Investigations of the physiology and genetics of the New Zealand conifers : rimu, kahikatea and totara (1988)
AuthorsHawkins, Barbara Janeshow all
Several experiments were undertaken to examine some of the physiological and genetic factors which affect the distribution of rimu, kahikatea and totara, and which contribute to the overall geographic dominance of rimu in New Zealand.
A nutrition experiment, examining the growth of several provenances of each of the three species at six levels of nutrient supply, was carried out to explore species' differences in the demand for and utilization of nutrients. This work indicated that there is some differentiation among rimu, kahikatea and totara in their ability to utilize and respond to nutrients. Rimu appeared to have a greater ability to take up and store nutrients, while kahikatea and totara were more responsive to improved nutrition.
A second nutrition experiment repeated three of the treatments in the first experiment, with half of the seedlings in each nutrient treatment inoculated with mycorrhizas. The results of this experiment showed there to be a significant improvement in the growth of all species due to mycorrhizal infection, with rimu and kahikatea being more mycotrophic than totara. Growth was improved in all nutrient treatments, and this appeared to be due primarily to the enhanced uptake of phosphorous and, to a lesser extent, nitrogen by mycorrhizal fungi.
A germination experiment and two controlled environment experiments, germinating seeds and growing seedlings of the three species at different temperatures, were conducted to discover the optimum temperatures for germination, growth and net photosynthesis. The temperatures producing maximum rates for these parameters (around 27°C) were much higher than anticipated for temperate species, and a third controlled environment experiment was then carried out with a subalpine podocarp species. The purpose of that experiment was to examine the optimum growing temperature of a species which must have been under relatively severe selection pressures to adapt to its environment.
Snow totara, the subalpine species, had the same optimum temperature for growth and net photosynthesis (27°C) as did the podocarp species from lower elevations. It is suggested that the high optimum temperatures for growth evolved in the New Zealand species during the subtropical conditions of the Miocene, and have been retained as a relic trait to the present. This would imply that selection pressures since that time have been acting upon attributes other than growth rate.
If selection pressures have not been acting upon growth rates in the recent past, significant provenance differentiation with respect to optimum growing temperature would not be expected. An experiment growing seedlings of seven totara provenances in two temperature regimes was undertaken to discover whether optimum growing temperature was correlated with the climate of seed origin. No such correlation could be found.
Isozyme analysis of seed of several populations of rimu, kahikatea and four species of Podocarpus was carried out to study the amount and partitioning of genetic variation within and between the species. Results from this work indicated that all species have lower levels of genetic variation than most northern hemisphere conifers. Rimu and kahikatea are also significantly less variable than the Podocarpus species. It is suggested that variability in the New Zealand species may have been lost through genetic drift in restricted populations during the cold periods of the glacial/interglacial cycles of the last two million years. The differences between the species may be due to their different chromosome numbers and the greater number of species in the genus Podocarpus which hybridise and thus maintain variability.