Micromechanical modeling of hollow cylinder torsional shear test on sand using discrete element method

Previous studies on the hollow cylinder torsional shear test (HCTST) have mainly focused on the macroscopic behavior, while the micromechanical responses in soil specimens with shaped particles have rarely been investigated. This paper develops a numerical model of the HCTST using the discrete element method (DEM). The method of bonded spheres in a hexagonal arrangement is proposed to generate flexible boundaries that can achieve real-time adjustment of the internal and external cell pressures and capture the inhomogeneous deformation in the radial direction during shearing. Representative angular particles are selected from Toyoura sand and reproduced in this model to approximate real sand particles. The model is then validated by comparing numerical and experimental results of HCTSTs on Toyoura sand with different major principal stress directions. Next, a series of HCTSTs with different combinations of major principal stress direction (α) and intermediate principal stress ratio (b) is simulated to quantitatively characterize the sand behavior under different shear conditions. The results show that the shaped particles are horizontally distributed before shearing, and the initial anisotropic packing structure further results in different stress–strain curves in cases with different α and b values. The distribution of force chains is affected by both α and b during the shear process, together with the formation of the shear bands in different patterns. The contact normal anisotropy and contact force anisotropy show different evolution patterns when either α or b varies, resulting in the differences in the non-coaxiality and other macroscopic responses. This study improves the understanding of the macroscopic response of sand from a microscopic perspective and provides valuable insights for the constitutive modeling of sand

Photosynthetic Product Allocations to the Organs of Pinus massoniana Are Not Affected by Differences in Synthesis or Temporal Variations in Translocation Rates

Photosynthesis and the allocation of photosynthetic products are the two main factors that determine plant growth. To understand the growth and productivity of Pinus massoniana Lamb., the diurnal changes in photosynthetic rate were continuously monitored. Furthermore, the translocation and allocation of the photosynthetic products synthesized in the morning and afternoon were explored using 13C pulse labeling. The results showed that: (1) on sunny days, the diurnal variation of the net photosynthetic rate showed a “double peak” curve, with an obvious “a depression” when temperatures were highest and humidity lowest. On cloudy days, it showed an irregular “jagged” curve, which was curve consistent with the variations in photosynthetically active radiation (PAR). Meanwhile, the photosynthetic rate changed with the transient changes in environmental factors such as PAR, temperature, and humidity. (2) The mean value of the net photosynthetic rate in the morning was higher than in the afternoon, and the response of the net photosynthetic rate to environmental change (PAR, temperature, humidity, and CO2 concentration) in the morning was greater than that in the afternoon. (3) The translocation of photosynthetic products synthesized in the afternoon was faster than that in the morning. Shortly after synthesis of photosynthetic products, the translocation of products synthesized in the morning tended toward upper organs (including current-year leaves and 1-year leaves), while the translocation of products synthesized in the afternoon decreased in the upper organs. However, after 15 days of 13C pulse labeling, the allocation of the photosynthetic products synthesized in the morning and afternoon tended to be the same. These results indicate that the differences in the photosynthetic products synthesized and the temporal differences in the translocation rates did not affect the final allocation of the photosynthetic products in the various organs of the P. massoniana. These results improve our knowledge of the functional phases of P. massoniana during the diurnal cycle

Ecological Footprint Model of Cultivated Land Based on Ecosystem Services in Beijing

International audienceThe existing ecological footprint model is limited to the defects of material production and consumption. The researchers proposed an ecological footprint model based on ecosystem services (EF-ES) to strengthen research into the services provided by ecosystems, and calculated and analyzed the cultivated land in Beijing from 1995 to 2010. The results shows that: (1) from 1995 to 2000, the equilibrium factors of cultivated land in Beijing were relatively stable in EF-ES, but the yield factors in 2000 were significantly lower than that of the other years; (2) the equilibrium factors based on EF-ES were far less than other methods results. It was mainly due to ecosystem services provided by farmland ecosystem which is far less than the other ecosystem types. The yield factor based on EF-ES was similar to the result of EF-NPP; (3) the annual ecological carrying capacity was the lowest, and the ecological deficit was the highest in 2000. The ecological deficit gradually decreased from 1995 to 2010. Compared with EF-NPP, the ecological deficit of EF-ES was small, which showed that the ecological services provided by cultivated land can compensate for the ecological deficit caused by material production. The calculation results based on the model accord with the development of urban agriculture in Beijing. So EF-ES has an objective and comprehensive response to the ecological security of cultivated land, and it is a good expression for the localization of ecological footprint

Maxent modeling for predicting impacts of climate change on the potential distribution of Thuja sutchuenensis Franch., an extremely endangered conifer from southwestern China

Objectives Detailed and reliable information about the spatial distribution of species provides important information for species conservation management, especially in the case of rare species of conservation interest. We aimed to study the consequences of climate change on geographical distributions of the tertiary rare tree species Thuja sutchuenensis Franch. (Cupressaceae) to provide reference for conservation management of this species, including priority area selection for introduction and cultivation of the species. We expect that this approach could be promising in predicting the potential distribution of other rare tree species, and as such can be an effective tool in rare tree species restoration and conservation planning, especially species with narrow distribution or raw presence-only occurrence data. Methods 107 records covering the whole distribution range of T. sutchuenensis in the Daba Mountains were obtained during a 3-year field survey. The principle of maximum entropy (Maxent) was used to model the species' potential distribution area under paleoclimate, current and future climate background. Results The Maxent model was highly accurate with a statistically significant AUC value of 0.998, which is higher than 0.5 of a null model; The location of the potential distribution for the last interglacial period is in southeastern China, with the largest optimal habitat area being only 1666 km(2). In other periods, the central location of the potential distribution is accordant with the real present distribution, but the model's predicted optimal habitat area is outside the current distribution. Conclusions Our findings can be applied in various ways such as the identification of additional localities where T. sutchuenensis may already exist, but has not yet been detected; the recognition of localities where it is likely to spread to; the priority selection area for introduction and cultivation and the conservation management of such rare tree species. (C) 2017 The Authors. Published by Elsevier B.V

Maxent modeling for predicting impacts of climate change on the potential distribution of Thuja sutchuenensis Franch., an extremely endangered conifer from southwestern China

Objectives: Detailed and reliable information about the spatial distribution of species provides important information for species conservation management, especially in the case of rare species of conservation interest. We aimed to study the consequences of climate change on geographical distributions of the tertiary rare tree species Thuja sutchuenensis Franch. (Cupressaceae) to provide reference for conservation management of this species, including priority area selection for introduction and cultivation of the species. We expect that this approach could be promising in predicting the potential distribution of other rare tree species, and as such can be an effective tool in rare tree species restoration and conservation planning, especially species with narrow distribution or raw presence-only occurrence data. Methods: 107 records covering the whole distribution range of T. sutchuenensis in the Daba Mountains were obtained during a 3-year field survey. The principle of maximum entropy (Maxent) was used to model the species’ potential distribution area under paleoclimate, current and future climate background. Results: The Maxent model was highly accurate with a statistically significant AUC value of 0.998, which is higher than 0.5 of a null model; The location of the potential distribution for the last interglacial period is in southeastern China, with the largest optimal habitat area being only 1666 km2. In other periods, the central location of the potential distribution is accordant with the real present distribution, but the model’s predicted optimal habitat area is outside the current distribution. Conclusions: Our findings can be applied in various ways such as the identification of additional localities where T. sutchuenensis may already exist, but has not yet been detected; the recognition of localities where it is likely to spread to; the priority selection area for introduction and cultivation and the conservation management of such rare tree species