Callitris

Abstract: White cypress pine (Callitris glaucophylla) is a drought-tolerant evergreen conifer, which is a member of the Australian C. columellaris species complex. The complex is comprised of five closely related morphospecies that occur in a wide range of bioclimatic regions in Australia. Ecological genomics of the complex provides an opportunity to identify markers associated with environmental adaptation and is expected to broaden our understanding of its speciation process. We adopted a single-tree linkage mapping approach combined with high-throughput restriction site associated DNA (RAD) sequencing and expressed sequence tag-simple sequence repeat (EST-SSR) genotyping to set up a baseline genetic map for C. glaucophylla. The generated linkage map consisted of 4284 markers positioned on 11 linkage groups, corresponding to the haploid chromosome number of Callitris (2n = 22). The spatial distribution of markers was uneven compared to random expectation with significant clustering in central positions of some linkage groups, which may be associated with recombination cold spots of pericentromere regions. Allelic segregation was shown to be distorted in particular regions of four linkage groups, where selection may have operated on viability genes, leaving allelic distortion in surrounding linked markers. We then tested RAD single nucleotide polymorphisms (RAD-SNP) marker recovery and transferability of the linkage map to population genomic data collected for a related species, Callitris gracilis. Of the linkage map markers, 1257 markers (ca. 30 %) were recovered in independent RAD sequencing of population samples of C. glaucophylla. Genetic diversity and differentiation evaluated using mapped markers reflected ascertainment bias slightly; a decrease in Hs (absolute difference of −0.018) for a related species (C. gracilis) and an increase in F ST between C. glaucophylla and C. gracilis (+0.018) were detected. Although care should be taken given such biases in cross-species transfer, this study demonstrated that the RAD-SNP-based linkage map is essentially useful when combined with population genomic analysis of this conifer lineage.

Abstract: In the last few years there has been a marked growth of interest in the recent history of climate. The evidence of secular variations in climatic parameters during the last few centuries has implications for ecologists studying forests containing long-lived species. Such species record aspects of past climate in their annual growth-rings and in their population structures. The study of annual growth rings in Australian trees is still in its infancy. The concepts and techniques of dendrochronology and its sub-fields dendroecology and dendroclimatology are outlined. A species by species account of dendrochronological studies employing Australian trees is presented. As most of the results are unpublished, the treatment is selective and intended to illustrate the potential of, or progress with, a particular tree species. The greatest potential for dendrochronology in Australia lies with the endemic Tasmanian conifers in the genera Athrotaxis, Dacrydium and Phyllocladus. Not only do these contain very long lived individuals (e.g. > 2000yr, Dacrydium franklinii) but also they have the most suitable ring characteristics. Chronologies of about 1000 yr and 780 yr are being prepared for A. cupressoides and Phyllocladus aspleniifolius respectively. There is good regional cross-dating within species, and to a lesser extent between species, giving promise of a detailed reconstruction of past climate for Tasmania. Relatively short chronologies may be developed on the mainland using Callitris. This species may provide a proxy rainfall record for semi-arid areas which would be invaluable to their future management. The possibilities for developing chronologies using long-lived tropical rain-forest species in Queensland are discussed. The most abundant tree species of the continent as a whole, in the genera Eucalyptus and Acacia, appear to be generally unsuitable for standard dendrochronological methods aimed at climatic reconstruction although there are some exceptions. For many dendroecological studies Eucalyptus will be the only species available, and it should certainly not be disregarded.

Abstract: In temperate regions the ages of trees can be measured by counting the annual rings revealed on an increment core. The central principle of dendro- chronology - crossdating - simply ensures greater accuracy. In the tropics and sub-tropics growth periodicity may not be clearly limited to a particular season, so that it is necessary first to ascertain whether any anatomical structures delimit annual periods. This problem is discussed with particular reference to the Austra- lian tropics. The climate and forest vegetation of tropical Australia are described. Studies on Eucalyptus and Callitris in open woodlands in the monsoon climates of northern Australia indicate that trunk diameter growth is confined to the wet season. The resulting growth bands, although predominantly annual, are difficult to count, but the trees are not long-lived. However, trees 1000 years old, and possibly considerably older, do occur in the tropical rain forests of eastern Australia. A variety of techniques for measuring age is discussed and it is con- cluded that in the case of Araucaria, and probably some other conifers, the growth bands are approximately annual. Discrepancies between radiocarbon dates and ages derived by other techniques do occur however, and indicate a need for further research. Dendrochronological studies on Pisonia grandis are described in detail to illustrate how ring-width measurements and correlation with climatic factors can sometimes be used to verify the annual nature of the growth rings.