Environmental Engineering Reference
In-Depth Information
However, there is evidence for changing hydrological conditions with relatively low
lake levels (and probably higher salinities) and drier conditions being characteris-
tic of this late-glacial cold period, as Roberts and Reed (2009) have indicated on
the basis of palaeoenvironmental research in Italy and Turkey, though the sites de-
scribed are generally at low altitude. However, Hajar
et al.
(2008) have recorded the
Younger Dryas in sediments from the Aammiq wetland at 865m asl in the Bekaa
Valley, Lebanon. Here an expansion of cedar and oak was halted c. 12 200 years
ago when a chenopod-rich steppe developed due to cooling. Similarly, at the Lago
di Mezzano at 452m asl in central Italy, forest expansion was curtailed c. 12 000
years ago (Ramrath
et al.,
2000). The patterns indicated by these sites may reflect
an east-west gradient, an altitudinal gradient, or both.
Between the last glacial maximum and the beginning of the Holocene, Mediter-
ranean mountain environments below the snow line were most probably composed
of mosaics of vegetation types. Open habitat shrub and grass communities domi-
nated exposed slopes, with scattered trees on less exposed slopes and mixed wood-
lands in sheltered valleys. Shifts in community locations and composition no doubt
occurred following temperature changes that crossed thresholds of tolerance for
some but not all species. It is likely that earlier glacial periods were similarly char-
acterized. The late glacial was a complex period climatically with evidence for
widespread forest development, which was interrupted as steppe returned and/or
forest declined during the Younger Dryas for a few thousand years prior to the start
of the Holocene.
2.4 The Holocene
There is a considerable body of evidence for environmental change during the
Holocene - the last 10 000 years - in the Mediterranean mountains, especially from
lake sediments. The literature is vast and some of it has recently been reviewed by
Tzedakis (2009) and Roberts and Reed (2009). Broadly, a distinction can be made
between the early and late Holocene, which reflects climatic change and intensify-
ing human impact due to the initiation of agriculture. This, plus a transitional zone,
has been proposed by Jalut
et al.
(2009) whose synthesis is based on the published
pollen analysis of 23 sites within the Mediterranean Basin, though Tigalmamine is
the only site in the southern basin; three sites are from the east and 19 are from the
north. A summary is given in Table 2.3. A significant implication of this synthesis
is that aridification began as a response to natural interglacial climatic change and
was later amplified by human activity.
Another factor that affected Holocene vegetation development was fire. More-
over, there is considerable debate as to how significant fire was in comparison with
climate as a driver of early Holocene forest change, notably the development of ev-
ergreen oak (
Quercus ilex
) communities beginning c. 8500 cal. years BP (calibrated
years before present). Addressing this issue, Colombaroli
et al.
(2009) examined
the pollen and charcoal profiles in a number of lake sites in the Adriatic region
and concluded that climatic change was the primary driving force on vegetation
