The role of temperature in the onset of the Olea europaea L. pollen seasonin southwestern Spain

Temperature is one of the main factors affecting the flowering of Mediterra nean trees. In the case of Olea europaea L., a low-temperature period prior to bud development is essential to interrupt dormancy. After that, and onc e a base temperature is reached, the plant accumulates heat until flowering starts. Different methods of obtaining the best-forecast model for the ons et date of the O. europaea pollen season, using temperature as the predicti ve parameter, are proposed in this paper. An 18-year pollen and climatic da ta series (1982-1999) from Cordoba (Spain) was used to perform the study. F irst a multiple-regression analysis using 15-day average temperatures from the period prior to flowering time was tested. Second, three heat-summation methods were used, determining the the quantities heat units (HU): accumul ated daily mean temperature after deducting a threshold, growing degree-day s (GDD): proposed by Snyder [J Agric Meteorol 35:353-358 (1985)] as a measu re of physiological time, and accumulated maximum temperature. In the first two, the optimum base temperature selected for heat accumulation was 12.5 degreesC. The multiple-regression equation for 1999 gives a 7-day delay fro m the observed date. The most accurate results were obtained with the GDD m ethod, with a difference of only 4.7 days between predicted and observed da tes. The average heat accumulation expressed as GDD was 209.9 degreesC days . The HU method also gives good results, with no significant statistical di fferences between predictions and observations.