Sap flow and water transfer in the Garonne river riparian woodland, France : first results on poplar and willow

This work is the first attempt at using Granier sap sensors on Populus nigra, Populus x euramericana cv I45/51 and Salix alba for the monitoring of sap flows in an active floodplain over two consecutive years. The main characteristic of these diffuse porous trees is their capacity to use several tree rings for xylem sap transfer. Results showed that the sap flux densities remained homogeneous on the external 4 cm of the trunk, then decreased with depth. For young trees, the active sapwood can represent half of the trunk. Results indicated that in the same environment and at the same age, daily differences existed between the two major native riparian tree species, the black poplar and the white willow. Their maximal sap flux density (2.6-3.6 dm 3 dm -2 h -1) was similar to other fast growing trees. The influence of age was the third important screened factor. Sap flow measurements over several months indicated that water uptake was variable throughout the season, depending on water availability, and was more pronounced for older trees. The sap flux densities for the planted poplar (I45/51) ranged from 2.2-2.6 dm 3 dm -2 h -1 (about 90 dm 3 day -1) in the wetter spring conditions and dropped to 1.6-1.7 dm 3 dm -2 h -1 (about 60 dm 3 day -1) in less favourable conditions. Under the worst conditions, e.g., the especially long drought in the summer of 1998, these values dropped to 1.0-1.2 (about 40 dm 3 day -1), and even to 0.35 dm 3 dm -2 h -1 (about 12 dm 3 day -1) for a few days. Complementary long-term studies are needed to better understand the complex sap flow changes and to be able to relate them to significant environmental factors. Priority should be given to the long-term monitoring of sap flows at different depths for a correct estimation of actual daily water uptakes by riparian softwood trees

Monitoring of water from the underground to the tree: first results with a new sap extractor on a riparian woodland

Riparian woodlands are characterized by variable hydrological conditions. Following the mapping of the complex underground water circulation of the wetlands, we studied the water uptake by trees. Although there are numerous analytical techniques available to monitor the water origin and water fluxes, no rapid technique for the extraction of xylem sap exists on the market. For this reason we designed and built a unique machine able to extract sap directly in the field from wood cores in a few minutes. A short description of the machine and its performance is given, prior to reporting the first experimental results obtained in a young riparian woodland along the Garonne River. The results compare the vertical water profile of the soil and the corresponding xylem sap at different roots horizons and in the trunk

Daily variations of water use with vapor pressure deficit in a plantation of I214 poplars.

Daily rates of whole tree water use were measured in a homogeneous 7-year-old plantation over 84 days. Two trees were selected in the two most contrasting ends, i.e., with mean water table at -1.5 m and -3.5 m. Results showed that the mean tree water use was close to 80 dm3 day–1 with a small mean difference between the two sites (5.4 dm3 day–1, about 7%). Higher daily variations were observed on a single tree over the season (from 17 to 138 dm3 day–1). These fluctuations could be modeled using linear regressions on the vapor pressure deficit (VPD). Results also indicated decreasing sap flux densities with radial sapwood depth and that the whole tree water use could be estimated from the sap flux densities measured at the 0–2 cm depth

Mélange des eaux de la nappe phréatique et de la rivière

Mélange des eaux de la nappe phréatique et de la rivièr

Correlation of conductivity and stable isotope 18O for the assessment of water origin in river system

When working on wetlands, or other surface and subsurface water systems with multiple water sources, it is not always easy to identify the mixing between the different waters. Here, we have tried to adapt an easy technique, i.e., conductivity, but regularly standardised against stable natural isotope 18O, to immediately give a glimpse on the field site of the percentage the two kinds of water. In certain cases, this method is also valid to discriminate a third source of water. The first experimental site to be described is in a meander of the Garonne river in south-west France, where there exists an interesting case of active paleo-channels. The second site is located in North India, in the Himalayas mountains, Garhwal, at the source of the Ganges river. Here, the samples were taken to discriminate between snow and glacier melt

The Guianese paradox: How can the freshwater outflow from the Amazon increase the salinity of the Guianan shore?

French Guiana is notable for the extent of its rain forests, which occupy 97% of the country, and the influence of the Amazon along its shores. In fact, the shores and estuaries support a mangrove forest typical of saline conditions. This paper reports the chemical characteristics, conductivity and salinity and the stable isotopes (oxygen and deuterium) of the rivers and shores between the Cayenne area and the border with Surinam. The results show a quite homogenous freshwater pool over the country. However, the low slope of the coast, a result of the wide mud banks deposited by the Amazonian plume, have turned the mouths of the smaller rivers to the northwest, creating large salty areas where mangroves grow several kilometers inland. Despite the large amount of Amazonian water, the Guianan coast exhibits high salinity. In fact, the freshwater itself remains far from the shore, following the north Brazilian current, while only the mud plume arrives at the coast, creating this paradox

Interactions between groundwater and surface water at river banks and the confluence of rivers

Riparian vegetation depends on hydrological resources and has to adapt to changes in water levels and soil moisture conditions. The origin and mixing of water in the streamside corridor were studied in detail. The development of riparian woodland often reflects the evolution of hydrological events. River water levels and topography are certainly the main causes of the exchange between groundwater and river water through the riverbank. Stable isotopes, such as 18O, are useful tools that allow water movement to be traced. Two main water sources are typically present: (i) river water, depleted of heavy isotopes, originating upstream, and (ii) groundwater, which comes mainly from the local rainfall. On the Garonne River bank field site downstream of Toulouse, the mixing of these two waters is variable, and depends mainly on the river level and the geographical position. The output of the groundwater into the river water is not diffuse on a large scale, but localised at few places. At the confluence of two rivers, the water-mixing area is more complex because of the presence of a third source of water. In this situation, groundwater supports the hydrologic pressure of both rivers until they merge, this pressure could influence its outflow. Two cases will be presented. The first is the confluence of the Garonne and the Ariège Rivers in the south-west of France, both rivers coming from the slopes of the Pyrénées mountains. Localised groundwater outputs have been detected about 200 m before the confluence. The second case presented is the confluence of the Ganges and the Yamuna Rivers in the north of India, downstream of the city of Allahabad. These rivers are the two main tributaries of the Ganges, and both originate in the Himalayas. A strong stream of groundwater output was measured at the point of confluence

The use of the stable oxygen isotope (18O) to trace the distribution and uptake of water in riparian woodlands

Streamside vegetation forming narrow "corridors" in temperate regions, is typically dominated by deciduous tree species reflecting strong influences by human activities. Riparian woodlands depend on hydrological resources and have to adapt to rapid changes in water levels and soil moisture conditions. Three main water sources are typically present in the riparian zone: river water originating in the mountains, ground water and rainfall. Stable isotopes, such as oxygen-18, are useful tools which allow for water movement to be traced within the riparian zone and which help to identify water sources utilised by the trees growing in these areas

Climate change and water availability in north-west Algeria: investigation by stable water isotopes and dendrochronology

Since the 1970s, rainfall has declined along the North African coast, while the demographic pressure has increased. Supplementing the rainfall data and water level of the Béni Bahdel dam, water isotopic signature and tree ring analyses were used to better understand the effects of climate change (lower rainfall, higher summer temperature) and the water circulation in the Tafna River basin in north-west Algeria. Changes are recommended in water storage and afforestation policies and irrigation techniques

Sap flow measurements of Ceriops tagal and Rhizophora mucronata mangrove trees by deuterium tracing and lysimetry

Mangrove forest trees grow in severe conditions such as diurnal submersion and high salinity surface and subsurface waters. This study focuses on two species on Mayotte Island, i.e. Ceriops tagal and Rhizophora mucronata, living in the middle range of the coastal mangrove. The seedlings of these trees were planted in a tropical greenhouse with an original pump system built to reproduce the natural tidal effect. The water used by these saplings, in two contrasted salinity conditions, was measured by lysimetry. For adult species, the trees’ water consumption was measured on the field side after being injected with heavy water (D2O). Our work shows that this isotopic technique also works in saline conditions, and a water consumption of around 1 ± 0.2 L per day and per centimeter of diameter was found. These values are discussed as follows: the techniques used, the distinctive features of the mangrove trees, and other factors affecting the water absorption