Aims The field of ecohydrology is providing new theoretical frameworks and methodological approaches for understanding the complex interactions and feedbacks between vegetation and hydrologic flows at multiple scales. Here we review some of the major scientific and technological advances in ecohydrology as related to understanding the mechanisms by which plant–water relations influence water fluxes at ecosystem, watershed and landscape scales.
Important findings We identify several cross-cutting themes related to the role of plant–water relations in the ecohydrological literature, including the contrasting dynamics of water-limited and water-abundant ecosystems, transferring information about water fluxes across scales, understanding spatiotemporal heterogeneity and complexity, ecohydrological triggers associated with threshold behavior and shifts between alternative stable states and the need for long-term data sets at multiple scales. We then show how these themes are embedded within three key research areas where improved understanding of the linkages between plant–water relations and the hydrologic cycle have led to important advances in the field of ecohydrology: upscaling water fluxes from the leaf to the watershed and landscape, effects of plant–soil interactions on soil moisture dynamics and controls exerted by plant water use patterns and mechanisms on streamflow regime. In particular, we highlight several pressing environmental challenges facing society today where ecohydrology can contribute to the scientific knowledge for developing sound management and policy solutions. We conclude by identifying key challenges and opportunities for advancing contributions of plant–water relations research to ecohydrology in the future.

Aims Beech (Fagus sylvatica L.) is an important species in natural and managed forests in Europe. This drought-sensitive species dominates even-aged stands as well more natural stands composed of a mixture of tree species, age and size classes. This study evaluates the extent that heterogeneity in spacing and tree diameter affect the seasonal availability and use of water.
Methods Two stands were evaluated: (i) a heterogeneous forest remnant (NAT) with trees up to ~300 years old, a mean top height of 28.4 m and a total of 733 stems ha^-1 with stem diameters averaging 18 cm and (ii) an even-aged 80-year old stand (MAN), with a height of 25 m, and a total of 283 stems ha^-1 with diameters averaging 38 cm. Stem sap flow, J s (g m ?2 s^-1), was continuously measured in 12 (MAN) and 13 (NAT) trees using 20-mm long heat dissipation sensors. Individual tree measures of sap flow were correlated using non-linear statistical methods with air vapour pressure deficit (D, hPa) and global radiation (R g, J m ?2 day^-1), along with constraints imposed by reductions in soil water content (SWC). SWC was measured as volumetric % using time domain reflectometry.
Important findings The daily integrated J s (J s-sum) for trees growing in the evenly spaced MAN stand and trees in canopy and closed forest positions in NAT stand decreased as the availability of soil moisture was reduced. In the heterogeneous NAT stand, SWC in a recently formed canopy gap remained high throughout the vegetation period. Based on regression models, the predicted relative decrease in J s-sum for dry relative to moist soil water conditions in the closed forest (at mean daily D = 10 hPa) was 7–11% for trees near the gap and 39–42% for trees in the closed forest. In MAN, the reduction in J s -sum was 29% in dry relative to moist conditions. J s -sum in the outer 20 mm of the xylem in NAT was lower than that in MAN and the rate of decline in J s with xylem depth was less in NAT than in MAN. In MAN, J s -sum in deep and outer xylem was negatively affected at low soil moisture availability; in NAT, this was the case for only the outer xylem indicating that deep roots could be important in supplying water at times of low soil moisture in the upper soil.

Aims In the mid- and high-latitude regions, three quarters of the land surface is covered by boreal conifer forests, and snow lasts for 6–8 months of the year. Correctly modeling surface energy balance and snowmelt at mid- and high-latitudes has a significant influence on climate and hydrological processes. However, the heterogeneous and clumped forest structure exerts important control over the radiative energy at the forest floor, which results in large variations of underneath snow cover and snowmelt rate. The goal of this study is to investigate the impact of hierarchically clumped vegetation structure in boreal forest on snowmelt and exchanges of energy and water.
Methods We used a simple Clumped Canopy Scheme (CCS) for canopy radiation transfer to characterize the impact of the clumped forest structure on net radiation at the snow surface underneath forests. The CCS was integrated with the Variable Infiltration Capacity macroscale hydrological model (herein referred to as VIC-CCS) to characterize the impact of clumped vegetation structure on surface energy balance and snowmelt during the snow season. A twin simulation, VIC-CCS and the standard VIC model, was performed to isolate the impact of CCS on the energy and water fluxes and snowmelt rates. The simulation results were compared to in situ measurements at four different forest stands: old aspen forest in the Southern Study Area (SOA), black spruce forests in the Southern and Northern Study Areas (SOBS and NOBS) and fen wetland in the Northern Study Area (NFEN) within the Boreal Ecosystem–Atmosphere Study (BOREAS) region in central Canada during 1994 to1996.
Important findings Simulations showed that the implementation of CCS has reduced incoming long-wave radiation at the underlying snow surface and, thereby, lowered the snowmelt rate. Comparison against ground observations of net radiation and surface flux rates showed a reasonable agreement while demonstrating implementation of CCS can markedly improve model surface energy budget and energy inputs computation for snowmelt. The modeled snowmelt matches reasonably well with observations with root mean square error (RMSE) ranging from 16.51 to 19.81 mm using VIC-CCS versus 29.86 to 32.61 mm for VIC only in the four forest sites. The improvement is the most significant for the deciduous forest (old aspen) site, reducing RMSE by16 mm. This study demonstrates that taking into account the effect of the clumped forest structure in land surface parameterization schemes is critical for snowmelt prediction in the boreal regions.

Aims Evapotranspiration (ET) is a key component of water balance and is closely linked to ecosystem productivity. In arid regions, large proportion of precipitation (PPT) is returned to the atmosphere through ET, with only a small amount available to plants. Our objective was to examine the variability in ET–soil water relationship based on a set of ecosystems that are representative for semi-arid Inner Mongolia and its main land use practices.
Methods This study used Eddy covariance (EC) data of water vapor (i.e. ET, mm), PPT (mm), soil volumetric water content (VWC, %), root biomass density and soil properties from three paired sites in semi-arid Inner Mongolia: cropland (Cropland-D) versus undisturbed grassland (Steppe-D), grazed grassland (Grazed Steppe-X) versus fenced grassland (Fenced Steppe-X) and poplar plantation (Poplar-K) versus undisturbed shrubland (Shrubland-K). The paired sites experienced similar climate conditions and were equipped with the same monitoring systems.
Important findings The ET/PPT ratio was significantly lower at Cropland-D and Grazed Steppe-X in comparison to the undisturbed grasslands, Steppe-D and Fenced Steppe-X. These differences are in part explained by the lower VWC in the upper soil layers associated with compaction of surface soil in heavily grazed and fallow fields. In contrast, the ET/PPT ratio was much higher at the poplar plantation compared to the undisturbed shrubland because poplar roots tap groundwater. The VWC of different soil layers responded differently to rainfall events across the six study sites. Except for Poplar-K, ET was significantly constrained by VWC at the other five sites, although the correlation coefficients varied among soil layers. The relative contribution of soil water to ET correlated with the density of root biomass in the soil (R 2 = 0.67, P < 0.01). The soil water storage in the upper 50 cm of soil contributed 59, 43, 64 and 23% of total water loss as ET at Steppe-D, Cropland-D, Shrubland-K and Poplar-K, respectively. Our across-site analysis indicates that the site level of soil water for ET differs between land use and land cover type due to altered root distribution and/or soil physical properties. As a result, we recommend that ecosystem models designed to predict the response of a wide variety of vegetation to climatic variation in arid regions include more detail in defining soil layers and interactions between evaporation, infiltration and root distribution patterns.

Aims The Miyun Reservoir is the most important drinking water source for Beijing—the capital of China with a population of more than 16 million. Since the 1980s, the inflow to the reservoir has been decreasing, which seriously threatens the security of water use in Beijing. Our goal was to analyze the impact of land use and cover change (LUCC) on run-off yield in the upstream of the Miyun Reservoir.
Methods In this study, the Soil andWater Assessment Tool (SWAT) was used to simulate the impacts of LUCC on the run-off yield in the Bai River catchment—upstream of the Miyun Reservoir basin in northern China. The investigation was conducted using two 6-year historical streamflow records: from 1986 to 1991 and from 2000 to 2005. A split sample procedure was used for model calibration and validation. The data from 1986 to 1988 and from 2000 to 2002 were used for calibration, while those from 1989 to 1991 and from 2003 to 2005 for validation. The SWAT calibration was based on monthly measured discharge at Zhangjiafen station at the catchment outlet from Bai River catchment. Additionally, the influence of LUCC on the surface run-off was distinguished from that of climate change on the surface runoff through SWAT scenarios modeling, the two-way analysis of variance (ANOVA), and the rainfall–run-off double-mass analysis in the Bai River catchment.
Important findings We found that the SWAT model could be used successfully to accurately simulate run-off yield and different LUCC patterns affecting water quantity in this catchment. During calibraion for the two periods the simulated monthly run-off satisfactorily matched the observed values, with the Nash–Sutcliffe coefficient>0.9 and 0.7 and a coefficient of determination of 0.9 and 0.65 at the outlet station (Zhangjiafen station), while during validation for the two periods the obtained values were 0.85, 0.65 and 0.9, 0.65, respectively. During the period of 1986–91, both the SWAT scenarios modeling and the analysis of the two-way ANOVA method showed that LUCC and climate change had some impact on run-off, and the impact of climate change was more significant than that of LUCC. Compared with the period during 1986–91, the run-off yield in the period during 2000–05 significantly decreased. The obtained results from the rainfall–run-off double-mass analysis indicate that since 1998 LUCC has had an increasing influence on the run-off, while the response of the run-off to rainfall has been decreasing. Since 1998, the LUCC has been a major driving force for run-off change in Bai River catchment.

Aims In recent years, there has been an increased interest in examining changes in forest systems in response to drought, flooding, hurricanes and climate change. In the southern United States, forested wetlands are of special interest because of the extent of these forests. Coastal plain forested wetlands are among the most vulnerable to these climatic impacts. One of the problems in developing management practices for these coastal areas is the difficulty in adequately describing productivity relations and predicting how the structure and function of these communities might be affected by natural or anthropogenic disturbances. Community response to environmental change often occurs over a period of years, and the majority of reported studies are for 1–3 years in duration. This study documents long-term changes (10 years) in structure, composition and growth along a catena of high water table forested sites of an ancient beach ridge landscape in coastal South Carolina.
Methods Aboveground net primary production (ANPP) of trees was monitored from 2000 to 2009 on three sites within a longleaf pine-swamp blackgum forest system on the southern end of the Waccamaw Neck area of Georgetown County, SC. Permanent study plots (20 × 25 m) were established across a moisture gradient (Dry, Intermediate, and Wet). Water levels were continuously monitored, litterfall was measured monthly and growth of trees ≥10 cm diameter at breast height was monitored on an annual basis. Annual litterfall and tree production values were summed to provide estimates of ANPP.
Important findings The study site was under severe drought conditions July 2001 through late summer 2002 and again in 2007. Diameter growth was affected in all three sites, but with different patterns. It seems that diameter growth in the Wet site was more sensitive to drought conditions in 2001–02 and 2007 than either Dry or Intermediate sites. While droughts did not seem to have a significant impact on litterfall in the Wet site, litterfall in the Intermediate site was more sensitive to the drought than either Dry or Wet sites. ANPP was significantly lower in both Intermediate and Wet sites in 2001 at ≤602 g/m 2. Highest ANPP (>1?000 g/m 2) occurred in the Intermediate and Wet sites in 2003 following a return to more normal water levels at the end of the drought. Maximum tree production occurred on the Wet site in 2003 (657 g/m 2), which exceeded total ANPP of any site in 2001. In the Dry site, ANPP remained relatively consistent throughout the study when compared to Wet and Intermediate sites. While litterfall estimates are well defined with 3–5 years of data, data collection is continuing to assess impact of drought on stem growth across the gradient, which is still not clear with 10 years of data.

Aims Since 2000, the environmental flow controls project has been implemented in the lower Heihe River Basin, a typical arid inland river basin in northwest China, to restore the deteriorated ecological environment in this region. The aim of this study was to explore the impacts of groundwater fluctuations on vegetation dynamics. Our results can be used as a reference for water resources planning and management to maintain proper environmental flows in arid areas.
Methods The location (by Global Positioning System) and depth of the monitoring wells, as well as groundwater table depth and salinity were measured in situ at each site from July to August 2009. Based on the measurements of the groundwater table depth and salinity following the implementation of environmental flow controls project (EFCP) in the lower Heihe River Basin, the groundwater fluctuations during the period from 2001 to 2009 were analyzed. Descriptive statistics and Pearson's correlation were used to analyze the relationship between vegetation changes and groundwater table fluctuations. Additionally, the spatial distributions of the groundwater table depth and salinity were interpolated using the simple kriging method. Trend analysis was applied to the time series of integrated Moderate Resolution Imaging Spectroradiometer normalized difference vegetation index data to identify interannual vegetation dynamics. The relationship between vegetation status and groundwater environment was investigated at different spatial scales by analyzing and comparing the time series and trends.
Important findings (i) The groundwater table and salinity increased significantly in most of the study area with spatial heterogeneity. On average, the groundwater table rose ~0.5 and 1.5 m in the upper and lower Ejina Basin, respectively, and the groundwater salinity increased across the study area by 0–4%. (ii) A notable correlation between the vegetation status and the groundwater table was revealed when the groundwater table depth fluctuated between 1.8 and 3.5 m, whereas the vegetation did not show an obvious response to groundwater table changes when the groundwater table depth was more than 5–6 m. (iii) Vegetation restoration mainly occurred in riparian areas within 500–1 000 m of from natural rivers, where the groundwater table depth varied from 2 to 4 m, and salinity was <5%, whereas vegetation degradation appeared at some locations where groundwater environment had deteriorated.

Aims The Amazon basin plays an important role in the global carbon budget. Interannual climate variability associated with El Ni?o can affect the Amazon ecosystem carbon balance. In recent years, studies have suggested that there are two different types of El Ninos: eastern-Pacific (EP) El Ni?o and central-Pacific (CP) El Ni?o. The impacts of two types of El Ni?o on the Amazon climate and Amazon ecosystem are analyzed in the study.
Methods A composite method has been applied to highlight the common features for the EP- and CP-El Ni?o events using observational data, IPCC-AR4 model output. Potential impacts of the two different types of El Ni?o on ecosystem carbon sequestration over the Amazon have been investigated using a process-based biogeochemical model, the Biome–BioGeochemical Cycles model (Biome–BGC).
Important findings Below-normal rainfall is observed year round in northern, central and eastern Amazonia during EP-El Ni?o years. During CP-El Ni?o years, negative rainfall anomalies are observed in most of the Amazon during the austral summer wet season, while there is average or above-average precipitation in other seasons. EP- and CP-El Ni?o events produce strikingly different precipitation anomaly pattern in the tropical and subtropical Andes during the austral fall season: wetter conditions prevail during EP-El Ni?o years and drier conditions during CP-El Ni?o years. Temperatures are above-average year round throughout tropical South America during EP-El Ni?o events, especially during austral summer. During CP-El Ni?o events, average or slightly above-average temperatures prevail in the tropics, but these temperatures are less extreme than EP year's temperature except in austral fall. These precipitation and temperature anomalies influence ecosystem productivity and carbon sequestration throughout the Amazon. Using the Biome–BGC model, we find that net ecosystem production (NEP) in the EP-El Ni?o years is below average, in agreement with most previous studies; such results indicate that the Amazon region acts as a net carbon source to the atmosphere during EP-El Ni?o years. In the CP-El Ni?o years, NEP does not differ significantly from its climatological value, suggesting that the Amazon forest remains a carbon sink for the atmosphere. Thus, even if CP-El Ni?o events increase in frequency or amplitude under global warming climate as predicted in some Global Climate Models, the Amazon rainforest may remain a carbon sink to the atmosphere during El Ni?o years in the near future.

Aims Changing climate and land use patterns make it increasingly important that the hydrology of catchments and ecosystems can be reliably characterized. The aim of this paper is to identify the biophysical factors that determine the rates of water vapor loss from different types of vegetation, and to seek, from an array of currently available satellite-borne sensors, those that might be used to initialize and drive landscape-level hydrologic models.
Important findings Spatial variation in the mean heights, crowd widths, and leaf area indices (LAI) of plant communities are important structural variables that affect the hydrology of landscapes. Canopy stomatal conductance (G) imposes physiological limitation on transpiration by vegetation. The maximum value of G (G max) is closely linked to canopy photosynthetic capacity, which can be estimated via remote sensing of foliar chlorophyll or nitrogen contents. G can be modeled as a nonlinear multipliable function of: (i) leaf–air vapor pressure deficit, (ii) water potential gradient between soil and leaves, (iii) photosynthetically active radiation absorbed by the canopy, (iv) plant nutrition, (v) temperature and (vi) the CO₂ concentration of the air. Periodic surveys with Light Detection and Ranging (LiDAR) and interferometric RADAR, along with high-resolution spectral coverage in the visible, near-infrared, and thermal infrared bands, provide, along with meteorological data gathered from weather satellites, the kind of information required to model seasonal and interannual variation in transpiration and evaporation from landscapes with diverse and dynamic vegetation.