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Global change, structure and dynamics at the
upper forest limit in the Aleve forest (Varaita
Valley, Piedmont, Italy)
Renzo Motta, Paola Nola Dep. AGROSELVITER,
University of Turin, Via Leonardo da Vinci 44,
10095 Grugliasco (TO), Italy. Email
rmotta.selv_at_iol.it Dep. ECOTER, University
of Pavia, Via S. Epifanio, 27100 Pavia, Italy.
Email pnola_at_ipv36.unipv.it
Introduction Subalpine forests, forest and
treelines are important indicators of
environmental changes because they represent
natural or artificial features, highly sensitive
to environmental conditions and changes. During
the last decades, an increasing trend of forest
and tree-lines has been observed in North
America, Scandinavia, European Alps, Russia and
in Asia. The recent rises observed in Europe and
North America have not reached the same extent as
in the early mid-holocene, anyway the phenomenon
is very widespread and has been frequently
related to the global change. The first authors
that have been related the rise of the radial
tree growth with the increase of CO2 have been La
Marche et al. in 1984. Since this study many
others studies have been carried out in North
America, Europe, Asia, and in New Zealand. Most
of these studies have confirmed the rise of
growth rate with same exceptions. In the European
Alps, anyway, to study global change temporal and
spatial effects on the forest stands can results
very difficult due to the strong antropogenic
influences. In fact, in the Alps human activity
has radically influenced and changed subalpine
forests in various ways large areas of forest
were destroyed, lowering the upper forest limit
to create pasture or as the result of
indiscriminate logging, the natural composition
and the structure of forests were modified,
forest stands were eliminated to provide wood and
fuel for mining and other industrial activities,
and forest litter was collected from time to
time. In this contest human activities can mask
the impact of the global change and they are a
source of confusion in studies attempting to
forecast the temperature trend. For these reasons
in the European Alps is necessary to adopt
simultaneous analysis of multiple lines of
evidence. The purposes of this study are a )to
describe forest and treelines dynamics b )to
study structures and spatio-temporal growth
dynamics in forest stands located in subalpine
forests, at the forest limits and at the
treeline c)to study the growth trends in stone
pine and larch at the upper forest limits.
In order to give a better impression of net
productivity of a tree the simple tree ring width
were converted into basal area increments (BAI)
and filtered with a low-pass filter . The BAI
data from the cores collected in the subalpine
forests have been averaged decade by decade,
separately, for different age groups of trees so
that only BAI derived from rings within a
specific age range have been averaged for each
decade in succession. This gives tree-growth
estimates within which the age of trees is held
roughly constant through time.
Results
Fig. 2 Low frequencies of BAI chronology. Data
are plotted as deviation from the mean for the
period 1550-1995. Tree growth is commonly used as
a measure of fitness relative to climate and
environment regimes in forest ecosystems. BAI is
only an index of the net primary production and
doesnt take into account of possible changes in
wood density that can compensate volume changes.
However, even allowing for the uncertainties, the
data show an evident recent stone pine increased
growth at least in the context of changes over
1550 to date. The increased growth in stone pine
has been evident since about 1870. The larch
doesnt show the same trend.

Table 1 Forest and tree-limits in the studied
area
Methods Forest and tree-lines have been
measured in the whole study area. Stand
structures have been studied in two aspects
within six plots located at the tree limit (2),
at the forest limit (2) and inside the subalpine
forest (2). Within each plot, one increment core
was taken upslope at 50 cm height from each tree
with dbh gt 5 cm (a total of 603 cores) in order
to calculate the age structure and to analyse
growth trends. Additional cores were taken from
23 stone pines and 19 larches at the forest line
within the studied area, in order to have a
reference chronology for each species. In this
case two or three cores per tree were taken at
breast height (the first one upslope and the
other ones at 90-120 from the first one) and
only the largest trees, apparently healthy and
presently dominant were sampled.
Fig. 1 Age class distributions in the six
plots. The study of age structure revealed that
the current subalpine forest stands (C1 and V1)
were established about 200 years ago, probably
following a clearcut and a reduction in human
activity (felling and grazing). At the forest
limit the stands (C2 and V2) are younger but some
trees have more than five centuries and at the
tree limit (C3 and V3) most of the trees (almost
exclusively stone pine) are younger than a
century. The features of the curves indicate a
natural regeneration process.
Fig. 3 Decadal averages of BAI for different
age groups of trees. The growth trends in
dominant and codominant trees provides clear
evidence of increasing in the growth of both
stone pine and larch trees, thought this increase
is more evident in the stone pine. On the
contrary, the young trees (lt100 years) shown a
reduction in ring width in the last decades this
could be due to the enforcement of the
concurrence caused by high densities.
Main conclusions In the studied site there is a
clear evidence that there is a widespread rise in
forest and tree limit and that growth rates of
stone pine and larch (only among dominant trees)
are increasing. This enhancement is more evident
for the stone pine than to the larch. It is
difficult to infer a definitive cause-and effect
relationships from this site but the most likely
causes of these trends are both the land use and
management changes and the climatic change.
Discriminating the effect of a normal response
of tree and forest resources to climatic
variation from human caused responses is very
difficult but is the greatest challenge facing
mountain ecologists.
Poster presented at the International Conference
on Dendrochronology for the third millennium
2-7 April 2000, Mendoza, Argentina. The
contribution is part of the Project
ENV4-CT97-0641 UE-FORMAT