Shiqin Xu

Abstract Although there is mounting concern about how high vapor pressure deficit (VPD) and low soil moisture (SM) affect ecosystem productivity, their relative importance is still under debate. Here, we comprehensively quantified the relative impacts of these two factors on ecosystem gross primary production (GPP) using observations from a global network of eddy‐covariance towers and two approaches (sensitivity analysis and linear regression model). Both approaches agree that a higher percentage of sites experience GPP reduction from high VPD than from low SM over the growing season. However, the constraint of high VPD and low SM on GPP reduction is tightly linked with climates and plant functional types. Humid and mesic ecosystems including forests and grasslands are dominated by VPD, while the semi‐arid and arid ecosystems including shrublands and savannas are dominated by SM. The varying dominant role of these two factors on GPP is closely related to plant stomatal behavior, as predicted by a stomatal conductance model. Additionally, we highlight the non‐linear impact of SM on GPP during droughts and the possible underestimation of the SM effects for deep‐rooted plants when only using surface‐layer SM. Our results shed light on a better understanding of the impacts of VPD and SM on vegetation productivity, with important implications for modeling the response and feedback of ecosystem dynamics to current and future climates.

Mountain regions are experiencing more pronounced climate change than other global land areas. How have vegetation dynamics responded to these changes and what are the implications for hydrology? To answer these questions, we examine the impacts of changes in mean air temperature ( T _mean ), precipitation ( P ) and winter snow cover extent ( SCE ) in the headwaters of the Yellow River basin (HYRB) on two important vegetation dynamic metrics: (i) the maximum growing-season greenness (represented by the monthly maximum NDVI); and (ii) the beginning of growing season (BGS). Satellite-derived NDVI and SCE , along with observation-based gridded climate data, show that during the past 34 years (1982–2015) the HYRB experienced widespread vegetation greening, while no significant trend in BGS was observed. Spring greenness and phenology were significantly affected by SCE change, highlighting the importance of snow-related process to spring vegetation activity. We observed a clear signal of elevation-dependent warming below 4300 m elevation, which is absent at higher elevations. Changes in NDVI and BGS are elevation-dependent, and trends in T _mean , P, and SCE with elevation play different roles in this dependence. Both observed and estimated watershed annual evapotranspiration series show increasing trends, suggesting that vegetation greening imposes positive effects on evaporative fluxes. Given steady-state and non-stationary hydrological conditions, increasing evapotranspiration should result in runoff reduction, which agrees with catchment-scale runoff observations across the HYRB. These findings represent new knowledge regarding the vegetation response to climate change in alpine environments which has important implications for the hydrology of the region and for other high-water yielding mountainous regions worldwide.

Abstract Conservation management for the water dependent desert‐oasis ecotone in arid northwest China requires information on the water use of the dominant species. However, no studies have quantified their combined water use or linked species composition to ecotone transpiration. Here, the water use of three dominant shelterbelt shrubs ( Haloxylon ammodendron , Nitraria tangutorum , and Calligonum mongolicum ) within an ecotone was measured throughout the full leaf‐out period for three shrub species from 30 May to 16 October 2014, with sap flow gauges using the stem heat balance approach. Species‐specific transpiration was estimated by scaling up sap flow velocities measured in individual stems, to stand area level, using the frequency distribution of stem diameter and assuming a constant proportionality between sap flow velocity and basal cross‐sectional area for all stems. The mean peak sap flux densities ( J sn ) for H . ammodendron , N. tangutorum , and C. mongolicum , were 40.12 g cm −2 h −1 , 71.33 g cm −2 h −1 , and 60.34 g cm −2 h −1 , respectively, and the mean estimated daily area‐averaged transpiration rates ( T daily ) for the same species were 0.56 mm day −1 , 0.34 mm day −1 , and 0.11 mm day −1 . The accumulative stand transpiration was approximately 140.8 mm throughout the measurement period, exceeding precipitation by as much as 42.1 mm. Furthermore, T daily of these shrubs appeared to be much less sensitive to soil moisture as compared to atmospheric drivers, and the relationship between J sn and atmospheric drivers was likely uninfluenced by soil moisture regimes in the whole profile (to 1‐m depth), especially for H . ammodendron and C. mongolicum . Results indicate that these shrubs may use deep soil water recharged by capillary rise, or may directly access shallow groundwater. This study provides quantitative data offering important implications for ecotone conservation and water and land resource management. Copyright © 2016 John Wiley & Sons, Ltd.