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 atthe extremes of a floristic-vegetational gradient between an evergreenMediterranean maquis on the coast and a winter deciduoussubmediterranean woodland on the Karst plateau, where Q. ilex persistsonly sporadically but is still a co-dominant tree.Field data were recorded from sunrise to sunset during the periods ofhighest productivity and of maximum physiological stress. i.e. hightemperatures and water deficit in summer, and low temperatures andstrong, dry winds in winter. Photosynthesis was measured with a portableInfrared Gas Analyzer.Microclimatic data showed that summer and winter stress were moreintense on the Karst plateau. Assimilation of the coastal population washigher because a positive CO2 balance was maintained during both stressperiods. During summer drought stress diurnal assimilation of the inlandpopulation was just in balance with nocturnal leaf respiration. Inwinter, on the contrary, photosynthetic activity ceased almostcompletely for almost a week, due to the lower temperature regime and tostrong, dry winter winds that caused severe mechanical damages and, moregenerally, a drastic shortening of the leaf life span.The competition ability of Q. ilex in ecotonal areas is brieflydiscussed on the basis of these results. Winter stress seems to be themain factor limiting the distribution of Q. ilex northwards.