A Comparison of Water Uptake by Transpiration from Different Soil Depths among Three Land Cover Types in the Arid Northwest of China

In recent decades, the frequency, intensity, and extent of extreme drought events have posed serious threats to ecosystems in vulnerable regions. With low annual precipitation, the arid area in northwest China is a typical ecologically fragile area, and extreme drought events will aggravate desertification in this area. In order to control desertification, various experimental plantations have been established in Northwest China. However, there is no consensus on which plantations are more suitable to become widespread. To explore this, we conducted a comparative study on different plantations from the perspective of long-term deep (100 cm depth) soil moisture balance. In our study, six typical ecosystems were selected for comparison of the variation of soil moisture and control factors. The results showed three main findings. First, the soil moisture of all six ecosystems showed a similar hierarchy of increasing moisture with the increasing depth of the soil layer. However, the deep layer soil moisture (mean = 0.33 ± 0.22 cm3·cm−3) of the artificial poplar (Populus alba) forest exhibited a downward trend over time after the fifth year, but did not at the shallow layer for this ecosystem. Second, the trends of the maximum canopy coverage between the different ecosystems from 2010 to 2019 showed significant differences from one another, with the maximum value of the leaf area index for the poplar forest being the highest (Maximum = 7.13). Third, a negative correlation (R2 = 0.52) was found between deep soil moisture and transpiration for the poplar forest, and a positive correlation (R2 ≥ 0.23) between these two metrics was found for the other five ecosystems. The results revealed that transpiration processes had a different consumption of deep soil moisture due to the differences in the root and canopy density of several plantations. Among these ecosystems, the transpiration of the artificial poplar forest is noticeably large, resulting in a unilateral decline in soil moisture