The latitudinal and yearly variation in the timing of microsporogenesis in Alnus, Betula and Corylus

Abstract

The timing of the tetrad phase of microsporogenesis in five Betulaceae species growing in natural conditions in Finland was studied by calendar days and by period unit and degree-day heat sums. Latitudinal variation and variation between successive years were examined. To account for the variations found, models using heat sums, critical daylength and joint effects of daylength and temperature sums were tested. The total lengths of meioses and tetrad phases were measured in four tree species. Variation in the phenology of tree species and the underlying different timing systems are discussed with reference to the annual cycle of the generative development in trees. The tetrad phase of the microsporogenesis was usually reached in August in all the species studied. There were no significant differences between the species studied, but latitudinal differences were significant. In contrast to previously studied species (the meioses of which occur in the spring), the meiosis on site occurred earlier the higher the latitude. Within Finland a latitudinal range of about 10 days was observed. Moderate variation between years and high dispersion within trees and populations were also characteristic of Betulaccae. Neither a simple temperature-sum or critical-day timing model nor a model on the joint effect of daylight and the temperature could account for all the variation found. Duration of daylight is found to be the most important factor in the timing of Betulaccae meioses. Temperature is considered to be a modifying factor here in contrast to meioses in previously studied species in which the temperature was the main factor. This difference in timing systems is found to exist in adaptative strategies between different parts of the annual cycle rather than between tree species or genera.