LAUSR

Abstract In the semiarid sandy region of Northeast China, non-native woody species such as Pinus sylvestris var. mongolica L. and Populus x xiaozhuanica W.Y. Hsu & Liang frequently suffer dieback during years of extreme drought events due to water deficiency. In contrast, dieback does not usually occur in native woody species, e.g., Pinus tabuliformis Carr., Ulmus pumila L., and shrub Caragana microphylla Lam. However, the mechanism of water-use patterns between the non-native and native woody species remain unclear, which limits our understanding of the mechanisms underlying dieback of non-native woody species. During two consecutive growing seasons in 2014 and 2015, sources of water uptake by two non-native (P. mongolica and P. xiaozhuanica) and three native (P. tabuliformis, U. pumila and C. microphylla) woody species were determined using the stable isotopes of δ2H and δ18O in the twig xylem water, soil water at various depths (0–200 cm), groundwater (water from capillary fringe or beneath the water table) and precipitation, coupled with soil moisture and groundwater level measurements. Results showed that both P. mongolica and P. tabuliformis trees used the 0–100 cm soil water in the spring and the 0–40 cm and 0–100 cm soil water in summer of 2014 and autumn of 2015, respectively, when soil moisture was high. In contrast, in summer of 2015 and autumn of 2014, when soil moisture was low, P. mongolica trees used both the 0–100 cm soil water and groundwater, whereas P. tabuliformis trees used the 0–200 cm soil water and both the 40–200 cm soil water and relatively limited groundwater, respectively. The P. xiaozhuanica trees used the 40–200 cm or the 100–200 cm soil water and groundwater, whereas U. pumila trees mainly used the 0–100 cm or 0–200 cm soil water, except in autumn when soil moisture was high. P. xiaozhuanica trees used the 0–200 cm soil water, while U. pumila trees used the 0–40 cm soil water. The shrub C. microphylla mainly used the 0–200 cm soil water during the measurement period irrespective of soil moisture conditions. Compared with the native woody species, which mainly used soil water, the non-native tree species relied on groundwater besides soil water, especially under low soil moisture conditions. Therefore, a sharp decline in groundwater level during extreme drought years might have a strong negative impact on the growth and survival of non-native woody species.