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Soil moisture measurements provide valuable information about the possible occurrence of drought episodes specific for tree species and are also used to assess the performance of water budget models. Soil temperature measurements provide information on the soil thermal characteristics such as the depth and duration of frost in soils, and can also be used to evaluate the simulation of heat transport throughout the soil profile.
In the five level II plots with intensive monitoring we measure soil moisture and soil temperature:
During spring 2010 CS616-moisture content reflectometers have been installed in three locations within every plot. At each location the sensors have been installed within the main rooting zone in fixed depth intervals of 0 - 20, 20 - 40 and 40 - 80 cm. At locations with a forest floor thickness of more than 5 cm, an extra sensor was installed in the forest floor layer. When forest floor thickness is less then 5 cm the extra sensor was installed in the first depth interval of 0 – 20 cm instead. At one of the three locations four temperature sensors were installed at the same depths as the moisture sensors. All sensors are connected to a central data logger that automatically registers measurements with a time interval of 6 hours.
Older sets of TDR-probes are present since 1997-2000 at all plots at one location (Wijnendale, Ravels, Groenendaal) or two locations (Brasschaat, Gontrode) within the plot. At every location TDR-probes have been installed at regular depth intervals with 2 probes per depth and with 5 to 7 depths per location. Measurements with these older TDR-probes are carried out manually using a Tektronix cable tester. Over time the measuring frequency varied. During the last years there was 1 measurement every 2 weeks.
Samplers for soil solution are installed at 3 randomly selected locations in each level II plot. Sampling is performed at 4 depths, corresponding with a different soil horizon: soil organic layer and mineral soil (A-horizon, B-horizon and C-horizon).
Soil solution from the soil organic layer is sampled with 4-6 zero-tension lysimeters per plot. A zero-tension lysimeter samples water percolating through the organic layer by gravitation. A zero-tension lysimeter consists of a PE tray installed below the organic layer, which is connected to a 2 litre PE collecting bottle by a silicone tube.
2. Mineral soil
Soil solution in the mineral soil is sampled at three depths (A-horizon, B-horizon and C-horizon) with at least 2 suction-cup lysimeters per depth installed at each of the 3 locations. A suction-cup lysimeter consists of a vertically placed PE tube with a porous ceramic cup at the bottom and a 2 litre collection bottle made from dark coloured glass, which is connected to the tube by two silicone tubes. The lysimeters are placed to underpressure (60 cbar) with a vacuum pump two days before sampling.
Soil solution samples are collected two times per month. The volume collected by each individual sampler is measured. Then for each of the 4 depths a mixed sample is composed by mixing the total volume of all samplers. From the total volume a 500 ml sample is transported to the laboratory for chemical analysis.
The water budget of forests is one of the most important factors concerning tree vitality and forest condition. Moreover, the determination of soil water fluxes is of major importance in understanding a number of physiological processes like nutrient uptake, growth and response to biotic stress factors. The impact of environmental stressors like drought may result in reduced tree growth, increased occurrence of pests and diseases as well as deterioration of crown condition. Especially in a changing climate, water availability together with nutrient supply and the detection of drought stress are important issues for forest health.
Water budget models are commonly used to quantify water availability and the different components of the water balance in forest ecosystems. In the framework of the Life+ FutMon Project INBO participated in the development and implementation of water budget models on intensive monitoring plots. A water budget model comparison action was carried out in order to standardise the calculation of water budgets and provide recommendations and guidelines for the appropriate use of water budget models in European forests. The implementation of such models in Flemish level II plots is currently ongoing. Once available, the results of these models will help to gain insight in the relations between soil hydrology, tree vitality and forest condition.
A first detailed pedological characterisation was carried out in 1991 and profiles were classified according to Soil Taxonomy (Soil Survey Staff, 1991) and the revised FAO legend (FAO 1988). Laboratory analyses were restricted to organic matter content, pH and textural analysis.
A second detailed profile description, chemical analyses of the horizon samples and classification following the World Reference Base for Soil Resources on 10 Level II plots was made by J.H. Mikkelsen in 2007 during the BioSoil Inventory (Mikkelsen et al. 2008).
In spring-summer 1991, the organic layer and the mineral soil at each observation plot were sampled according horizons. Samples were taken around 3 trees in the plot. They were taken in 4 directions around the tree at a distance of 1 crown radius from the stem, ½ crown radius and ¼ crown radius. The total number of samples for each horizon was 36. L, F and H layers were sampled together. Chemical analyses were carried out according to the reference methods of the ICP Forests soil Submanual (PCC – UNEP – UN/ECE, 1993), except for total nitrogen, for which Kjeldahl digestion was used.
In 2004, composite samples of the organic layers (OL, OF and OH) and at fixed depth intervals (0 – 5 cm, 5 – 10 cm, 10 – 20 cm, 20 – 40 cm and 40 – 80 cm) were taken. Analytical methods follow the reference methods described in the ICP Forests manual (FSCC, 2006).
Profile descriptions and analytical results have been reported to the central ICP Forests database. The Second European Forest Soil Condition report by the FSCC (De Vos and Cools, 2011) describes the results.