Effetti di carenze idriche prolungate su pinete mediterranee: insegnamenti da due esperimenti in Italia meridionale

A synthesis is presented of two studies carried out on the effects of imposed water shortage in two Mediterranean pine stands growing in Southern Italy, highlighting how physiological responses and structural acclimation can combine in the overall response of mature pine trees to long-term drought. The impact of a severe reduction in water availability on the water relations, transpiration and growth of 50-year-old Pinus halepensis trees (growing along the Ionian coast, Puglia, experiment 1) and 35-year-old Pinus laricio trees (growing in Sila, Calabria, experiment 2) was studied in two replicated experiments, in which the water available to trees was restricted by suspending a `roof’ made up by transparent polyethylene sheet. The experimental effects were followed for 12 months in experiment 1 and for 36 months in experiment 2, comparing “covered” plots and “uncovered” control plots. In experiment 1 trees experienced a severe water stress during the experimental period; a direct and rapid impact of drought was evident on the water relations and gas exchanges; there was an evident decrease of predawn water potential and transpiration was strongly reduced by stomatal closure (short-term physiological adjustment). A few months after the removal of the covering, no difference in whole plant hydraulic resistance was observed between droughted and control trees. Needle and shoot elongation and stem radial growth were considerably reduced in droughted trees, but no strategy of trees to allocate carbon preferentially to the stem conducting tissues was apparent throughout the experiment. In experiment 2, albeit rather extreme at the first sight, the treatment resulted in a realistic simulations of water availability as predicted by climate change scenarios; a 35% reduction in terms of timber yield and a 30% reduction of leaf area was estimated over the experiment in covered plots; however, the reduction of radial growth over the three years translated into a mere 2% reduction in sapwood area, so that a shift of resources from leaves to sapwood was apparent after three years (structural adjustment). As for the effect of treatment on transpiration, a 43% reduction of transpiration was observed in covered trees over the summer, but recovering to the same control values was measured over the autumn when, due to differences in leaf area, leaf specific transpiration was greater in covered trees; no significant differences was found for leaf hydraulic resistance and minimum daily values of water potential between covered and control plots; this suggests that stomatal closure was effective in counterbalancing the effects of soil dryness, maintaining minimum water potential values rather constant (homeostasis of water potential), largely decoupled from predawn water potential (isohydric behaviour).