The influence of air temperature on the starting date of Quercus pollination in the South of Europe

In temperate zones the air temperature influences many aspects of the plant growth and also the time of flowering is often correlated with this environmental parameter. It is a generally accepted idea that higher temperatures in the period preceding ripening of the flowers determine earlier pollination. To verify if a correlation between the air temperature and the date of onset of the pollination period of Quercus spp. exists, a comparative study was carried out over 7 years (1995–2001) in two South-European towns: Vigo (Spain) and Perugia (Italy). Quercus pollen is released in the atmosphere of Perugia on average in the last two weeks of April while in the Spanish region the pollination occurs on average one month before.

In order to overcome the dormancy period Perugia requires 1110 Chilling Hours (CH)-884 Growth Degree-Days (GDD°C) and Vigo 709 CH-861 GDD°C. With the Ashcroft method Perugia needs 1075 CH-1000 GDD°C and Vigo 625 CH-1512 GDD°C. Heat accumulation from the end of winter dormancy to the onset of pollination, showed the highest significance when mean temperature in Perugia and maximum temperature in Vigo were used. Every year we have found that the colder station needed a lower heat accumulation: Perugia required a higher quantity of chilling and heat than Vigo. However, the correlation detected between temperature and flowering was, on average, less significant that those found in the same regions for other arboreal taxa that present winter pollination (e.g. Corylus, Alnus). This preliminary study suggests that there is an effect of air temperature on Quercus pollination, but other environmental factors, such as photoperiod, hours of light, rainfall, relative humidity, may be of great influence in determining the onset of pollination in plants with a spring flowering.