Rainfall pattern greatly affects water use by Mongolian Scots pine on a sandy soil, in a semi-arid climate

Abstract. We report new information on tree water use by Mongolian Scots pine (Pinus sylvestris var. mongolica) growing on a sandy soil, in a region characterised by an erratic rainfall pattern. Measurements were made over three successive years of contrasting annual rainfall – a wet year (2013), a dry year (2014), and a second dry year (2015). The result was the development of worsening levels of drought year by year. Over the three years, sap flux density (J_s) was measured at individual tree level in up to 25 trees. The sap flux density values were up-scaled to estimate tree water use at plot level (T_s). Our measurements follow forest plot response to increasing levels of drought which developed over a three-year period as soil moisture conditions gradually worsened from wet, to moderate-drought, to severe-drought, to extreme-drought, in response to the dynamics of a variable rainfall pattern. Values of T_s did not exceed 3.03 mm day⁻¹ (2013), 1.75 mm day⁻¹ (2014) and 1.59 mm day⁻¹ (2015) during the three growing seasons. Total annual stand transpiration over the same three years declined progressively from 290 mm (2013), to 182 mm (2014) and to 175 mm (2015). Satisfactory power-function relationships (R² = 0.64) between daily T_s and the product of ET₀ and the relative extractable soil water (REW) were found. This study helps elucidate the interplay between the effects of the atmosphere and soil moisture on tree water use. Tree water use responded to drought, with daily T_s values decreasing by 5–46 % in response to moderate drought, by 48–62 % in response to severe drought and by 65 % in response to extreme drought. Upon release of moderate drought by heavy rainfall in 2013, daily T_s recovered completely. However, under the severe and extreme droughts in the subsequent dry years, recovery of T_s following heavy rainfall was incomplete (57–58 %). Our results highlight the negative effects of water stress on the growth of mature forest trees, in a sandy soil, in a climate characterised by large intra- and inter-annual variances in rainfall. When the erratic rainfall and sandy soil were also coupled with a declining groundwater table, the result was tree water use fluctuated widely over quite short time scales (months or weeks). Overall, our findings account for the observed premature degradation of these MP plantations in terms of an eco-hydrological perspective.