The Holocene period, Climax and anthropocene

After experiencing a near-polar atmosphere,  global warming is setting up in the Alps about 10 000 years ago,  this is the Holocene or climatic optimum. this is the reclaiming plant, plants migrate from their meridional shelters to territories abandoned by ice. This is also the period where we find the first traces of human activities. The settlement of men begins to take precedence over nature. they exploit the land for agricultural purposes but also for the frame.
The Holocene  known to be stable also had some climatic variations period . According to a report from Bintz et al (1989) this period is subdivised in some others ones.

• Preboreal (10000 - 9000 BP): the transition from late glacial warming and this results in regression of herbaceous and shrub steppe taxa in favor of Betula and Pinus. Changes in vegetation during the Preboreal emphasizes improving the climate. Rising temperatures, entails a substantial increase in forest communities. Although any estimate altitudinal be difficult, it seems that the upper limit of the forest has puatteindre 1900 m in the area of intra-Alpine.
• Boreal (9000 - 8000 BP): The Boreal is the period of the peak of Corylus (hazel), which extends roughly in the floors and hill and mountain, quickly eliminating Pinus and Betula.The hazels are in competition witha "mixed oak" in which Ulmus (elm) plays an important role, rising to 1500 m above sea level. Other elements of the package deciduous Quercus and Tilia (lime) are well represented, Acer (maple) is rare, Fraxinus (ash) extends gradually. The appearance of Hedera (ivy) and Viscum (mistletoe) may reflect relatively high temperatures. In the Montane level , the Hazelnut trees still dominate, constituting a narrow belt between the Quercetum mixtum and pine forests. Groups of subalpine Pinus uncinata (pin hook), Pinus cembra (arolla pine) and Benda to develop expense of shrubland in the massive internal, the tree probably reached at the end of this period, 2 100 meters. The margins have a southern vegetation dynamics close to that of the Southern Alps. Corylus is very discreet, Quercus, Ulmus and Tilia are still poorly represented.
• Atlantic (8000 - 4700 BP):The Atlantic is a time of profound changes in the vegetation cover in the Northern Alps. The Quercetum mixtum wins in  collinean stage . Its high percentages are due mainly to the development of Fraxinus, but also Tilia and Ulmus, Acer achieved its highest values. Then, in mid- old Atlantic  Abies huge extends  in the Montane stage and has a significant wealth  on all the North Alps.The Pine Forest shows a remarkable altitudinal range, from 1 000 to 2 000 m, pushing below the "mixed oak" and spreading largely on the slopes at the expense of groups alticola Pins. The sharp increase causes a sharp rise in the upper limit. In Oisans, Abies colonized slopes above 2100m. If the massive western wetlands, Pine Forest is fast in the dry inland valleys, this training is confined to a narrow strip in shady, and the pine forests continue to dominate the intra-Alpine mountain scenery. Other distinctive features of the Atlantic are the strong increase of Alnus glutinosalincana in wetlands, the place not negligible of Taxus in the vegetation of Piemont Dauphinois and the Vercors, the expansion of Pinus cembra in the subalpine and the presence of fairly regular Alnus viridis (green alder) in altitude sites. Finally, in the external massifs. Fagus extends gradually from the 6th millennium BP. With the Neolithic, the first demonstrations of farming activities occur in the pollen diagrams. The oldest clearings, along with cereal crops, have been recognized Pluvis (210 m), Francine(288 m), Charavines-les-Bathers (495 m), Choranche(500 m), Saint-Thibaud-de-Couz (500 m) and Luitel Col (1250 m).
• Subboreal (4700 - 2700 BP): From the  end of the Atlantic , around 5000 BP, Fagus begins a continuous increase in the higher collinean and lower montane, between 400 and 1200m, . However, this species has never transgress the limits of current area that is both the south. In the Mountane stage, beech could be associated to the fir tree, it is also possible that the distributionof these two species resulted from the influence of local  and ecological factors. Abies colonizing cool, moist slopes, and Fagus hot and dry slopes. The competition factor between different tree species only may explain the regression of hardwood forests. At low altitude,elm, ash and linden and oak disappear,may be promoted by clearing the  final Neolithic and and the Bronze Age, becomes the constitutional main groups killing hill and forest.
  In the subalpine zone of massive internal Alnus viridis, Larix and Picea, which appeared in the Atlantic are quite regularly but moderately represented. The moors and montane grasslands are
  colonized by Arolles. In Oisans, for against the Pinus cembra pollen is rare. The upper limit
  Procedure of the forest rises over this chronozone, In Maurienne, it probably reached2 400-2 500 m, well above the stage. Clearing of the Neolithic Age and the Bronze, more or less marked depending on the site,reflect the growing influence of man oncover, the intensity of settlement and the colonization of territories with little or no popular before.
• Subatlantic (2700 BP to Present):Temporary colonization by the tree orBeech at the top of hill and at the  transition Subboreal-subatlantic, report wetter conditions and cooler. Moreover,  many authors have hypothesized aclimatic deterioration at the end of the Bronze Age.Until then mainly determined by the great migrations of arboréenes species that  punctuate the forest dynamics and postglacial climate change, the composition and physiognomy of the plant cover in the Northern Alps  changes depending on the needs of people. The evolution of the vegetation is deeplyupset, at all altitudes, by clearing, agricultural and pastoral practices,logging and the introduction of new species.  In collinean area ,  succesive clarifications  of oak woods have indirectly but undoubtedly favored the development of hornbeam, Walnut and Chestnut. Their introduction dates, traditionally reported to the Roman times, are not known. The major eventis the expansion of Picea in the Subalpine basis, to the detriment of the Pine Forest and Arolles, Pins hook and larches.The extension of this came later - around 2000 BP in the central area at the medieval time in Oisan- was probably favored by the increasing human pressure. A high altitude, a consequence of  the cembra degradation   and other groups  in subalpine forest , the secondary stands of Larix, moors in Ericaceae or Juniperus (Juniper), of Alnus viridis bush to spread on surfaces deforested. Finally, the chronology of the various episodes of the Little Ice Age in the MassifDes Ecrins was established on the basis of a comparison of dendroclimatic and pollen data.

References :http://www.persee.fr/web/revues/home/prescript/article/bspf_0249 7638_1989_num_86_2_9362
Consulté le 16 novembre 2011