Water limitation of understorey and canopy of a maritime pine stand
Plants react to water limitation species dependent. Isohydric plants are able to maintain nearly constant water potential while anisohydric plants change their water potential with respect to evaporative demand and soil moisture. Forest ecosystems often consist out of understory with anisohydric grasses and canopy with isohydric trees. Therefore, it is expected, that the compartments of a forest respond differently to drought conditions.
To address this question we examine a maritime pine stand (Pinus Pinaster Ait.) with purple moor grass (Molinia caerulea L. Moench) in the understorey. It is situated at the Le Bray site, near Bordeau and maintained by our project partners at INRA Bordeaux. Two eddy-covariance towers were installed at 7 and 40m and measured water vapour, carbon dioxide and sensible heat fluxes. Additional soil moisture measurements were carried out at several depths.
After data filtering, gap filling and partitioning of water and carbon fluxes water use efficiency will be calculated. First results show a clear water limitation during summer 2006 and suggest root systems with different depths of understorey and canopy. Further examinations will check hydraulic lift at this site, since tree root system reaches near a shallow ground water table.
Drought effects on primary production and water use efficiency of a mediterraneansavannah-type oak forest
The productivity of terrestrial ecosystems is limited by the availability of water in large areas of the world due to the close coupling of the water cycle to the carbon cycle by photosynthesis. Especially in arid regions the water availability is subject to severe fluctuations and limits the biomass production.
In our research we focus on the process based understanding of water and carbon fluxes of a typic mediterranean cork oak stand. Canopy and understorey will be treated as a two component system. Therefor, two Eddy Covariance towers in a portugese cork oak savannah acquire ecosystem flux data.
The soil-vegetation-atmosphere transfer model "CANVEG" is adapted concidering soil and plant parameters measured on the site and validated with the flux data.
Evapotranspiration partitioning using stable isotope ratios
The evapotranspiration (ET) of an ecosystem consists of transpiration through the stomata of plants and of evaporation from soil and plant surfaces. By determination of δH218O ratios of source and ET water the contribution of each process can be quantified.
In a portuguese cork oak stand we measure stable isotope ratios with on-site cavity ring-down spectrometry at different phenological stages and under changing environmental conditions using open soil and branch chambers.
LAI measurement and method development in an open, mediterranean cork oak stand
The leaf area index (LAI) is defined as the one sided leaf area per unit ground area of a single plant, canopy or whole ecosystem. It is one of the most important ecosytem characteristics determining ecosystem functions and properties, e.g. photosynthetic performance, aerodynamic roughness and so on. It is widely used in canopy models for the purpose of light distribution and scaling of carbon and water fluxes.
In an open, mediterranean cork oak stand we measure the LAI indirectly with digital photography based methods. The gap-probability theory derived from the Lambert-Beers-Law is applied on cover photos of the canopy resulting in gap-probability distributions. Embracing results from leaf inclination photos the LAI can be estimated.
The measurements are conducted in several heights and along different zenith angles within the canopy. The photos are processed with methods of object based image analysis explicitly concidering large in between tree gaps and branch and trunk area. The results are compared with the established LAI-2000 device.