Airborne pollen of Olea europaea L. in Tetouan (NW Morocco): heat requirements and forecasts

Abstract

The northwest of Morocco is one of the major areas dedicated to olive tree crop in Morocco. Airborne pollen of Olea was recorded during 2008–2012 to determine its annual load and variation. Aerobiological sampling was undertaken over five seasons in Tetouan using the volumetric method. This study used also a thermal model to calculate growing degree-days required until the start of the pollen season and the peak day. For this purpose, a range of the threshold temperatures between 0 and 10 °C was tested. The pollen season started from the second decade of March and showed the highest pollen index in May or April. The annual pollen score recorded varied from year to year between 4,168 and 1,953. The main pollen season lasted 62–105 days, with peak days occurring in April or May; the highest concentration reached 379 pollen grains/m3. The sum of the difference between mean temperature and threshold temperatures of 10 and 7 °C from 51 and 61 days before the season start and peak day accumulated averages heat units of 278 and 624 growing degree days, respectively. In order to validate the forecast of these parameters of the pollen season, we have used the year 2012 as external data. The dates predicted did not differ from those observed when threshold temperatures 10 and 7 °C were tested to forecast the onset of the Olea pollen season and the full anthesis, respectively. The method performed in this study showed a high coincidence between expected and observed data and should be useful to allow olive crop yield management in the area of Tetouan.