Естественное старение литейного сплава AlMg₅Si₂Mn

Наведено литу структуру та структуру після гомогенізації виливків зі сплаву AlMg₅Si₂Mn, отриманих литтям у кокіль і за допомогою лиття під високим тиском. Зразки було досліджено методами диференціальної скануючої калориметрії, трансмісійної електронної мікроскопії, енергодисперсійного рентгенівського аналізу та визначення мікротвердості. Усередині зерен α-Al в обох сплавах було виявлено вигнуті пластинчаті виділення. Дослідженням цих виділень встановлено ряд особливостей, а саме: 1) їх склад дуже близький до стехіометричного складу Mg₂Si; 2) включення розподіляються уздовж дислокацій; 3) щільність включень значно вища для виливків, відлитих під високим тиском, оскільки в них матриця α-Al містить більше дислокацій, ніж у виливках, отриманих литтям у постійну форму; 4) включення випадковим чином розподілені в середині зерен α-Al, між ламелями Mg₂Si включень не спостерігалося; 5) гомогенізація сплаву призводить до повного розчинення включень, і при штучному старінні формуються нові голчасті виділення.The as-cast and heat treated structure of permanent mould and high pressure die castings of the AlMg₅Si₂Mn alloy has been investigated by differential scanning calorimetry, microhardness measurements, transmission electron microscopy and energy dispersive X-ray analysis. Curved plate-like precipitates were detected for both alloys inside the α-Al grains. Examination of these precipitates revealed a number of features, such as: 1) the composition of the precipitates is very close to the stoichiometric Mg₂Si compound; 2) precipitates are aligned along dislocations; 3) the precipitate density is much higher for the high pressure die castings where the α-Al matrix contains more dislocations than in permanent mould castings; 4) precipitates lie inside the α-Al grains where they are randomly distributed. The Mg₂Si lamellas were not observed between precipitates; 5) homogenization of the alloy results in complete dissolution of the precipitates and during artificial aging new needle-shaped precipitates are formed.Представлены литая структура и структура после гомогенизации отливок из сплава AlMg₅Si₂Mn, полученных литьем в кокиль и с помощью литья под высоким давлением. Образцы были исследованы методами дифференциальной сканирующей калориметрии, трансмиссионной электронной микроскопии, энергодисперсионного рентгеновского анализа и определения микротвердости. Внутри зерен α-Al в обоих сплавах были обнаружены изогнутые пластинчатые выделения. Исследование этих выделений выявило ряд особенностей, а именно: 1) их состав очень близок к стехиометрическому составу Mg₂Si; 2) включение распределяются вдоль дислокаций; 3) плотность включений значительно выше для отливок, отлитых под высоким давлением, так как в них матрица α-Al содержит больше дислокаций, чем в отливках, полученных литьем в постоянную форму; 4) включения случайным образом распределены в середине зерен α-A, между ламелями Mg₂Si включений не наблюдалось; 5) гомогенизация сплава приводит к полному растворению включений, и при искусственном старении формируются новые игольчатые выделения

Pollination and dispersal trait spectra recover faster than the growth form spectrum during spontaneous succession in sandy old‐fields

Question: Spontaneous succession is the most natural and cost‐effective solution for grassland restoration. However, little is known about the time required for the recovery of grassland functionality, i.e., for the recovery of reproductive and vegetative processes typical of pristine grasslands. Since these processes operate at different scales, we addressed the question: do reproductive and vegetative processes require different recovery times during spontaneous succession? Location: Kiskunság sand region (Central Hungary). Methods: As combinations of plant traits can be used to highlight general patterns in ecological processes, we compared reproductive (pollination‐ and dispersal‐related) and vegetative (growth form) traits between recovered grasslands of different age (<10 years old; 10–20 years old; 20–40 years old) and pristine grasslands. Results: During spontaneous succession, the reproductive trait spectra became similar to those of pristine grasslands earlier than the vegetative ones. In arable land abandoned for 10 years, pollination‐ and dispersal‐related trait spectra did not show significant difference to those of pristine grasslands; anemophily and anemochory were the prevailing strategies. Contrarily, significant differences in the growth form spectrum could be observed even after 40 years of abandonment; in recovered grasslands erect leafy species prevailed, while the fraction of dwarf shrubs and tussock‐forming species was significantly lower than in pristine grasslands. Conclusions: The recovery of the ecological processes of pristine grasslands might require different amounts of time, depending on the spatial scale at which they operate. The reproductive trait spectra recovered earlier than the vegetative one, since reproductive attributes first determine plant species sorting at the regional level towards their respective habitats. The recovery of the vegetative trait spectrum needs more time as vegetative‐based interactions operate on a smaller spatial scale. Thus, vegetative traits might be more effective in the long‐term assessment of restoration success than the reproductive ones

Soil conditions and phylogenetic relatedness influence total community trait space during early plant succession

This study is part of the Transregional Collaborative Research Centre 38 (SFB/TRR 38: ecosystem assembly and succession). The authors thank the working group Z1 (monitoring) members of the SFB/TRR 38 who helped us to perform this study and the Vattenfall Europe Mining A.G. for providing the research site.Aims The total space of traits covered by the members of plant com- munities is an important parameter of ecosystem functioning and complexity. We trace the variability of trait space during early plant succession and ask how trait space co-varies with phylogenetic community structure and soil conditions. Particularly, we are inter - ested in the small-scale variability in trait space and the influence of biotic and abiotic filters. Methods We use data on species richness and soil conditions from the first 7 years of initial succession of an artificial catchment in north-east- ern Germany. Total functional attribute diversity serves as a proxy to total trait space. Important Findings Total trait space steadily increased during succession. We observed high small-scale variability in total trait space that was positively cor - related with species richness and phylogenetic segregation and nega- tively correlated with total plant cover. Trait space increased with soil carbonate content, while pH and the fraction of sandy material behaved indifferently. Our results indicate that during early succession, habitat filtering processes gain importance leading to a lesser increase in trait space than expected from the increase in species richness alone

Effects of Water and Nitrogen Addition on Species Turnover in Temperate Grasslands in Northern China

Global nitrogen (N) deposition and climate change have been identified as two of the most important causes of current plant diversity loss. However, temporal patterns of species turnover underlying diversity changes in response to changing precipitation regimes and atmospheric N deposition have received inadequate attention. We carried out a manipulation experiment in a steppe and an old-field in North China from 2005 to 2009, to test the hypothesis that water addition enhances plant species richness through increase in the rate of species gain and decrease in the rate of species loss, while N addition has opposite effects on species changes. Our results showed that water addition increased the rate of species gain in both the steppe and the old field but decreased the rates of species loss and turnover in the old field. In contrast, N addition increased the rates of species loss and turnover in the steppe but decreased the rate of species gain in the old field. The rate of species change was greater in the old field than in the steppe. Water interacted with N to affect species richness and species turnover, indicating that the impacts of N on semi-arid grasslands were largely mediated by water availability. The temporal stability of communities was negatively correlated with rates of species loss and turnover, suggesting that water addition might enhance, but N addition would reduce the compositional stability of grasslands. Experimental results support our initial hypothesis and demonstrate that water and N availabilities differed in the effects on rate of species change in the temperate grasslands, and these effects also depend on grassland types and/or land-use history. Species gain and loss together contribute to the dynamic change of species richness in semi-arid grasslands under future climate change

Patterns of ash (Fraxinus excelsior L.) colonization in mountain grasslands: the importance of management practices

International audienceWoody colonization of grasslands is often associated with changes in abiotic or biotic conditions or a combination of both. Widely used as fodder and litter in the past traditional agro-pastoral system, ash (Fraxinus excelsior L.) has now become a colonizing species of mountain grasslands in the French Pyrenees. Its present distribution is dependent on past human activities and it is locally controlled by propagule pressure and abiotic conditions. However, even when all favourable conditions are met, all the potentially colonizable grasslands are not invaded. We hypothesize that management practices should play a crucial role in the control of ash colonization. From empirical field surveys we have compared the botanical composition of a set of grasslands (present and former) differing in management practices and level of ash colonization. We have displayed a kind of successional gradient positively linked to both ash cover and height but not to the age of trees. We have tested the relationships between ash presence in grassland and management types i.e. cutting and/or grazing, management intensity and some grassland communities' features i.e. total and local specific richness and species heterogeneity. Mixed use (cutting and grazing) is negatively linked to ash presence in grassland whereas grazing alone positively. Mixed use and high grazing intensity are directly preventing ash seedlings establishment, when low grazing intensity is allowing ash seedlings establishment indirectly through herbaceous vegetation neglected by livestock. Our results show the existence of a limit between grasslands with and without established ashes corresponding to a threshold in the intensity of use. Under this threshold, when ash is established, the colonization process seems to become irreversible. Ash possesses the ability of compensatory growth and therefore under a high grazing intensity develops a subterranean vegetative reproduction. However the question remains at which stage of seedling development and grazing intensity these strategies could occur

Biological soil crusts of Arctic Svalbard and of Livingston Island, Antarctica

Biological soil crusts (BSCs) occur in arid and semi-arid regions worldwide including the Polar Regions. They are important ecosystem engineers, and their composition and areal coverage should be understood before assessing key current functional questions such as their role in biogeochemical nutrient cycles and possible climate change scenarios. Our aim was to investigate the variability of BSCs from Arctic Svalbard and the Antarctic Island, Livingston, using vegetation surveys based on classification by functional group. An additional aim was to describe the structure of BSCs and represent a classification system that can be used in future studies to provide a fast and efficient way to define vegetation type and areal coverage. Firstly, this study demonstrates huge areas occupied by BSCs in Arctic Svalbard, with up to 90 % of soil surface covered, dominated by bryophytes and cyanobacteria, and showing an unexpectedly high variability in many areas. Livingston Island has lower percentage coverage, up to 55 %, but is dominated by lichens. Our findings show that both Polar Regions have varied BSC coverage, within the sites and between them, especially considering their harsh climates and latitudinal positions. Secondly, we have classified the BSCs of both areas into a system that describes the dominant functional groups and local geography, creating a simple scheme that allows easy identification of the prevailing vegetation type. Our results represent the first contribution to the description of BSCs based on their functional group composition in Polar Regions

Invasive alien aquatic plants in South African freshwater ecosystems

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Deagrarianisation and forest revegetation in a biodiversity hotspot on the Wild Coast, South Africa

Deagraianisation is a worldwide phenomenon with widespread social, ecological and economic effects yet with little consensus on the local or higher level causes. There have been contested views on the causes and consequences of deagrarianisation on South Africa’s Wild Coast, which is an international biodiversity hotspot. Using GIS, household interviews and ecological sampling, we compared the perspectives of current and former cultivators as to why some have abandoned farming, whilst also tracking the uses and woody plant cover and composition of fields abandoned at different periods. The GIS analysis showed that field abandonment had been ongoing over several decades, with a decline from 12.5 % field cover in 1961 to 2.7 % in 2009. The area of forests and woodlands almost doubled in the corresponding period. There was a distinct peak in field abandonment during the time of political transition at the national level in the early 1990s. This political change led to a decrease in government support for livestock farming, which in turn resulted in reduced animal draught power at the household and community level, and hence reduced cropping. The study showed it is largely the wealthier households that have remained in arable agriculture and that the poorer households have abandoned farming. The abandoned fields show a distinct trend of increasing woody biomass and species richness with length of time since abandonment, with approximately three woody plant species added per decade. Most local respondents dislike the increases in forest and woodland extent and density because of anxiety about wild animals causing harm to crops and even humans, and the loss of an agricultural identity to livelihoods and the landscape