Fine root distributions of shelterbelt trees and their water sources in an oasis of arid northwestern China

Shelterbelt trees play an important role in maintaining the sustainability of oases agricultural ecosystems, but the trees require a considerable amount of water for survival. The objectives of this study were to investigate the root distributions, transpiration and water sources of shelterbelt trees (Gansu Poplar; Populus gansuensis) in order to improve water management efficiency. Fine root and soil water distributions were investigated along three transects that passed through cropland and an adjacent shelterbelt, while sap flow measurements were conducted on six Gansu Poplar trees. Results showed that roots were mainly distributed within 5 m of both sides of an irrigation channel passing between the first and second tree rows. The maximum distance to which trees extended fine roots horizontally was about 18 m from the shelterbelt. In 2-m soil profiles, fine roots were mainly distributed in the 1.4-2.0 m and 0-0.4 m layers depending on the available water sources. A positive relationship was observed between soil water and fine root mass density. Trees grown near the cropland-shelterbelt border exploited water from cropland irrigation and irrigation channel leakage, greatly enhancing their transpiration. During the growing season of 2013, the mean total transpiration of trees grown farther away from the border (10.75 and 17.45 m) was 216.9 mm, whereas for trees grown nearer to the border (0.85 and 6.30 m) the amounts were 670.1 and 488.7 mm, respectively. If the trees were assumed to absorb the same amount of water from soil, rainfall and groundwater sources, then irrigation water sources provided 67.6% and 55.6% of the water meeting the transpiration requirements of the trees closest to the border. The results have important implications for water management in oasis agricultural areas by limiting the extension of shelterbelt tree roots into adjacent cropland in order to improve irrigation water use efficiency.