SEASONAL TRENDS IN CARBON ASSIMILATION, STOMATAL CONDUCTANCE, PRE-DAWN LEAF WATER POTENTIAL AND GROWTH IN TERMINALIA-FERDINANDIANA, A DECIDUOUS TREE OF NORTHERN AUSTRALIAN SAVANNAS

Seasonal trends in pre-dawn leaf water potential and morning and after noon rates of light-saturated assimilation and stomatal conductance we re studied in saplings of the deciduous tree Terminalia ferdinandiana Excell. Mean daily maximum assimilation rates ranged from 11 mu mol m( -2) S-1 in the wet season to 8 mu mol m(-2) s(-1) during the transitio n from the wet to the dry season. Saplings were without leaves from Ju ne to October inclusive (dry season). There was a log-linear relations hip between stomatal conductance and pre-dawn leaf water potential (r = 0.76, n = 325), and a weak linear relationship between daily maximum assimilation and pre-dawn leaf water potential (r = 0.39, n = 184). A ssimilation rates were higher in the morning than in the afternoon in April and May, but were similar throughout the day from December to Ma rch. Seasonal trends in assimilation could be attributed primarily to stomatal closure, but diurnal differences could not. High leaf tempera tures may have been responsible for observed lower assimilation rates in the afternoon in April and May. Assimilation and stomatal conductan ce decreased when leaf temperatures rose above 38 degrees C and/or the leaf-to-air vapour pressure difference exceeded 4-4.5 kPa. Pre-dawn l eaf water potentials decreased more quickly, and stomatal conductance was more sensitive to this decrease, in T. ferdinandiana saplings than in saplings of Eucalyptus tetrodanta F.Muell. a co-occurring evergree n tree. Specific leaf area and assimilation per unit dry weight were h igher in T. ferdinandiana than in E. tetrodonta which is consistent wi th other studies of costs and benefits of deciduousness.