Growth of 16 provenances of Eucalyptus microtheca in a regularly irrigated plantation in eastern Kenya

Growth and survival of 16 provenances of Eucalyptus microtheca were studied to assess their adaptability to regularly irrigated conditions, A further aim of the study was to find out which climatic variables in the natural habitat of E. microtheca contribute to adaptation to prevailing conditions. The study area was located in Bura, Tana River District, Kenya. The trial was irrigated at approximately 7 week intervals throughout the experimental period, 1984-1992. The total water input during the experiment was equal to about 5500 mm of rainfall (680 mm year(-1)), Survival rate ranged from 60 to 100%, and mean annual increment (MAI) from 0.8 to 23.5 m(3) ha(-1) at 8 years of age, Statistically significant differences between provenances were found in MAI and basal area, Thr eastern Australian provenances (New South Wales and Queensland) exhibited better growth rate than the northern (Northern Territory) and western (Western Australia) ones. Out of nine climatic variables tested, precipitation of the driest quarter was the best predictor for basal area growth at the age of 8 years, The results indicate that the growth capacity of provenances can be attributed to the precipitation of the driest quarter in their natural habitats in Australia. The frequent mild water stress that occurs in habitats of the eastern provenances may contribute to their prodigal water-use strategy. These provenances have physiological and morphological adaptations that assist quick water harvesting, enabling the plant to maintain a high daily gas exchange rate. The long, dry period that occurs in habitats of the western and northern Australian provenances may contribute towards their conservative water-use strategy, These provenances have mechanisms that enable the plant to conserve water and maintain good water status throughout the day. The provenances could be divided into two categories at the age of 1 year, suggesting that approximate ranking can be carried out at an early stage, thus speeding up final provenance testing. However, the best provenance could be detected as late as 5 years of age, emphasising the importance of long lasting experiments in provenance selection. We conclude that the eastern Australian provenances are most suitable for regularly irrigated conditions owing their great capacity to harvest available water very quickly, Western and northern provenances may be most suitable for areas where long drought periods dominate.