Positive and negative effects of exotic Erodium cicutarium on an arid ecosystem

Many exotic species negatively affect native species and alter ecosystem function. Erodium cicutarium, an exotic annual plant, can attain high densities, but little is known about its effects on native plant communities. We first examined patterns of abundance of E. cicutarium and native annuals over a 16-year period at a long-term study site in southeastern Arizona. In years of high E. cicutarium abundance, the correlations between the abundance of E. cicutarium and native annuals in small-scale local communities were typically negative, suggesting a competitive interaction. To further examine the interaction between E cicutarium and native annuals, we conducted a short-term field experiment at the same location using plots that contained pairs of quadrats. One quadrat in each pair was subjected to E. cicutarium removal during the winter of 2003-2004 while the other served as a control. At the end of the growing season, E. cicutarium removal quadrats contained significantly higher abundance and richness of native annual plants. However, control plots contained significantly higher abundance of all annuals due to the presence of E. cicutarium. Thus, in the single growing season examined, while E. cicutarium appears to suppress the diversity and abundance of native species, its presence significantly increases community productivity.     

A comparison of methods for estimating fractional vegetation cover in arid regions

We compared a set of methods for estimating the fractional vegetation cover (f_c) of sparse desert vegetation over an arid region of southern Xinjiang, China. Six kinds of remote sensing inversion models (an NDVI regression, a spectral mixture analysis (SMA), a pixel dichotomy model, a three-band maximal gradient difference (TGDVI) model and two modified TGDVI models) were used to derive f_c from remote sensing data, and the results were compared with f_c values measured in the field to select an appropriate model to derive the fractional cover of sparse desert vegetation in arid regions. The NDVI regression based on field f_c and the NDVI for the sampled pixels in September 2006 showed the highest precision, while the results of 2007 showed that the NDVI regression method is inappropriate for depicting vegetation characteristics in other growing season because the empirical model highly depend on the specified in situ measurement. The SMA approaches yielded higher precision than the other models, indicating that it is applicable for analysing the coverage of sparse desert vegetation. The pixel dichotomy model can yield a high precision based on finely detailed vegetation maps. However, it requires the measurement of many parameters. The TGDVI model is simple and easy to implement, and the values that it predicted for the coverage of high-density vegetation and barren areas were close to those measured in the field, but the f_c values of sparsely vegetated areas were underestimated. The predictions of the modified TGDVI models were close to the values measured in the field, indicating that these modified models can reliably and effectively extract information on the fractional cover of sparse vegetation in an arid region. We analyzed the models’ sensitivity with respect to rainfall because the short-wavelength infrared bands used in the two TGDVI models proposed in this study are sensitive to moisture. The results showed that the modified TGDVI models’ accuracy was not affected by increasing soil moisture content caused by rain. However, the NDVI regression, SMA and TGDVI were sensitive to the change of soil moisture content. Moreover, the two modified TGDVI models yielded negative values for water sources, such as reservoirs and rivers, implying that they are effective for characterising water bodies. However, the modified TGDVI models cannot predict f_c in snow- and glacier-covered regions, producing abnormally high rather than zero values. Additionally, the predictions before and after snowfall on the top of a mountain show a linear increasing relationship, suggesting that the short-wavelength infrared band may be useful to predict snow depth.     

Mercury concentrations in an aquatic ecosystem during twenty years following abatement the pollution source

Phenyl Hg was widely used as a slimicide in Finnish pulp industry until the end of 1967. The use of Hg caused a significant increase of Hg levels in fish in several areas. High concentrations were measured in Lake Kirkkojärvi in Hämeenkyrö, SW Finland. Vast amounts of Hg are still present in the lake sediments. Since 1968 uncontaminated fibres have partly covered the contaminated layers. Since 1971 Hg has been monitored in fish, sediments and aquatic plants in the water course downstream from the pulp and paper factory. The Hg concentration of a 1-kg pike (Esox lucius) has decreased from 1.5 µg g^?1 in the years 1971–74 to 0.8 µg g^?1 in 1990.     

The response of plant and insect assemblages to the loss of Calluna vulgaris from upland vegetation

Across large areas of upland Britain Molinia caerulea and Nardus stricta are replacing Calluna vulgaris as the dominant plant species of the vegetation assemblage. Associated changes in the composition of the whole plant assemblage and a phytophagous insect group, the Hemiptera, were investigated. Vegetation and hemipteran assemblages were sampled at six moorland sites in northern England and Scotland. Sampling at each site was carried out in Calluna vulgaris-dominated areas and grass-dominated areas that had previously been dominated by C. vulgaris. Vegetation assemblages of heath and grass sites differed significantly even when species by which sites were selected for sampling (C. vulgaris, M. caerulea and N. stricta) were removed from the analysis with the highest species richness in dry grassland sites. Hemipteran assemblages differed significantly between heath and grass sites with greater species richness also being in grass sites. Plant species composition was the most important explanatory variable of hemipteran assemblages with temperature, latitude, soil loss on ignition and vegetation height also explaining a significant proportion of the variation. Overall a range of vegetation composition and structural variables explained 40.8% of variation in the Hemiptera species data with ranges of soil and biogeographic variables explaining an additional 8.7%.This research demonstrates that the replacement of one dominant plant species by another is associated with a change in other aspects of moorland biodiversity. In particular the loss of C. vulgaris actually leads to an increase in the species richness of Hemiptera and plants although the number of specialist species declines.     

Mechanisms of pH change in wood ant (Formica polyctena) nests

We conducted a study to determine why the pH of wood ant nest materials is typically higher than that of the surrounding forest soil. An experiment with litter bags demonstrated that the pH of litter increased significantly (after only 7 months) in ant nests. Because the food that foraging ants bring into the nest contains easily decomposed carbohydrates and basic cations (largely in the form of honeydew and prey) that can cause increases in pH, we then estimated the amount of this influ of basic cations and easily decomposed carbohydrates. Based on these estimates, we conducted a second experiment to determine whether addition of field-determined quantities of an easily decomposed carbohydrate (glucose) or a basic cation (Ca²⁺) would increase the pH of the litter in artificial ant nests in the forest. Both glucose and Ca²⁺ additions significantly increased the pH of the litter but the increase was greater with Ca²⁺. The rate of pH increase in the artificial nests was similar to the rate of increase in natural nests. According to our study, ants collect substantial amounts of mineral-enriched materials in this ecosystem, which collectively increase the pH of nest material.     

Photoacceleration of plant litter decomposition in an arid environment

In arid ecosystems, abiotic processes facilitate the physical and chemical degradation of plant litter to the extent that decomposition models that use climatic and litter composition variables as surrogates for microbial activity are not predictive. The purpose of this study was to estimate the potential contribution of photodegradation to the decomposition of plant litters that varies in architecture and chemical composition. Litter of Pinus edulis, Juniperus monosperma and Populus deltoides were exposed to ambient and attenuated sunlight, with and without supplemental water additions, at a riparian forest site along the Middle Rio Grande (New Mexico, USA). Mass loss, elemental composition, and microbial extracellular enzyme activities (EEA) were measured over 639 days. The composition of the fungal communities associated with the decomposing litters was compared by analyses of fungal ITS nrDNA sequences. Litter exposed to ambient sunlight had greater mass loss rates than shaded litter, independent of the water treatment: Populus increased by 100%, Pinus by 86% and Juniperus by 46%. The increases were proportional to exposed litter surface area per g dry mass. EEA potentials, particularly oxidative activities, were low in comparison to those measured in mesic ecosystems. For Populus litter, the principal driver of photoacceleration appeared to be photodegradation of cellulose; for Pinus, it was photodegradation of polyphenols; for Juniperus accelerated mass loss was associated with photodegradation of both polysaccharides and polyphenols. Fungal community composition varied by litter type, but the dominant colonizers were yeasts and dark-septate hyphal taxa; a finding consistent with the low enzymatic oxidation potential. This study shows that photochemical oxidation can supplement enzymatic oxidation and increase decomposition rates. As a result, organic matter decomposition in arid ecosystems is not restricted to periods of high moisture availability as is plant production. This decoupling may partly account for the low soil organic matter content of these ecosystems.     

Runoff generation in a degraded Andean ecosystem: Interaction of vegetation cover and land use

Tropical mountain regions are affected by rapid land use/-cover change, which may threaten their (eco-)hydrological functions. Although there is a growing interest in evaluating the effect of land use/-cover change on mountain hydrology, quantitative assessments of the impact of land use/-cover on hydrological processes are hampered by the lack of field measurements characterizing runoff generation processes. In this paper, we present results from field experiments of rainfall runoff mechanisms in the southern Ecuadorian Andes. A rainfall simulator was used to quantify the hydrological response of distinct land use/-cover types to intense rainfall (about 40 mm/h). The rainfall runoff experiments indicate that degraded and abandoned land generate surface runoff within a few minutes after the start of the rainfall event. These lands have a very rapid and sharp hillslope hydrological response, as Hortonian overland flow is the dominant runoff generation mechanism. In contrast, surface runoff on arable and rangelands is rare, as their soils are characterized by a high infiltration capacity (i.e. > 29 mm/h). Our experiments provide evidence that runoff generation in degraded Andean ecosystems is mainly controlled by the surface vegetation cover and land management. When reducing the surface vegetation cover, the soil is increasingly affected by rapid hillslope runoff as the presence of large amounts of smectites in the outcropping soft rocks makes the material very prone to sealing and crusting, thereby enhancing runoff generation.     

广西钟山主要水稻土肥力变化浅析

连续十年定位监测结果表明,土壤有机质、钾素养分含量逐年下降,土壤氮素、磷素养分含量则逐年上升.主要原因是有机肥和钾肥投入量少,钾肥施用比例低.钾肥施用比例与作物产量呈极显著正相关,增施钾肥和有机肥是改善水稻土肥力的主要措施.     

Assessing the degradation of saline wetlands in an arid agricultural region in Spain

A constellation of saline wetlands in the Spanish Monegros Desert is currently subjected to increasing agricultural pressure. Until now, no systematic study of their degradation status has been performed, and no comprehensive map of these wetlands has been available. Both subjects are addressed for the first time in this study. In addition, we set up a conceptual tool for assessing saline wetlands degradation that improves upon available methods within geomorphological, sedimentological or hydrological frameworks. A wetlands inventory was produced and updated using satellite data, field observations, and orthophotographs, together with the available disperse data about these wetlands' toponymy and location. The degraded landscape appearance of the surviving wetlands, mainly affected by agricultural intensification ― dumping and farming ― has been qualitatively assessed.     

The transient response of vegetation to climate change: A potential source of CO2 to the atmosphere

Global climate change as currently simulated could result in the broad-scale redistribution of vegetation across the planet. Vegetation change could occur through drought-induced dieback and fire. The direct combustion of vegetation and the decay of dead biomass could result in a release of carbon from the biosphere to the atmosphere over a 50- to 150-year time frame. A simple model that tracks dieback and regrowth of extra-tropical forests is used to estimate the possible magnitude of this carbon pulse to the atmosphere. Depending on the climate scenario and model assumptions, the carbon pulse could range from 0 to 3 Gt of C yr^?1. The wide range of pulse estimates is a function of uncertainties in the rate of future vegetation change and in the values of key model parameters.     

The Boreal Forest Transect Case Study: Global change effects on ecosystem processes and carbon dynamics in boreal Canada

The Boreal Forest Transect Case Study (BFTCS) is a multi-disciplinary ecological study organised around a 1000 km transect located in central Canada. The transect is oriented along an ecoclimatic gradient in a region likely to undergo significant environmental change within the next few decades, and crosses the climate-sensitive boreal forest biome, including the transitions north and south into tundra and grassland respectively. Originally conceived as an extension to the BOReal Ecosystem Atmosphere Study (BOREAS), the 10-year BFTCS project projects the intensive canopy-scale measurements and modelling advances obtained from BOREAS to a wider range of sites with a longer-term perspective. In addition to considering ecophysiological processes with time-frames of the order of one year or shorter, BFTCS addresses the effects of larger scale, longer term processes including vegetation succession and ecosystem disturbances. The BFTCS currently provides practical linkages among ecosystem monitoring, field experiments and regional scale modelling. It will ultimately provide a knowledge-base of key processes and their environmental sensitivities, and assessments of possible climate feedbacks, which can be used to assess the possible consequences of global change both regionally and globally.     

Distribution of alkaline-earth carbonates in soils of an arid constructional plain,Transcaucasus

Distribution of alkaline-earth carbonates, both lateral and vertical, in soils of the serozem type of the Kura—Araks constructional plain (eastern Transcaucasus, Soviet Union) was examined. Carbonate content in soils, formed in homogeneous alluvium under the same climatic condition, have a wide range (from 0.5 to 10%) depending on several factors. The main factors are: source of alluvium and conditions of accumulation, topography and ages of certain surfaces, availability of surplus run-off water and chemical composition of ground water.     

The impact of subdivision and sedentarization of pastoral lands on wildlife in an African savanna ecosystem

This study looks at the impact of subdivision and sedentarization of pastoral lands on wildlife numbers and production in a savanna ecosystem of southern Kenya. The study uses aerial counts over a period of 33 years to compare changes in wildlife populations on two adjacent and ecologically similar Maasai group ranches. During the period under study, one group ranch was subdivided and settled. The other remained communally owned under shifting seasonal use. Wildlife populations decreased sharply on the privatized ranch following subdivision and increased steadily on the adjacent ranch where pastoralists continued mobile pastoralism. The results of multivariate analysis show that sedentarization and settlement distribution accounts for wildlife declines on the subdivided ranch. Both the direct displacement of wildlife and the reduction in grass production following a switch from seasonal to permanent grazing associated with sedentarization are discussed as causes of wildlife loss. Given the demand for title deeds among pastoralists to counter land losses, the resulting sedentarization is likely to become the biggest threat to wildlife in the East African savannas.     

Potential effects of the increase in carbon dioxide and climate change on the dynamics of vegetation

The continued CO₂ loading of the atmosphere appears to be responsible for inducing three new force factors controlling dynamic changes in the world's vegetation. They come from (1) enhanced fertilization with the single most important plant nutrient, (2) the widely expected global temperature increase, and (3) aggravated weather disturbances. Increased CO₂ absorption may enhance plant growth but it may also increase soil-nutrient limitations. It surely will enhance the metabolism of forest trees similarly as global warming will enhance plant metabolism, but both factors may also shorten the lifespan of perennial plants. Increased weather disturbances can be expected to produce new physiological stresses on the standing vegetation, particularly on habitats with poor soils. Since wide-spread forest decline has been reported from both the Atlantic and Pacific region, it seems possible that the roughly synchronie mass mortality of trees during the past two decades is related to the global increase in CO₂. The paper gives an overview of forest decline and dieback as known from past and present research and suggests how the changing atmospheric environment may interact in this widely observed contemporary phenomenon of vegetation dynamics.     

Variation in soil chemical properties along toposequences in an arid region of the Levant

Detailed characterization of soils and their variation along different topography positions has not been investigated in depth for Mediterranean arid regions. There is a need to accurately understand the variation and the spatial distribution of soil properties within dry region of the Levant. Such understanding is required to optimize the use and management of scarce land and water resources. The objective of this study was to examine the effect of hillslope characteristics on the variation of selected soil chemical properties in an arid Mediterranean climate. At each of five selected transects four sites were chosen to represent four different topographic positions: summit, shoulder, backslope and toeslope. A soil profile was examined at each site and a representative sample from each horizon was withdrawn for chemical analyses. The analyses indicated that generally, the carbonate contents of the surface horizons decreased from higher to lower positions of the toposequence, the carbonate content increased with depth for profiles occupying the lower positions. This suggests more intense leaching within soil at lower positions. The effect of steepness and curvature on controlling the variation of soil properties was obvious at the summit and shoulder positions. Leaching process seems to hinder the effect of steepness and curvature for soils at lower positions. Lower positions receive runoff water and organic matter from upper positions, which complicate the relationship between landform shape and organic matter content. Continuous tillage resulted in lower organic matter contents for soils at lower positions. Higher pH values were reported for soils down along the transect due to the movement of soil material from upper hillslope positions. Electrical conductivity, exchangeable calcium and potassium contents decreased downslope due to higher moisture accumulation. However, no obvious relationship was found between the variation of pH or EC in one hand and the variation of steepness and curvature on the other. Potassium content was variable due to its greater mobility. The analyses indicated that variation in the soil CEC is governed by two factors: the leaching pattern, which is controlled by hillslope position, and the accumulation of Eolian carbonates at the soil surface. The distribution of iron oxides and types of clay minerals indicated more weathering in a descending direction and with soil depth, which is attributed to higher availability of soil moisture along the same direction. These relationships suggested systematic variation of chemical properties along toposequences in this arid environment.     

The effects of forest patch size and matrix type on changes in carabid beetle assemblages in an urbanized landscape

In this study we compared ground beetles (Carabidae) from a range of different forest fragments along an urbanization gradient in Brussels, Belgium. We address the following questions: (i) How does the degree of urbanization in the surrounding habitat affect forest beetles, and does it interact with the effects of patch size and distance to forest edge? (ii) Do these factors have a different effect at the level of individual species, habitat affinity groups or total community? During 2002 we sampled 13 forest plots in 10 forest patches, ranging in size from 5.27 to 4383 ha. The beetles were captured using transects of pitfall traps from the edge to a distance of 100 m into each woodland and identified to species level. Effects of urbanization, forest size and forest edge were evaluated on total species number, abundance and habitat affinity groups and ten abundant, widespread model carabid species. Overall, the effects of urbanization, forest size and edge effects slightly influenced total species richness and abundance but appeared to have a major effect on ground beetle assemblages through species specific responses. More urbanized sites had significantly fewer forest specialists and more generalist species. Large forest fragments were favoured by forest specialist species while generalist species and species frequently associated with forest (forest generalists) dominated the smaller forests. Forest edges mainly harboured generalist species while forest specialist species were more frequent into the forests if the forest patches were large enough, otherwise they disappeared due to the destruction or impoverishment of their habitat. Our results show the importance of differentiating between habitat affinity, especially habitat generalists versus specialists, the latter having a higher value in nature conservation, and merely the quantity of species represented in human-dominated areas.     

Modeling impacts of climate change on carbon dynamics in a steppe ecosystem in Inner Mongolia,China

Purpose  

In this study, a process-oriented biogeochemistry model, denitrification–decomposition (DNDC), was employed and adapted to interpret and integrate the field observations that the tested ecosystem was a weak sink of atmospheric carbon dioxide (CO₂) in 2004 but a strong source in 2005 during the growing seasons. Then we applied the model to predict long-term impacts of climate change on carbon (C) dynamics in the semiarid grassland.     

Severe declines of ant-following birds in African rainforest fragments are facilitated by a subtle change in army ant communities

Tropical rainforests are disappearing at a rapid rate. Although several studies have revealed the detrimental effects of forest fragmentation on tropical birds, the ecological mechanisms facilitating the decline of populations have been poorly documented. In the tropical rainforests of Africa and America, ant-following birds track the massive swarm raids of army ants and prey on animals which are flushed by the ants. We analyzed the persistence of five species of ant-following birds along a habitat fragmentation gradient in western Kenya and tested if bird populations are limited by the abundance of army ant colonies in forests. Abundances of four of five ant-followers declined by 52–100% in forest fragments 113 ha. Multi-model Bayesian inference suggests that the decline of the three most specialized ant-followers is facilitated by a decrease in the abundance of the army ant Dorylus wilverthi in small forest fragments. Our data suggests that a second, fragmentation-tolerant army ant species, Dorylus molestus, does not functionally compensate for the decline of D. wilverthi because, first, of a higher affinity of birds to D. wilverthi raids (found for one species). Second, because the daytime activity of D. molestus is dependent on high humidity conditions, a pattern which was not found for D. wilverthi. Consequently, specialized ant-followers in small fragments, where D. wilverthi is missing, probably suffer from food scarcity due to a cease of army ant foraging in the dry season. Our results suggest that a subtle alteration of army ant communities caused by habitat fragmentation may have large ecological consequences.