Photosynthetic activity of Quercus ilex at the extremes of a transect between Mediterranean and submediterranean vegetation (Trieste NE Italy)

Microclimatic data and CO2 gas exchange rates were measured in thecoastal area near Trieste (NE Italy) in two groups of plants rowing at
the extremes of a floristic-vegetational gradient between an evergreen
Mediterranean maquis on the coast and a winter deciduous
submediterranean woodland on the Karst plateau, where Q. ilex persists
only sporadically but is still a co-dominant tree.
Field data were recorded from sunrise to sunset during the periods of
highest productivity and of maximum physiological stress. i.e. high
temperatures and water deficit in summer, and low temperatures and
strong, dry winds in winter. Photosynthesis was measured with a portable
Infrared Gas Analyzer.
Microclimatic data showed that summer and winter stress were more
intense on the Karst plateau. Assimilation of the coastal population was
higher because a positive CO2 balance was maintained during both stress
periods. During summer drought stress diurnal assimilation of the inland
population was just in balance with nocturnal leaf respiration. In
winter, on the contrary, photosynthetic activity ceased almost
completely for almost a week, due to the lower temperature regime and to
strong, dry winter winds that caused severe mechanical damages and, more
generally, a drastic shortening of the leaf life span.
The competition ability of Q. ilex in ecotonal areas is briefly
discussed on the basis of these results. Winter stress seems to be the
main factor limiting the distribution of Q. ilex northwards.