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Drought-associated tree mortality and forest decline have been increasingly observed across the globe due to warmer and drier climates, particularly in drought-prone environments. Understanding how forests respond to the increasing inter-annual climate variability and frequent severe drought events in the context of global change is of great significance for predicting the dynamics of forest ecosystems under future warming-drying climate scenarios. This study combined tree-ring series, xylem anatomy, wood isotope signatures, and remote sensing methods to examine the physiological mechanisms underlying the response of natural H. davidii forests in Horqin Sandy Land, a water-limited area in northern China, to the identified severe drought events and long-term climatic variability. Our results indicated that the climatic variables related to environmental moisture conditions exert the most profound influence on the performance of the studied species, with severe drought events significantly reducing growth vigor. The concurrent reduction in water-transporting conductivity and increase in water-use efficiency signified the prevalence of a more conservative water-use strategy in response to severe droughts at the holistic level of the whole tree, which physiologically accounts for the increased hydraulic safety and diminished radial growth during the period. Remotely sensed vegetation indices appeared to respond less conspicuously to severe drought events compared to radial growth, xylem anatomical traits, and water use efficiency. Nonetheless, a coordinated response to climatic variability between tree canopy and individual-level performance was found over the long-term time series, implying that a multiscale approach enhances our comprehension of the long-term forest responses to climatic variability. In short, this study highlights the role of water-related physiology in modulating the performance of H. davidii trees during severe droughts by adjusting xylem hydraulic functioning and tree water use efficiency. The findings lay a theoretical foundation for the use of native tree species for sustainable afforestation in vulnerable sandy land environments. • Multiple approaches were used to examine Hemiptelea davidii forest responses. • Inter-annual responses to climate at individual and stand levels were coordinated. • Water exerts the strongest influence on forest performance of different levels. • Xylem hydraulics and water use efficiency adjusted significantly during droughts. [ABSTRACT FROM AUTHOR]

• Hydraulic limitation underlies the crown dieback of Populus trees in windbreaks. • Higher branches show symptoms of greater drought stress. • Higher branches have lower stem hydraulic conductivity and increased embolism risk. • Higher branches show higher leaf and stem soluble sugar contents. Populus trees play an important role in windbreak construction in northern China, but widespread decline and mortality of Populus windbreaks have occurred in recent years. The phenomenon of die-backs from the top of Populus trees downwards is commonly found in declining windbreaks preceding tree deaths, particularly in water-limited areas. To provide a mechanistic explanation for such top-down diebacks, we analyzed the variation in xylem hydraulics, leaf water relations, and tissue non-structural carbohydrate contents (NSC) along the vertical axes of Populus pseudo-simonii trees, i.e. one of the most commonly used tree species for creating windbreaks in northern China. Consistent changes were observed along the height gradient of the tree crown, i.e. with the height increase branches showed a clear trend of decrease in hydraulic efficiency and leaves showed symptoms of greater drought stress. The stem hydraulic conductivity standardized using whole cross-sectional wood area decreased from 4.66 at the bottom to 1.69 kg m−1 s−1 MPa−1 at the top of the tree crown. Such a change in hydraulic efficiency was accompanied by significant increase in the degree of xylem embolism (i.e. from < 10% at the bottom to about 25% at the top of the tree crown) and an overall reduction in hydraulic safety margin with branch height increase. Lower xylem hydraulic efficiency and symptoms of less favorable water status, however, did not result in reduced contents of tissue NSC in branches of higher positions. Rather, the stem and leaf soluble sugar contents both showed significant positive correlations (P < 0.01, linear regressions) with sampling heights and the starch contents largely remained stable along the height gradient. Our results highlight that greater risk of hydraulic failure rather than carbon depletion is underlying the commonly observed crown dieback in Populus windbreaks. [ABSTRACT FROM AUTHOR]

• Soil moisture significantly decreased with the stabilization of sand dunes. • Shrubs of different dune fixation stages showed contrasting functional traits. • High hydraulic efficiency and prodigal water use allowed success in active dunes. • Strong hydraulic safety and water conservation enabled persistence in fixed dunes. Dominating sand-fixing shrubs play crucial roles in the stabilization and rehabilitation of sand dunes. Different sand-fixing shrub species often separate along the temporal-spatial environmental gradient during the sand dune stabilization process; however, the physiological mechanisms underlying such a separation remain poorly understood, which limits our ability to identify the causes of land desertification and the cruxes of rehabilitation. We investigated xylem hydraulics and water use characteristics of four important shrub species used for sand dune fixation projects in northern China that show distinct preferences to different stages of dune stabilization, i.e. two species succeed in active dunes and the other two in fixed dunes. The major aim was to examine the roles of xylem hydraulics, water use and the coordination of these two aspects in determining the habitat preferences of sand-fixing shrubs along the process of dune stabilization in water-limited environments. The two active-dune species consistently exhibited higher stem hydraulic conductivity but lower resistance to drought-induced xylem embolism than the two fixed-dune species, which reflects contrasting requirements to shrub hydraulic functionality in sand dunes of the two successional stages that differ substantially in soil water regimes. In coordination with contrasts in hydraulics, they also diverged clearly in water use strategies with the fixed-dune shrubs showing more conservative water use. Our results highlight the critical roles that hydraulics and water utilization play in determining the adaptation of dominating sand-fixing shrub species to their respective environments shaped by the plant-soil interactions during sand dune vegetation development. [ABSTRACT FROM AUTHOR]