A palaeoecological study of recent environmental change in the drainage basin of the Lac d'Annecy (France).
A palaeoecological study of sediments from the Lac d'Annecy has
been undertaken in order to assess the impact of man on soil vegetation
systems in the lake basin.
The geology of the drainage basin is dominated by weakly magnetic
calcareous Jurassic and Cretaceous deposits, but natural soil forming
processes and artificial burning have produced assemblages of
strongly magnetic minerals within topsoil's of the catchment area.
These can be characterized by mineral magnetic parameters which are
preserved as the eroded soil fractions become incorporated within the
Man's impact on the landscape appears to have been most
dramatically felt from c. 1100 AD; fossil pollen assemblages give
evidence for widespread forest clearance, associated with arable and
pastoral farming activities, which are thought to reflect the earliest
intensive agricultural development of the higher slopes within the
catchment area. At the same time there is a marked change in the
nature of the sedimentary matrix. An increase in the concentration of
major cations and magnetic minerals indicates a regime of more
intensive soil erosion and a change in the magnetic mineral assemblage
itself indicates a shift in the relative importance of different
catchment sources to the total allochthonous material flux.
Reconstruction of environmental change during more recent
centuries has been aided by reference to primary and secondary
documentary sources of evidence relating to past patterns of land-use.
The mixed farming system of the eighteenth century, characterized by a
regime of relatively intensive arable cultivation, was not
particularly well-suited to the natural environment. A decrease in
the concentration of major cations and magnetic minerals, together
with a decline in the total sediment accumulation rate from the midnineteenth
century onwards, is thought to reflect a fall in the rate
of loss of material from catchment surfaces. It has been suggested
this was related to a shift in focus of the rural economy from the
semi-arable, semi-pastoral subsistence agricultural system to one
which concentrated increasingly on the breeding of livestock for the
local dairying industry.