Assessing the impact of extreme air temperature on fruit trees by modeling weather dependent phenology with variety-specific thermal requirements

Extremely high and extremely low temperature may have a terminal impact on the productivity of fruit tree ifoccurring at critical phases of development. Notorious examples are frost during flowering or extremely hightemperature during fruit setting. The dates of occurrence of such critical phenological stages depend on theweather history from the start of the yearly development cycle in late autumn, thus the impact of climate extremescan only be evaluated correctly if the phenological development is modeled taking into account the weatherhistory of the specific year being evaluated. Climate change impact may lead to a shift in timing of phenologicalstages and change in the duration of vegetative and reproductive phases. A changing climate can also exhibit agreater climatic variability producing quite large changes in the frequency of extreme climatic events.We propose a two-stage approach to evaluate the impact of predicted future climate on the productivity offruit trees. The phenological development is modeled using phase - specific thermal times and variety specificthermal requirements for several cultivars of pear, apricot and peach. These requirements were estimated usingphenological observations over several years in Emilia Romagna region and scientific literature. We calculatedthe dates of start and end of rest completion, bud swell, flowering, fruit setting and ripening stages , from lateautumn through late summer. Then phase-specific minimum and maximum cardinal temperature were evaluatedfor present and future climate to estimate how frequently they occur during any critically sensitive phenologicalphase. This analysis has been done for past climate (1961 - 1990) and fifty realizations of a year representative offuture climate (2021 - 2050).A delay in rest completion of about 10-20 days has been predicted for future climate for most of the cultivars.On the other hand the predicted rise in air temperature causes an earlier development of crops thus a reduction inthe length of the different phenological stages. Despite the earlier timing of phenological phases may expose thecrops to frost hazard, the mean increase of air temperature avoids relevant impacts on crops. The frequency of airtemperatures higher than the cardinal temperatures is expected to increase by 5% compared with the reference1961 - 1990 climate.The work was carried out within the Italian national project AGROSCENARI funded by the Ministry forAgricultural, Food and Forest Policies (MIPAAF, D.M. 8608/7303/2008)