Temperature effects on eucalypt shoot growth in the Brisbane region

In this thesis the ecophysiological basis of seasonal shoot growth of Eucalyptus in subtropical southeast Queensland is investigated via studies in the field and in controlled environments. Field observations of tagged branches of mature trees of E. intermedia R.T. Baker established the phenology of the leaf canopy of that species. Litter studies in a forest dominated by E. drepanophylla F. Muell. ex Benth. and E. maculata Hook, and including trees of E. intermedia extended the single-species observations. The results were analysed with respect to possible environmental effectors. Temperature appeared to be most highly related to the phenology of the leaf canopy. The phenology of leaf production showed a marked relationship to rising temperatures in spring, and a lower, hysteresis type response, to falling temperatures in autumn. A temperature threshold of 16-18oC for new shoot growth was apparent. Leaf fall appeared to be partially related to patterns of leaf production and fruiting. Quantification of the relationship between leaf production and temperature was attempted in two sets of controlled environment studies. Two species of Eucalyptus with contrasting distributions, both occurring in the field studies, were used; E. drepanophylla and E. maculata. The first set of observations was of the growth of the plants under constant temperature regimes during naturally increasing solar radiation in spring. From these observations, basic relationships between temperature regime and biomass accumulation, leaf production, height and leaf area were established. Temperature optima of 24/19oC and 27/22oC were estimated for E. maculata and E. drepanophylla respectively. The plants were moved factorially between temperature regimes in mid-late summer, and the relationships between temperature and the various growth criteria observed prior to the temperature change were not evident. The second set of observations of plant growth under constant temperature regimes was made in late autumn-winter. The main trends between growth and temperature established in the first experiment were confirmed. The growing temperatures were changed in mid-winter, and it was found that the relationships between the various growth criteria and temperature were unaltered. The field and experimental results are then compared and the results analysed using a general growth index model. It is concluded that incoming levels of solar radiation and a temperature-dependent rate of respiration must be considered when predicting leaf production. A simple descriptive model is developed using the information derived from the thesis and the literature. It describes the pattern of seasonal shoot growth in the field using simple functional axioms. In this way an attempt to define the mode of action of temperature and other crucial environmental factors is made. It is concluded that leaf production is limited by the net assimilate available for new growth (the balance between amount of solar radiation received and the losses due to maintenance respiration), this potential being modified by the temperature tolerances of the species. A shift in partitioning rates to the leaf canopy is assumed when temperatures lie above the lower threshold temperature. This model remains to be tested directly, but provides a useful analytical tool.