Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms

It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystemclimate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.This is the publisher’s final pdf. The published article is copyrighted by New Phytologist Trust and can be found at: http://www.newphytologist.org/Keywords: Climate change, Temperature acclimation, Optimum temperature, Thermal optimality, Temperature adaptatio

Numerical Simulation of Bedrock Sagging Sinkholes in Strain-Softening Rock Induced by Embankment Construction

A bedrock sagging sinkhole occurred in Jiangxi Province of China when constructing the Changli freeway above shallow karst caves. It was chosen as a case to investigate the failure mechanism and potential evolution. The in situ stress of the study area was measured and numerically reproduced. The Hoek–Brown strength parameters were obtained by laboratory tests. A strain-softening constitutive model was established according to the strain-softening behaviour exhibited by the specimens in the triaxial test. The stress-strain curves of the specimens were reproduced by numerical methods. Then, bedrock sagging sinkholes in strain-softening rock induced by embankment construction were simulated. The occurrence of the strain-softening zone and its transition to the residual zone were observed and classified into four stages. The stress paths of the four stages were analysed. Interestingly, in this case, the supports at both ends of the bedrock began to yield from the top and extended downward, while the midspan position began to yield from the bottom and extended upward, and the reasons for yielding were related to tension. Further analysis found that the failure mode was basically consistent with the size and direction of the bending moment. In fact, this failure mode was quite similar to a fixed supported beam. Then, the feasibility of calculating the stability of karst caves based on beam assumptions was discussed. Finally, potential evolution of the bedrock sagging sinkhole was discussed

Constraint Embankment Construction to Prevent the Collapse of Underground Caves

Dozens of underground karst caves were found before constructing the Changli highway. The thickness-to-span ratio of nearly half of the caves is less than 0.05, and the greatest ratio is only 0.35, far less than the value demanded by local construction specifications (0.8). The caves located at K50 + 700 and K178 + 800 are by far the only two caves that have become unstable. Only one passive measure was taken when constructing the highway, i.e., building 0.5 m thick continuous reinforced concrete slabs above the embankment; this measure did not contribute to the improvement of the stability of the underground caves. Numerical solutions based on strength reduction and analytical solutions based on the beam hypothesis are used to assess the stability of underground caves. The capacity of an underground cave to bear embankment construction is observed to be proportional to the tensile strength of the rock mass and the square of the thickness-to-span ratio of the cave roof. The tensile strength of the rock mass is ψ times lower than that of the intact rock. The value of ψ is mainly determined by the geological strength index (GSI). To prevent instability of underground caves, the embankment height should be reasonably controlled. However, local construction specifications requiring that the thickness-to-span ratio of underground cave be greater than 0.8 are conservative