PHOTOSYNTHESIS AND TRANSPIRATION OF EVERGREEN MEDITERRANEAN AND DECIDUOUS TREES IN AN ECOTONE DURING A GROWING-SEASON

Microclimatic data and gas exchange rates of Carpinus orientalis, Fraxinus ornus, Phillyrea latifolia, Pistacia terebinthus, Quercus ilex and Q. pubescens, were measured in an ecotone between a Mediterranean maquis and a deciduous thermophilous woodland, near Trieste (NE Italy). The field data were recorded during eleven months, from January 1990 to September 1991, in three-four sunny days per month, from sunrise to sunset. Photosynthesis and transpiration were measured with a portable Infrared Gas Analyzer. The results show that: 1) the two evergreen species, Quercus ilex and Phillyrea latifolia, have different sensitivity to low winter temperatures; this accords well with their different distribution patterns in the northern Adriatic region: the more sensitive Ph. latifolia is restricted to the coast, while and. ilex reaches the prealpine mountains; 2) during the summer drought the photosynthetic activity of C. orientalis, e. ilex and a pubescens is strongly reduced, that of P. terebinthus and, to a lesser extent, of Ph. latifolia, reaches the yearly maximum; 3) deciduous species (except P. terebinthus) reach the yearly maximum of photosynthetic activity in June, evergreen species in the equinoctial periods; 4) Q. ilex has the highest photosynthetic rate and diumal net assimilation in the year; 5) during the warmest hours of the summer, Q. pubescens closes the stomata, like Q. ilex. Evergreen Mediterranean species are highly competitive in the study area, due to the high assimilation during the whole year; winter stress is the main factor limiting their distribution northwards.