Les sols forestiers

• In forest environments, humus—layers of dead plant fragments in various stages of decomposition—are closely linked to soil types. In 2015, nearly half of forest soils, brunified soils (7.4 Mha), are generally moderately acidic and highly aerated due to intense activity of fauna and microorganisms (Corsica, Massif Central, Pyrenees, Vosges). Conversely, excess water in valley soils (2.5 Mha) slows biological activity, hinders plant rooting, and constrains forestry operations. Humus in these two soil types consists of a litter layer of dead leaves when they are evolved and highly fertile (mull and dysmull), or more rarely a dark brown layer of fine organic matter and litter from previous years (moder).

• Soils from calcareous rocks associated with carbonated humus predominate in the Paris Basin, the Alps, and the Mediterranean (2.3 Mha). Nutritional constraints may hinder the development of certain species, such as Scots pine and maritime pine, Douglas fir, or chestnut. Very acidic and poor soils (1.3 Mha) have developed on the most acidic sandy substrates (Gascony Moors, Sologne, sandstone Vosges, etc.). Generally, the organic layers of their humus are even thicker (dysmoder, mor).

Humus types in production forest excluding poplar plantations Illustration 108 - Humus types in production forest excluding poplar plantations

Soil properties determine the development and growth of forest stands by affecting their rooting, nutrient uptake, and water supply. Rainfall and water stored in soil provide trees with water. When during drought conditions the soil's usable water reserve falls below a given threshold (40% of maximum reserve for an adult maritime pine for example), the tree reduces its photosynthetic activity and thus its growth.

Water availability of species as indicated by forest soil flora Illustration 110

• A quarter of French forest stands (4.1 Mha) experience water deficit linked to climate (Mediterranean) or low soil water reserves (xerorhine or xerophile environments). This applies to three-quarters of the Mediterranean ecological region. Its soils are also particularly exposed to erosion, a risk heightened by fires in summer and autumn. Forests in dry environments are also found in Corsica, along the Atlantic coast (southwest), in the Causses, eastern Pyrenees, southern Alps and Jura, and on Burgundian plateaus.

• Conversely, excess water in riparian soils or linked to water tables perched on impermeable ground favors species adapted to wet environments (hygrophile or mesohygrophile). Forest stands developing under these conditions are particularly productive (6.4 m³/ha/year). They occupy 0.8 Mha, or 5% of French forest.