Weather variability effect on phenological events in a Mediterranean-type climate

The Mediterranean climate presents a distinct seasonality that greatly affects phenological development of natural vegetation. In a Mediterranean climate, summers are hot and dry, whereas winters are cool and wet. Most phenological models use temperature-based heat unit accumulation (e. g., degree-days) as a measure of developmental or growth rates. However, the effect of rainfall and, hence, water stress on phenology is also important in arid, Mediterranean regions. The objectives of this study were to evaluate both temperature and soil water availability effects on the flowering date of several natural plant species growing in a Mediterranean-type climate. The study was conducted in the Phenological Research Garden located in Oristano (Sardinia), Italy. Phenological data were collected for five Mediterranean species and four species typical of higher latitudes. Temperature data were employed to calculate cumulated degree-days (CDD) from 1 January until flowering using hourly mean temperature values obtained from an empirical model that approximates diurnal temperature curve from daily maximum and minimum air temperature. Drought conditions of the experimental period (11 years) were evaluated by calculating the available water capacity for each day using a simple soil-water balance model based on measured rainfall and estimated evapotranspiration rate. The results showed that the accuracy of CDD model in predicting flowering dates was generally good for most species, although the performance of heat unit model was variable over years. A significant improvement in the prediction of the flowering date was obtained by adjusting the CDD model for available soil water content. In general, the flowering dates were postponed when the soil water was not limiting and flowering occurred before severe water stress during drought years.