The vegetation of the soil of France includes ancient elements and other more recent emigrants. The glacial period destroyed the flora on only about half of the Alps and the Jura, which were covered with ice, and on the highest peaks of the Massif Central, the Vosges and the Pyrenees. Undoubtedly, the accompanying cooling caused a large number of species to flow southwards, which were later able to resume their journey towards the north. Some remains of the vegetation that adapted to the colder and humid climate remain, both on the peaks of the Alps and in the moors and peat bogs of north-western France, such as: Viola palustris, Vaccinium vitis idaea, Andromeda polifolia, etc.
The period of relatively warmer and drier climate, of which there is indisputable evidence in Central Europe (optimum post glacial) and which must have been felt also in France, is to explain the surviving stations of steppe plants, of which we have Examples on the most arid lands: savarts Champagne, causses of the Massif Central; and so too can be explained the plants with Mediterranean affinities, which are observed in certain sheltered valleys of the Alps and also in certain localities of the West. These provisions of xerothermic species are to be found especially on limestone and on slopes facing south. The Astragalus monspessulanus, which goes up the Rhône corridor to Bresse, can also be seen on the limestones of the Berry and on the Cretaceous slopes of the Lower Seine valley.
Presently, most of the French territory is included in the domain of the Atlantic flora, characterized by the pedunculate oak (Quercus pedunculata), the predominant essence of the forests, associated with the beech (Fagus silvatica) in the hills and in the more humid regions, the hornbeam (Carpinus betulus) and elm (Ulmus campestris) in the driest places. Only the Mediterranean region and the mountains above 500 meters (Vosges, Central Massif, Jura, Alps and Pyrenees) remain excluded from this domain, which, in its natural state, would present almost everywhere the forest aspect.
The ruins of the Court of Auditors, left abandoned for about thirty years in the heart of Paris, were already invaded by a forest flora, which has been studied. However, certain soils that are dry and almost devoid of vegetable soil are considered, even in the north of France, to be very difficult to afforest: such are the clay, when it is not covered with siliceous clay, and the harder banks of the coarse Eocene limestone, which emerges on steep slopes. For several centuries, the Savarts of Champagne Pouilleuse were used only for grazing or hunting; the eye could only see grassy clods and, in scattered spots here and there, xerophilous grasses (Nardus stricta) with a small sedge (Carex humilis), some phanerogams such as Coronilla minima, Phyteuma orbiculare, thickets of wild plum trees (Prunus spinosa), dogwoods (Cornus sanguinea), junipers (Juniperus communis). But on a large part of these savarts since the early years of the century. XIX they began to plant pine trees, which have completely changed the appearance (Pinus silvestris and P. Austria). A similar association is observed on the steep slopes of the recessed meanders of the Seine, shaped in the Normandy clay and in the limestone cliffs of the Vexin, where it was studied by the Allorge (Juniperus communis, Sesleria caerulea,Prunus spinosa, Prunus mahaleb, Crateaegus monogyna, Genista tinctoria, Daphne laureola, etc.).
A large part of the muddy soils of the Parisian basin, now covered with rich cereal crops, must have had a not very dense coating of oaks, with clearings that were enlarged by man. It would be difficult to establish whether the herbaceous species with oriental affinities, which can be seen there, are leftovers from the xerothermic period or have instead been introduced there favored by tillage.
According to TOPB2BWEBSITES, the beech forest, which is more common in the north and on the rather high hills of the east of the Paris Basin (Coasts of the Meuse and Moselle, the first buttresses of the Vosges), is poorer in undergrowth than the oak wood, in which hazelnuts, rowan (Sorbus aucuparia), boxwood (Buxus sempervirens), wild roses etc. they often appear at the edge of grassy clearings. The wet floods of the valleys originally had to be occupied almost everywhere by alders (Alnus glutinosa) and sometimes marshy meadows with rushes and sedges, which man often transformed into cultivated land or meadows. The aquatic flora, on the other hand, could not be changed; and the banks of the ponds are generally surrounded by a belt of straws (Phragmites communis). Their filling led to a succession of associations, which were studied by Allorge and others: the scirpeto takes the place of the fragmiteto with Scirpus lacustris, Glyceria aquatica, Sparganium ramosum, Typha latifolia, Carex riparia; later willows and alders appeared and, lastly, birches. The regions of the Atlantic coast, especially the Armorican Massif, have a particular aspect, due to the humidity of the climate and impermeable soil, the extreme rarity of limestone and, to a certain extent, the influence of man. The elevated parts of the hills, especially the quartzite ridges and the granite domes, are often covered with an association of gorse (Sarothamnus scoparius), rushes (Ulex europea) and heather (Calluna vulgaris), among which some grasses grow xerophile. They make up what is commonly called the Land. In badly drained depressions the moor is losing its characteristics;Erica tetralix indicates a more humid soil, and the multiplication of sphagnum and moss finally leads to the peat bog. In Brittany, in the Vendée and in Lower Normandy, there are undoubted cases of extension of the lands following the clearing of forests or the abandonment of crops; but the elevated places, which are beaten by the winds and have a siliceous soil, have probably never been covered with thin woods of oaks and pines, interspersed with patches of rushes and heather. There are also peat bogs in the Parisian Basin, in certain wet valleys or depressions (the Somma valley in Picardy, the Saint-Gond marsh and other depressions in the Champagne plain).