Response to climate change of different tree species and NDVI variation since 1923 in the middle arid region of Ningxia,China

The normalized difference vegetation index （NDVI） is used extensively to describe vegetation cover and ecological environ- ment change. The purpose of this study was to contrast the response of different tree species growing in the same habitat to climate change and retrieve past NDVI using tree-ring width data from tree cores collected from the transitional zone of Pinus tabulaeformis and Picea crassifolia in the Luoshan Mountains in the middle arid region of Ningxia. Correlation analysis indi- cated that radial growth ofP tabulaeJbrmis is more sensitive to precipitation and temperature change than that ofP crassifolia. Natural factors such as water availability and heat at this elevation are more suited to the growth ofP crassifolia, and are more advantageous to its renewal and succession. P. crassifolia is probably the better of the two species for protecting the forest ecosystem and conserving water in the Luoshan desertification area. Ring width of P. crassifolia correlates significantly with average NDVI for April-May （r =0.641, p 〈0.01）, and both of them are influenced positively by precipitation in April-May. The reconstructed NDVI for 1923-2007 shows the relatively low vegetation cover occurred in the 1920s-1930s, the 1960s-1970s, and the early 21 st century. The reconstructed NDVI better reflected the drought climate in the study area.

Effects of Y and Zn additions on electrical conductivity and electromagnetic shielding effectiveness of Mg-Y-Zn alloys

Microstructure, electrical conductivity, and electromagnetic interference(EMI) shielding effectiveness(SE) of cast Mg-x Zn-y Y(x = 2–5, y = 1–10) alloys were systematically investigated to understand the effects of Zn and Y additions on electrical conductivity and electromagnetic shielding effectiveness of the alloys.Experimental results indicate that the electrical conductivity and SE of the Mg-x Zn-y Y alloys decrease with Y/Zn ratio. Electrical conductivity is the main factor that affects the electromagnetic shielding properties and the variation tendency of electromagnetic shielding properties of the Mg-x Zn-y Y alloys is consistent with conductivity. Valence of Y and Zn atoms, configuration of extranuclear electron and volumetric difference are main reasons for the variations in the electrical conductivity. A high density of second phase and the formation of semi-continuous network structure can also improve the SE value at high frequencies.

Strain hardening of as-extruded Mg-xZn(x=1, 2, 3 and 4 wt%) alloys

The influence of Zn on the strain hardening of as-extruded Mg-x Zn(x = 1, 2, 3 and 4 wt%) magnesium alloys was investigated using uniaxial tensile tests at 10^(-3)s^(-1) at room temperature. The strain hardening rate,the strain hardening exponent and the hardening capacity were obtained from true plastic stress-strain curves. There were almost no second phases in the as-extruded Mg-Zn magnesium alloys. Average grain sizes of the four as-extruded alloys were about 17.8 μm. With increasing Zn content from 1 to 4 wt%, the strain hardening rate increased from 2850 MPa to 6810 MPa at(б-б_(0.2)) = 60 MPa, the strain hardening exponent n increased from 0.160 to 0.203, and the hardening capacity, Hc increased from 1.17 to 2.34.The difference in strain hardening response of these Mg-Zn alloys might be mainly caused by weaker basal texture and more solute atoms in the α-Mg matrix with higher Zn content.