Southern Hemisphere conifers : distribution and history interpreted from a physiological perspective

An examination of the morphology and photosynthetic physiology of the two dominant families of conifers in the southern hemisphere, the Podocarpaceae and Cupressaceae, was undertaken here in order to describe the physiological limitations which constrain their distribution. A species specific index of drought tolerance was devised which quantified the maximum attainable water-use efficiency of photosynthesis during imposed drought. This index, (ci/c a)min, measured the potential of leaves to photosynthesize under water stress, comparing the maximum draw-down of internal CO2 concentration (ci) below ambient (ca). When operating at (ci/c a)min, the leaf was at the maximum water stress before leaf damage (indicated by an irreversible loss of variable fluorescence and photosynthetic rate) occurred. (ci/c a)min was found to be well correlated with the minimum rainfall within the natural range of the 12 species investigated, and this is strong evidence that the distribution of these species is largely determined by their drought tolerance. Further support for the drought limitation hypothesis came from a comparison of photosynthetic rates (as measured by chlorophyll fluorescence) of conifers in the field. The apparent photochemical electron transport rate (ETR) was found to be significantly reduced in a dry site when compared with a wet site. Most Podocarpaceae, and many Cupressaceae were found to be highly drought sensitive, and probably became increasingly susceptible to extinction during the substantial increase in aridity which occurred in Australia during the Tertiary. The influence of features considered as possible morphological drought adaptations on water loss were also examined. Wax plugs and imbricacy were found to substantially decrease maximum stomatal conductance, with the combination of wax plugs, imbricacy and epistomy reducing conductance to 17% of the value expected on leaves with exposed, unplugged stomata. Competition for light is believed to be another area where conifers suffer due to their lack of broad-leaves. Many podocarp genera appear to have converged with angiosperms, producing discrete, bilaterally flattened short shoots which seem to function as broad-leaves. The degree of shoot flattening, as measured by shoot width, was correlated with the leaf saturation light requirement, broad shoots requiring substantially lower light intensities for saturation. From this it was inferred that the bilaterally compressed short shoot was an adaptation to light competition with angiosperms. Conifers in the southern hemisphere form a surprisingly distinct group which has retained its character since the earliest Tertiary. The late Cretaceous- early Tertiary also saw the radiation of angiosperms, and it has been suggested that this event resulted in the global decline of the Coniferales. By determining how distributions of these taxa are limited today, it was possible to shed some light on whether conifers in the southern hemisphere were simply overrun and replaced by the more competitive angiosperms, or if other factors such as climate change are likely to have played a part. A brief summary of the evolutionary history of conifers in the southern hemisphere leads to the conclusion that decreasing rainfall and fire have been the main influences on conifer extinction.