Plant response to weather variation in Mediterranean area

Recently many studies were conducted to investigate the phenological behavior of various species in different Mediterranean climate (Mooney and Kummerow, 1981; Moll, 1987; de Lillis and Fontanella, 1992). However, there is limited information on the timing of the key development processes and the relationship between phenological stage occurrence and duration and intensity of drought period. Prediction of crop and natural plant development is widely made using degree-days (DD). Critical aspects of DD calculation are related to the methodology used to calculate degree-days, selection of an appropriate threshold or base temperature, and starting the accumulation on the correct date (Allen, 1976; Baskerville and Emin, 1969; Yang et al., 1995). In addition, modofications were suggested to enhance the canonical equation for calculating DD as incorporating an upper temperature threshold (McMaster and Smika, 1988) or functions for other environmental factors that affect phenology (Caprio, 1993; Wilhelm and McMaster, 1995). The objectives of this paper are (1) to present threshold temperatures that were derived for the computation of degree-days, and (2) to evaluate the sensitivity of several species to weather variability.