Abstract
The climate of the Top End of the Northern Territory is dominated by alternating wet and dry seasons, imposed on a background of high year-round temperatures. Trees of Northern Territory savannas (the most widespread vegetation type) have developed a variety of responses to this strongly seasonal climate, including a range of leaf phenologies, from evergreen to fully deciduous. Young trees are probably more vulnerable than mature trees to drought, probably because their root systems are not so extensive.Responses of juveniles of two common tree species of Northern Territory savannas, Eucalyptus tetrodonta (evergreen) and Terminalia ferdinandiana (deciduous) to diurnal and seasonal changes were investigated. Saplings 1-3m high, were studied at a field site, and potted seedlings 0.6 to 1.0 m high, were used in investigations under controlled conditions.
For E. tetrodonta saplings, progressive leaf loss throughout the dry season (more than 80%) was more extensive than in large trees, and represented the major constraint on whole tree carbon assimilation. Mean daily maximum assimilation rates on a leaf area basis ranged from 14.5 µmol m•2 s•1 in May to 4.8 µmol m-2 s-1 in October, and were linearly related to pre-dawn leaf water potentials. Light-saturated assimilation rates were higher during mornings than afternoons, when high temperatures limited assimilation, primarily because of non-stomatal factors . Osmotic adjustment allowed leaf turgor to recover overnight throughout the year. Extensive death of leaves and shoots may be related to xylem embolism.
Terminalia ferdinandiana saplings were completely leafless from June to October inclusive, longer than large trees were leafless. Mean daily maximum assimilation rates varied from 12 µmol m-2 s-1 in March to 8 µmol m-2 s-1 in May, mainly due to variations in stomatal conductance. Pre-dawn leaf water potentials decreased more quickly, and stomatal conductance was more sensitive to this decrease, in T. ferdinandiana saplings than in E. tetrodonta saplings.
| Date of Award | Apr 1997 |
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| Original language | English |
| Supervisor | Derek Eamus (Supervisor) |