone seasonal senescence. We found that δ ¹⁸O values of water in exotic annuals (e.g., −11.8 ± 0.4 ‰ for Bromus tectorum L.) were similar to δ ¹⁸O values of surface soil water (e.g., −13.3 ± 1.4 ‰ at −15 cm) suggesting that transpiration by these species explained early season, surface water use in exotic communities. We also found that δ ¹⁸O values of water in taprooted exotics (e.g., −17.4 ± 0.3 ‰ for Centaurea diffusa Lam.) were similar to δ ¹⁸O values of deep soil water (e.g., −18.4 ± 0.1 ‰ at −120 cm) suggesting that transpiration by these species explained late season, deep water use. The combination of early-season, shallow water-use by exotic winter-actives and late-season, deep water-use by taprooted perennials potentially explains how exotic communities resist establishment of native species that largely extracted soil water only in the middle of the growing season (i.e., May to June). Early season irrigation or the planting of natives with established root systems may allow native plant restoration.     

The Sensitivity of Evapotranspiration to Inter-Specific Plant Neighbor Interactions: Implications for Models

Evapotranspiration (ET) is an important water loss flux in ecosystem water cycles, and quantifying the spatial and temporal variation of ET can improve ecohydrological models in arid ecosystems. Plant neighbor interactions may be a source of spatial and temporal variation in ET due to their effects on the above- and belowground microclimate and increased water demand for transpiration. Over longer timescales (annual to multiple years), adjustments in plant physiological traits may occur in response to neighbor environments, potentially affecting the transpiration (T) component of ET. We used a dynamic soil water model to assess the sensitivity of ET and T estimates to neighbor effects on soil moisture via competition for water, aboveground microclimate effects via canopy shading, and physiological adjustments (specifically, root distribution, stomatal behavior, and canopy leaf area). We focus on a common desert shrub (Larrea tridentata) under different inter-specific neighbor environments and precipitation regimes. Neighbors impacted T of Larrea by as much as 75% at the patch scale (plant and surrounding soil) and 30% at the stand scale. Annual T estimates were highly sensitive to changes in soil moisture associated with competition for water, and the inclusion of physiological adjustments to neighbor environments significantly impacted seasonal T. Plant neighbor interactions can significantly influence ET and soil moisture, and their inclusion in models can help explain spatial and temporal variation in water fluxes in arid ecosystems. Furthermore, physiological adjustments to neighbor environments may be an important source of variation to include in models that operate over seasonal timescales or in studies focused on plant responses to precipitation under climate change.     

贝加尔针茅草原土壤真菌群落结构对氮素和水分添加的响应

研究氮沉降和降雨变化对土壤真菌群落结构的互作效应,对未来预测多个气候变化因子对草地生态系统的交互作用具有重要意义。以施氮和灌溉模拟氮沉降和降雨增加,采用裂区设计,应用高通量测序技术,研究8个氮添加水平(0、15、30、50、100、150、200、300 kg N hm~(-2)a~(-1))和2个水分添加水平(不灌溉、模拟夏季增雨100 mm灌溉)对土壤真菌群落结构的影响。结果表明,氮素和水分添加后,土壤真菌群落中占优势的门类分别为接合菌门Zygomycota(22.0%—48.9%)、担子菌门Basidiomycota(7.8%—18.5%)、子囊菌门Ascomycota(9.4%—20.1%)、球囊菌门Glomeromycota(0.7%—3.1%)、壶菌门Chytridiomycota(0.1%—1.3%)。常规降雨条件下,随着氮添加水平升高,接合菌门相对丰度呈现出先升高后降低的趋势,N50处理最高;子囊菌门相对丰度在高氮添加时(N100—N300)呈升高趋势。而在氮素和水分同时添加条件下,随着氮添加水平升高,接合菌门相对丰度呈降低趋势,子囊菌门相对丰度变化则不明显。在相同的氮添加水平下,水分添加使接合菌门相对丰度增加,而担子菌门、子囊菌门、球囊菌门和壶菌门的相对丰度降低。在不同氮素和水分添加条件下,有5个土壤真菌门类11个真菌纲相对丰度变化显著。接合菌门的Mortierella属,担子菌门的Entolomataceae科和Geastrum属相对丰度变化极显著,可作为土壤真菌群落结构变化的指示种。PCo A分析结果也表明氮素和水分添加改变了土壤真菌群落结构。植物-土壤-微生物系统的结构方程模型结果表明,植物群落组成及植物物种丰富度的变化是土壤真菌群落结构发生变化的主要影响因素,土壤无机氮及p H的变化主要通过影响植物群落间接影响真菌群落,其对真菌群落的直接影响则较小。综上,氮素和水分添加改变了土壤真菌群落结构,且两者存在明显的互作效应,水分添加可改变氮添加对土壤真菌群落的影响。     

The effect of earthworms and snails in a simple plant community

Snails and earthworms affected the dynamics of a simple, three-species plant community, in the Ecotron controlled environment facility. Earthworms enhanced the establishment, growth and cover of the legume Trifolium dubium, both via the soil and interactions with other plant species. Worms increased soil phosphates, increased root nodulation in T. dubium, and enabled T. dubium seedlings to establish in the presence of grass (Poa annua) litter, by increasing soil heterogeneity. Worms also buried the seeds of Poa annua and Senecio vulgaris, reducing the germination of new seedlings. Snails reduced nitrogen-fixing Trifolium dubium and increased cover of plant litter, thereby reducing ammonia-nitrogen concentrations in the soil. These effects and their interactions demonstrate that the detritivore food chain, and earthworms in particular, cannot be ignored if we are to understand the spatial and temporal dynamics of plant communities.     

Do changes in rainfall patterns affect semiarid annual plant communities?

Question: Climate change models forecast a reduction in annual precipitation and more extreme events (less rainy days and longer drought periods between rainfall events), which may have profound effects on terrestrial ecosystems. Plant growth, population and community dynamics in dry environments are likely to be affected by these changes since productivity is already limited by water availability. We tested the effects of reduced precipitation and fewer rain events on three semiarid plant communities dominated by annual species. Location: Three semiarid plant communities from Almería province (SE Spain). Methods: Rain‐out shelters were set up in each community and watering quantity and frequency were manipulated from autumn to early summer. Plant productivity, cover and diversity were measured at the end of the experimental period. Results: We found that a 50% reduction in watering reduced productivity, plant cover and diversity in all three communities. However, neither the 25% reduction in watering nor changes in the frequency of watering events affected these parameters. Conclusions: The lack of response to small reductions in water could be due to the identity and resistance of the plant communities involved, which are adapted to rainfall variability characteristic of arid environments. Therefore, a rainfall reduction of 25% or less may not affect these plant communities in the short term, although higher reductions or long‐term changes in water availability would probably reduce productivity and diversity in these communities.     

Rooting depth,water availability,and vegetation cover along an aridity gradient in Patagonia

Above-and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44–45°S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above-and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at-27), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80–0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2–3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2–3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.     

The invasive plant species Centaurea maculosa alters arbuscular mycorrhizal fungal communities in the field

While several recent studies have described changes in microbial communities associated with exotic plant invasion, how arbuscular mycorrhizal fungi (AMF) communities respond to exotic plant invasion is not well known, despite the salient role of this group in plant interactions. Here, we use molecular methods (terminal restriction fragment length polymorphism analyses based on the large subunit of the rRNA gene) to examine AMF community structure in sites dominated by the invasive mycorrhizal forb, Centaurea maculosa Lam. (spotted knapweed), and in adjacent native grassland sites. Our results indicate that significant AMF community alteration occurs following C. maculosa invasion. Moreover, a significant reduction in the number of restriction fragment sizes was found for samples collected in C. maculosa-dominated areas, suggesting reduced AMF diversity. Extraradical hyphal lengths exhibited a significant, on average 24%, reduction in C. maculosa-versus native grass-dominated sites. As both AMF community composition and abundance were altered by C.maculosa invasion, these data are strongly suggestive of potential impacts on AMF-mediated ecosystem processes. Given that the composition of AMF communities has the potential to differentially influence different plant species, our results may have important implications for site restoration after weed invasion.     

Seasonal Dynamics of Microbial Community Composition and Function in Oak Canopy and Open Grassland Soils

Soil microbial communities are closely associated with aboveground plant communities, with multiple potential drivers of this relationship. Plants can affect available soil carbon, temperature, and water content, which each have the potential to affect microbial community composition and function. These same variables change seasonally, and thus plant control on microbial community composition may be modulated or overshadowed by annual climatic patterns. We examined microbial community composition, C cycling processes, and environmental data in California annual grassland soils from beneath oak canopies and in open grassland areas to distinguish factors controlling microbial community composition and function seasonally and in association with the two plant overstory communities. Every 3 months for up to 2 years, we monitored microbial community composition using phospholipid fatty acid (PLFA) analysis, microbial biomass, respiration rates, microbial enzyme activities, and the activity of microbial groups using isotope labeling of PLFA biomarkers (¹³C-PLFA). Distinct microbial communities were associated with oak canopy soils and open grassland soils and microbial communities displayed seasonal patterns from year to year. The effects of plant species and seasonal climate on microbial community composition were similar in magnitude. In this Mediterranean ecosystem, plant control of microbial community composition was primarily due to effects on soil water content, whereas the changes in microbial community composition seasonally appeared to be due, in large part, to soil temperature. Available soil carbon was not a significant control on microbial community composition. Microbial community composition (PLFA) and ¹³C-PLFA ordination values were strongly related to intra-annual variability in soil enzyme activities and soil respiration, but microbial biomass was not. In this Mediterranean climate, soil microclimate appeared to be the master variable controlling microbial community composition and function.     

Postfire vegetation dynamics in a sagebrush steppe in southeastern Idaho,USA

Postburn vegetation in an Artemisia tripartita and A. tridentata sagebrush community one year after the burn was compared with unburned vegetation. While the vegetal cover amounted to 38.1% of the total area on the burned site, it was 91.1% on the unburned site. Dominance-diversity curves for plant communities on both sites approach the niche pre-emption hypothesis or geometric series. Cover values and soil residual propagule data were used to suggest mechanisms of persistence of the more prominent species through fire using Noble & Slatyer's (1980) Vital Attributes model. The first year postfire vegetation was dominated by forbs and grasses with vegetative and propagule storage mechanisms of persistence. Such information on succession mechanisms should be of benefit to range managers.