Workpackage 2: The influence of hydrology on water uptake and growth of woody riparian species
Participating partners:
Université Joseph Fourier – Grenoble 1(F) (3)
University of Lethbridge (Canada) (4)
Centre National de la Recherche Scientifique –Toulouse (F) (5)
3.1 Objectives
Comparison of the water consumption and water regulation of poplars in Grenoble and Toulouse over the 2002 growing season
3.2 Methodology and scientific achievements
Continuous monitoring of the sap flow and punctual measurement of the stomatal and hydraulic conductance were recorded under contrasting soil water conditions. For the black poplar, in Toulouse, the maximal Sap Flux Density is about 2.2 dm3/dm2/h, which mean 2.2 liters of water per a sap wood surface of 10x10 and per hour. In Grenoble , the sap wood density ranges between 2.4 dm3/dm2/h for the wet site and 1 to 0.5 dm3/dm2/h for the dry site, due to the construction of a dam. The drier conditions also drastically limit stomatal conductance and transpiration in poplar species (10 and 0.5 mmol/m2/s respectively under low soil water regime) relative to those found in humid riparian areas (270 and 4.25 mmol/m2/s in wet sites).
3.3 Socio-economic relevance and policy implication
Poplar wood lands occur in alluvial areas and are important for the water purification (both ground water and river water) and become buffer zones during the flood. On the fauna level, these wet areas are needed for fish and bird reproduction. The main problem is the diminution of the surface for these wet zone, whereas the quality of the ground water and river water are getting worst. Also river diversion and damming giving insufficient water during summer, can lead to willow and poplars dieback.
3.4 Discussion and conclusion
Poplars are thought to consume large amounts of water to support their rapid growth, but a closer analysis of their water use as seen by sap flow measurements modulates this notion. Their maximal sap flux density is about 2.5 dm3/dm2/h, which is not exceptional (some trees display up to 4 dm3/dm2/h), but they compensate by larger sapwood area. Their relative high water need can certainly explain their impact on alluvial water quality by capture of nutrients and minerals. Poplars control the water stress by stomatal regulation. It seems that this regulation can reduce the sap flux density by a factor of 4. But this diminution in evaporated water leads to the heating of the crown leaves. Poplars are very sensitive to cavitation, which lead to loss of leaves and branches during severe drought. Repetitive drastic conditions will altere population structure by reducing growth, performance and survival.
3.5 Plan and objectives for the next period
Comparisons between black poplar and white willow under the auspices of future research projects. Finance has been applied for under a proposed Integrated Project entitled RHEOSIM which has been submitted to FP6.