Grading Woodland Soil Water Productivity and Soil Bioavailability in the Semi-Arid Loess Plateau of China

In the semi-arid region of the Loess Plateau in China, a portable photosynthesis system (Li-6400) and a portable steady porometer (Li-1600) were used to study the quantitative relation between the soil water content (SWC) and trees' physiological parameters including net photosynthesis rate (Pn), carboxylation efficiency (CE), transpiration rate (Tr), water use efficiency of leaf (WUEL), stomatic conductivity (Gs), stomatal resistance (Rs), intercellular CO2 (Ci), and stomatal limitation (Ls). These are criteria for grading and evaluating soil water productivity and availability in forests of Black Locust (Robinia pseudoacacia) and Oriental Arborvitae (Platycladus orientalis). The results indicated: To the photosynthesis of Locust and Arborvitae, the SWC of less than 4.5 and 4.0% (relative water content (RWC) 21.5 and 19.0%) belong to non-productivity and non-efficiency water; the SWC of 4.510.0% (RWC 21.547.5%) and 4.08.5% (RWC 19.040.5%) belong to low productivity and low efficiency water; the SWC of 10.013.5% (RWC 47.564.0%) and 8.511.0% (RWC 40.552.0%) belong to middle productivity and high efficiency water; the SWC of 13.517.0% (RWC 64.081.0%) and 11.016.0% (RWC 52.076.0%) belong to high productivity and middle efficiency water; the SWC of 17.019.0% (RWC 81.090.5%) and 16.019.0% (RWC 76.090.5%) belong to middle productivity and low efficiency water; the SWC of more than 19.0% (RWC 90.5%) belongs to low productivity and low efficiency water. The SWC of about 13.5 and 11.0% (RWC 64.0 and 52.0%) are called high productivity and high efficiency water, which provides the further evidence for Locust and Arborvitae to get both higher productivity (Pn and CE) and the highest WUEL and adaptation to the local environment, respectively.