THE PREDICTION OF PHENOLOGICAL STAGES IN FOUR NAMAQUALAND EPHEMERAL SPECIES USING THERMAL UNIT INDICES

Abstract

Flowering time of ephemeral plant species in Namaqualand (South Africa) generally ranges from mid-July through September, depending on the timing of the first substantial winter rains. The precise timing of the flowering of species is determined by the individual's life history and the integrated effects of a complex of environmental factors. Phenological and climatic data were recorded and used to determine the number of thermal units required by four Namaqualand ephemeral species to reach a certain phenological stage. Twenty-three thermal unit indices were used to calculate the number of thermal units needed to reach a specific phenological stage. The index combining cold units from sowing until flower initiation, with Growing Degree Days (heat units) from flower initiation until anthesis flowering or display/peak flowering, gave the most accurate predictive values. Flowering time of Namaqualand ephemerals can therefore be predicted at the beginning of the growing season after the first substantial rainfall, and refinements made throughout the season by the use of actual temperature data.