We completed the data already available about Haut-Languedoc forest soils through the digging and the study of 100 soil pits, from which we determined the available water capacity (AWC) of the soil types we encountered. The AWC is the soils capacity to store water and return it to trees. This crucial data will help us to localize areas where forests are more vulnerable to climate change. It can be estimated through the soil properties (texture, organic matter proportion...) through pedotransfer functions. Such functions are widely used in the agriculture context but need to be precised for forest soils, and adapter to the Haut-Languedoc local context.
This is a very ambitious part of the project that is linked to a strong scientific issue!
Once we identified the main Haut-Languedoc Regional Nature Park pedo-climatic regions (see step 2), we scattered our soil pits across the territory so that they took place in topographic conditions as varied as possible, notably concerning slope and exposition.
Map of plot localizations in the Haut-Languedoc territory
In order to limit root architecture variability which could introduce non-desired noise in the results, all the sampling plots were located under the same forest type: Pseudotsuga menziesii (douglas fir) high forest. This tree species was chosen because 1) it is largely planted upon the studied territory and upon different soil types, 2) it is involved in a high production issue that is linked to climate change, and 3) such forests are sparse enough to allow the soil pits digging without damaging the forest population.
In each of our 100 plots, we gathered data at three different scales.
We formed forest plots by choosing 20 trees spirally dispersed from a centre localized near the soil pit. For each tree, we identified their sanitary status using the ARCHI (CNPF) and DEFIFOL (DSF) methods. We also measured tree characteristics such as diameter, elevation and age (see step 4).
We studied soil sub-surface using an auger and a pickaxe to describe the soil subsurface profile following the methodology usually practiced by the forest managers. We also recorded other surface characteristics such as soil humus identification, surface rock fragment concentration, field plot micro-topography and field plot geology.
Using an auger to analyse soil sub-surface (Juliane Casquet - Pnr HL)
We excavated soil pits using a 25T mechanical shovel. We made the soil digging progressively until we reached the geological substrate, or, when it did not happen, until we reached the deepest roots. Soil pits were made by GUTKIN TP and they were one to five meters deep. Then, with the help of GEOSOLeau, we described morphological characteristics of soils in the field and registered them in a soil database following the specifications of the French National soil database DONESOL. Finally, Auréa analysed these samples and determined the key soil properties for water storage, i.e. soil textural fractions, soil organic content and gravel contents. They also measured roots dispersion, diameter and sanitary status. We completed this data with a measure of the samples water contents that we did at the INRA LISAH laboratory, so that we could produce the pedo-transfer functions adapted to the Haut-Languedoc territory.
Soil pit digging by a 25T shovel (Elise Bourru - Pnr HL)
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