- Ingela M. B. Wiman –
TREESWe have already touched upon the presumptive tree felling in Neolithic times. It has
not always been easy to judge what caused a cessation in a diagram of pollen from
a specifi c tree. Parasites or fungi of various kinds may cause its absence in a pollen
sequence, as we witness today with the elms that are dying because of the Ophiostoma
type fungi spread by a small black beetle whose larvae feed on the tree. Generally,
however, declines of trees are caused either by climatic variations or human activity. In
the period after the last glaciations, cold steppe vegetation predominated in Italy but
from the mid Holocene c. 7000 years ago, large fi r forests dressed hills and coastland
in northern and western Italy.^22 At this time a major shift in the vegetation cover is
detected from pollen samples taken in the area around the mouth of the Arno near Pisa.
Large fl oods of the river, as well as sea level fl uctuations, indicate climate change.^23 This
caused the disappearance of conifer woods, especially the Abies Alba, common silver
fi r. Nowadays the fi r appears in mountainous terrains, in some small populations in
the Pyrenees, in the Alps and easterly to the Balkans. Today it occurs up to 1100 m
above sea level (asl). Temperature falls by one degree Celsius per 100 meters asl. With
a climate shift of minus one degree the fi r tree line would subsequently fall 100 meters
downhill and vice-versa as a result of climatic fl uctuations. In reality, however, pollen-
aerosol dynamics is a complex issue. Pollen from fi r, for example, is very susceptible to
winds and precipitation and thus the light pollen may travel long distances and the
catchment area can be diffi cult to estimate. By observing small particles of charcoal
in the sediments, scientists suggest that these were emitted through the fi res caused
by Neolithic man and thus are indicative of human deforestation.^24 Generally, sudden
shifts may indicate human agency (especially when combined with charcoal) whereas
changes of a longue durée often indicate climate change.^25 Eventually broadleaved trees
like oaks and beech succeeded the fi r. The former became dominant in the northern
Alps around 3000 years ago.^26 These are trees that like warmer climate and were favored
by rising temperatures. Further to the south in the Massetano, similar changes have
been observed in the pollen sequences. The area around Lago dell’Accesa, as touched
upon above, harbored large evergreen forests of Quercus ilex at the mid-Holocene. Quercus
ilex, holm oak, has leathery dark-green leaves and a black bark. It reaches about 25
meters and the leaves are a bit spiky on the edges, especially in younger trees, to protect
them from being eaten by hoofed animals. It is commonly believed that this was the
species predominant in the later Mediterranean Holocene before the changes induced
by humans were detectable. Taking good note of the diffi culties combined with pollen
spread discussed above, we have a series of investigations from a small lake in the Colline
Metallifere that, combined with results from other lake samples from Toscana and Lazio,
in various ways may shed new light on the issues problematical above.
DATA FROM LAGO DELL’ACCESA, MASSETANO, ITALYSeveral probing campaigns have been made in Lago dell’Accesa situated close to Massa
Marittima (42°59 ́N, 10°53 ́E). The objective of most of the soundings is to investigate
climate versus man induced impact on the environment. The fi rst of the series to be
discussed here had a somewhat different agenda and was undertaken by the present
author and the geologist Sten Ekman. Its specifi c objective was to investigate whether