No abstract

In the hyperarid region of Northwest China, frequent variations in hydrological environments present challenges to the persistence of riparian plants. The main objective of this study was to determine whether two desert riparian species (Populus euphratica and Tamarix ramosissima) differed in their water uptake patterns and ecophysiological responses to fluctuating groundwater depths (GWDs). This study was conducted in typical desert riparian ecosystems in the downstream Heihe River basin, Northwestern China, where the GWD continuously increases during growing season. Stable oxygen composition (δ 18 O) in xylem water, soil water, and groundwater, as well as leaf water potential and gas exchange were monitored. Results showed that P. euphratica used a higher ratio of soil water, whereas T. ramosissima relied more on groundwater and deep soil water. As the GWD increased during the growing season, both species modified their water use patterns, but they did so differently, P. euphratica extracted an increasing proportion of deep soil water and groundwater, whereas T. ramosissima took an increasing ratio of groundwater at critical growth stages. P. euphratica exhibited decreases in its daily maximum photosynthetic rate (A max ) and stomatal conductance (g max ) as the GWD increased, whereas those of T. ramosissima changed little. In summary, both species shift to use greater ratio of more reliable water sources with the increasing GWD, but the switching of water sources could not sufficiently compensate for the impact of drought stress on gas exchange for P. euphratica.

The arid and semi-arid northwestern China has been undergoing ecological degradation and the efforts to reverse the ecological degradation have been undertaken for many years. Some shifting dunes have been fixed and the vegetation has been partially recovered in certain areas and the Mu Us Sandy Land in the Ordos Plateau is an example of the success. The present study attempts to reveal the relationships between the vegetation restoration and ecohydrology in the Mu Us Sandy Land. We continuously measured soil water content at 10-min intervals under three vegetation types (i.e., shifting dune, shrub-dominated community, and herb-dominated community) in the Mu Us Sandy Land from April 2012 to October 2013. The results show the infiltration coefficient increased with increased rainfall amount and eventually reached a stable value. Infiltration coefficients were 0.91, 0.64, and 0.74 in the shifting dune, in the shrub-dominated community, and in the herb-dominated community, respectively. Cumulative infiltration and soil texture are two vital factors affecting the depths of rainfall penetration. Only rainfall events larger than 35.0 mm could recharge soil water at the 60-80 cm layer in the herb-dominated community. Our results imply that the expected forward succession of restored vegetation may be destined to deterioration after reaching the climax simply because of following two facts: (1) soil water is mainly retained at shallower layer and (2) plant fine roots mainly distribute in deeper layer in the herb-dominated community.

Plant community structure responds strongly to anthropogenic disturbances, which greatly influence community stability. The changes in community structure, aboveground biomass (AGB), biodiversity and community stability associated with different management practices were studied with a three-year field investigation in a temperate steppe of Inner Mongolia, China. The species richness, Shannon-Wiener index, evenness, plant functional type abundance, AGB, temporal community stability, summed covariance, scaling coefficient and dominant species stability were compared among areas subjected to long-term reservation (R), long-term grazing (G), mowing since enclosure in 2008 (M) and grazing enclosure since 2008 (E). Site R had higher perennial grass abundance and lower species richness than sites G, M and E, although the AGB was not significantly different among the four sites. The species structure varied from a single dominant species at site R to multiple dominant species at sites G, M and E. The long-term reservation grassland had lower biodiversity but higher stability, whereas the enclosed grassland with/without mowing had higher biodiversity but lower stability. Different stability mechanisms, such as the compensatory dynamics, mean-variance scaling and dominant species stability were examined. Results showed that community stability was most closely related to the relative stability of the dominant species, which supports the biomass ratio hypothesis proposed by Grime.

Understanding the seasonality of the transpiration fraction (T/ET) of total terrestrial evapotranspiration (ET) is vital for coupling ecological and hydrological systems and quantifying the heterogeneity among various ecosystems. In this study, a two‐source model was used to estimate T/ET in five ecosystems over the Heihe River Basin. In situ measurements of daily energy flux, sap flow, and surface soil temperature were compared with model outputs for 2014 and 2015. Agreement between model predictions and observations demonstrates good performance in capturing the ecosystem seasonality of T/ET. In addition, sensitivity analysis indicated that the model is insensitive to errors in measured input variables and parameters. T/ET among the five sites showed only slight interannual fluctuations while exhibited significant seasonality. All the ecosystems presented a single‐peak trend, reaching the maximum value in July and fluctuating day to day. During the growing season, average T/ET was the highest for the cropland ecosystem (0.80 ± 0.13), followed by the alpine meadow ecosystem (0.79 ± 0.12), the desert riparian forest Populus euphratica (0.67 ± 0.07), the Tamarix ramosissima Ledeb desert riparian shrub ecosystem (0.67 ± 0.06), and the alpine swamp meadow (0.55 ± 0.23). Leaf area index exerted a first‐order control on T/ET and showed divergence among the five ecosystems because of different vegetation dynamics and environmental conditions (e.g., water availability or vapor pressure deficits). This study quantified transpiration fraction across diverse ecosystems within the same water basin and emphasized the biotic controls on the seasonality of the transpiration fraction.