Deep soil water extraction helps to drought avoidance but shallow soil water uptake during dry season controls the inter-annual variation in tree growth in four subtropical plantations

Abstract

Some essential features of forest hydrologic cycles are based on observations of the soil water balance. However, measurements of soil water dynamics in subtropical forests have mostly been restricted to the upper 100cm of soil. The relative shallow depth in soil water measurement would bias the understanding how and to what extent soil water contributes to evapotranspiration. We investigated and compared the soil water dynamics down to 200cm depth over 7 years in four subtropical plantations, including one mixed stand type (Cunninghamia lanceolata mixed with Schima superba) and three pure stand types (C. lanceolata, Pinus elliottii, and Pinus massoniana). We also examined the variations in fine root distribution and tree basal area increment across stand types. We showed that, in contrast to the P. elliottii and P. massoniana stands, the C. lanceolata stand type extracted deep soil water when the top soil was still wet, depleted soil water over a longer season, but consumed less total soil water during the dry season. Our results revealed that the mixed stand type depleted more soil water during the dry season than the pure C. lanceolata stand type. In addition to the species-specific traits of water consumption, the different soil water depletion between stand types seems to be also accounted for by the different slope aspect and rock fragment content. During the entire dry season, the four stand types mainly relied on the soil water in the 100–200cm soil depth where fine root density was considerably low, however, the inter-annual variation in stem growth depended on the soil water depletion from 0 to 50cm soil layer. We conclude from these results that deep soil water is likely to make a significant contribution to drought avoidance over the dry period, but the available of shallow soil water to the fine roots during dry season may determine how well trees grow. This study highlights the important to consider the deep soil water extraction when explain the ecosystem evapotranspiration variations.