A Mediterranean conifer under vegetation shift: seasonal changes of photochemical activity in Cupressus sempervirens (L.) and evidence of correlation with temperature models

The Italian cypress (Cupressus sempervirens L.) is widespread in the entire Mediterranean region and reaches in the Province of Trento high latitude (46° 21’ North) and elevation (about 985 m a.s.l.). The human range expansion of this cypress has taken place at the Northern margin of the range in Italy in recent decades, driven by ornamental planting in spite of climatic constraints imposed by low temperature. The aim of the present work was to investigate the effect of low temperature in cypress in a nursery of the Autonomous Province of Trento (North-Eastern Italy). Over a period of two and half years, maximum quantum use efficiency of photosystem II (FV/FM) was monitored in order to study the effects of air temperature on the photosynthetic efficiency of 99 different clones and to derive information about the potential distribution of cypress in this province. A significant positive correlation (r = 0.64, p < 0.001) was found between air temperature and FV/FM over the 29-month period and especially in spring and autumn. Minimum values of FV/FM were observed in winter. Moreover, in order to assess critical environmental factors for cypress, mean values of FV/FM were correlated with daily air temperature and temperature-based indices to identify the driving factors of the physiological changes along the annual cycle. The maximum value of correlation (r = -0.85, p < 0.001) was found with 10-day Cold Degree-Day Running Sums before sampling and with a threshold of 2.0 °C for daily minimum air temperatur