Abstract
Success of afforestation efforts hinges on better understanding of roles of water in the deep unsaturated zone in plant water uptake. But previous studies have deep soil water measurement either not deep, or not long enough to understand annual deep soil depletion rate for reforested forests, especially the effect of stand age or precipitation year on annual deep soil water depletion rate. To address this question, we selected, in 2016, four apple orchards planted on cropland in 2008, 2005, 2001, and 1994 (i.e., young orchard AP2008, mature orchards AP2005 and AP2001, and old orchard AP1994, respectively) in a thick loess area. Soil water contents were measured in 2016 - 2020 to the depth of 19 m with an in-situ neutron probe technique. Meanwhile, leaf area indexes were measured in 2017 and 2019 to represent each orchard water demand. Results showed annual deep soil water in 5-19 m showed a peak depletion rate of 160 mm yr−1 at around 14 years of orchard age. After the age effects are removed, mature orchards AP2005 and AP2001 showed significant depletions in 5-19 m deep soil water in the dry year 2016; in normal years 2017/2018, the depletion rates were reduced; and no significant deep soil water depletions were found in all orchards in the wet year 2019. The greater deep soil water uses in mature orchards AP2005 and AP2008 agreed with greater leaf area indexes. The depleted deep soil water contributions to actual evapotranspiration (AET) were 27.0% and 13.6% in the dry year 2016 for orchards AP2005 and AP2001, respectively; but their respective contributions were 15.1% and 7.9% in normal years 2017/2018. This indicates both the rainwater supply and the stand age affect deep soil water use, which suggests that more rainwater saving strategies and branch pruning are needed to avoid excess deep soil water depletion in dry and normal precipitation years for the mature forest on the Chinese Loess Plateau.
Published Version
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