ORNAMENTAL ATTRIBUTES OF THE NATURAL VARIANTS OF CORDYLINE AUSTRALIS Warwick Harris Lincoln Botanical 27A Edward St Lincoln New Zealand [email protected] Keywords: Cabbage tree, New Zealand, plant form, cold hardiness, conservation Abstract Cordyline australis (New Zealand cabbage tree) is widely grown in temperate climates as a garden and landscape subject and as a tub plant. Diversification of its use as an ornamental has been based on discoveries of plants with unusual leaf colours and growth forms, and hybridization with the other New Zealand Cordyline species. A systematic assessment of the genetic variation of wild populations of C. australis began in 1994 with measurements of the phenotypes and collection of seed of trees in 29 natural stands located over 12º of latitude. Progeny of these stands have been cultivated under uniform garden conditions at Auckland (36º 53´ S), Christchurch (43º 38´ S), and Dunedin (45º 51´ S) and their growth and phenology recorded. This has shown variation of leaf characteristics, tree habit, growth rate, susceptibility to low temperature damage, and onset of flowering, much of which is related to the latitude of origin of the populations. As well unusual plants within populations provide material of ornamental interest. The study relates to the use of C. australis in ecological restoration plantings and threats to conservation of the genetic integrity of wild stands. 1. Introduction The widespread interest in Cordyline australis (New Zealand cabbage tree) both as a wild plant and as a domesticated ornamental plant is documented in the “Dancing Leaves” (Simpson, 2000). Heenan (1991a) prepared a checklist of 38 cultivar names of the five New Zealand species of Cordyline and more have been added since (Anon., 2000). The derivation of the cultivars of C. australis to the time of Heenan’s checklist shows them to be discoveries of unusual variants of leaf variegation and plant habit. C. australis ‘Albertii’, probably the best known variegated cultivar, dates back to a discovery made in Belgium in the 1890s (Metcalf, 1987). Marked variants of habit from the usual massive trunk form are provided by C. ‘Ti Tawhiti’ a dwarf shrub discovered by Mâori before European contact (Harris and Heenan, 1991), and C. australis ‘Karo Kiri’ a recent discovery with short horizontal rigid leaves (Heenan et al., 1994). Further diversification of ornamentals involving C. australis comes from chance and controlled hybridization with other species, notably the Carse Hybrids (Heenan, 1991b) involving the forest cabbage tree C. banksii. The best known cultivar of this hybrid group is C. ‘Purple Tower’. Beginning in the 1980s there was widespread death of cabbage trees in northern New Zealand, which caused public concern as many of the trees had cultural, historical and landscape significance (Simpson, 2000). The condition was named “sudden decline” and there is now sound evidence that it is caused by the parasitic bacteria “Candidatus Phytoplasma australiense” (Beever et al., 2000) transmitted by a sap sucking insect. Proc. XX EUCARPIA Symp. on New Ornamentals Eds. J. Van Huylenbroeck et al. Acta Hort. 552, ISHS 2001 185 The sudden decline epidemic stimulated research on C. australis and one concern requiring investigation was acceleration of the loss of genetic variation of the species through the rapid decline and further fragmentation of its wild populations. The experiment reported here was established to gain information on the intraspecific genetic variation of C. australis with the primary applied objective of conservation of the species in the wild. This objective is of particular relevance as C. australis is one of the most commonly used species in ecological restoration plantings in New Zealand. The experiment has also provided information that has application to the use of C. australis as an ornamental plant and it is mostly this use that is considered here. Information presented is drawn from both published results and ongoing data collection and analysis. 2. Materials and methods 2.1. Populations and phenotypic evaluation In autumn 1994 stand structure and the phenotypes of individual trees within stands of 29 wild populations of C. australis in New Zealand spanning 12º of latitude (Fig. 1) were described. Seed was collected from five trees per population to raise plants for the detection of genotypic differences between trees grown in uniform garden environments. Details of the sites and phenotypic variation of leaves and stem dimensions are presented in Harris et al. (1998). 2.2. Experimental layout The seed collected was sown in spring 1994 with the intention of raising 120 plants per population. This was achieved for 25 populations, one population failed to germinate (No. 23), and three populations provided less than 120 plants. Observations were made on the seedlings while they were being raised for planting in experimental gardens (Harris and Beever, 2000). The experimental layout consisted of trees at 2 m square spacing in randomized blocks with 20 trees per population at Mt Albert, Auckland (36º 53´ S), Lincoln near Christchurch (43º 38´ S), and Invermay near Dunedin (45º 51´ S). The trees were planted at Christchurch and Dunedin in spring 1995 and at Auckland in winter 1996. See Harris et al. (2001) for details. 2.3. Characters recorded Tree height and trunk circumference growth have been measured at regular intervals. Records have been made of the size, shape, and thickness of leaves, and colour variations of leaf bases, blades and ribs. Leaf arrangement on the trunk for the top, middle and lower portion of the leaf tufts of each tree was assessed according to whether the leaves were straight and stiff, curved and floppy, or intermediate between these states (Fig. 2). The shedding of dead leaves, the time trees first flower, and the duration of flowering and fruit ripening within each season have been noted. As trees flower details are taken of panicle structure and flower and fruit characteristics as well as branching patterns as these are strongly influenced by flowering. Cold damage to the young trees at Dunedin and Christchurch was assessed after the 1996 and 1997 winters (Harris et al., 2001). Incidences of pests and diseases have been monitored and special attention has been given to unusual plants. 3. Results 3.1. Tree growth and dimensions The populations differ significantly in their height and trunk growth, thus influencing the appearance of the trees. The ratio between mean height and mean trunk 186 circumference of the 5-year-old trees of the populations at Christchurch in spring 1999 plotted against their latitude of origin shows a trend for them to be relatively shorter and stouter the further south their origin (P < 0.001) (Fig. 3A). 3.2. Leaf size, shape, and arrangement in the crown The populations differed markedly in the mean size, shape, curvature and thickness of their leaves and these differences were related to the latitude of origin of the populations. For example, the trend for leaf length-to-width ratio was for leaves of populations to become broader the further south their origin (P< 0.01). Population 8 has distinctively broad leaves (Fig. 3B). The frequency of the leaf arrangements also differed significantly between populations but was not related to latitude of origin. Blade thickness was the only leaf character shown to have a significant effect on leaf arrangement (Fig. 4A) and this effect diminished progressively from the top to the lower sections of the leafy crown. Population 11 was distinctly stiff-leaved and was also characterized by marked transverse curvature of its leaf blades (Fig. 4B). At 6 years old most populations retain a covering of dead leaves on their trunks but two have a high proportion of trees with lower trunks mostly bare of leaves. 3.3. Leaf colour variation Patterns of red-brown colour of seedling leaves varied between populations and showed a latitudinal pattern (Harris and Beever, 2000). Leaf blades of trees also showed latitudinal variation between yellow-green and grey-green coloration. Yellow-green decreased in more southern populations (P < 0.001) and occurred particularly in populations 7 and 9 from the eastern North Island (Fig.3C). Grey-green showed the reverse latitudinal pattern with especially high incidence in northeast South Island populations. Large differences unrelated to latitude occurred between populations in the distinctness of the yellowing of the midrib. Populations 7 and 9 had very distinct yellow ribs whereas population 11 from a similar latitude (Fig. 1) had green midribs (Fig. 3D). The incidence of purple coloration at the base of the leaves varied from 0 to 74%, was also not related to latitude, and was especially low in populations 16, 17 and 18 in northeast South Island. One tree with distinctly variegated leaves occurred in the Auckland plantation. 3.4. Cold damage Patterns of cold damage arising from low temperatures in the 1996 and 1997 winters showed a strong relationship to the latitude of origin of the populations (Harris et al., 2001). Growth retardation of the northernmost populations at Dunedin from this damage has kept them in the ground frost layer and they continue to show cold damage. Branching from close-to-ground damage induced by the release from apical dominance of primary shoots damaged by freezing has had a marked influence on tree appearance at Dunedin. 3.5. Flowering, flowers, fruit and branching The first tree flowered in spring 1998 and most flowering has occurred in the Christchurch planting. By spring 2000 all trees at Christchurch of population 27 had flowered whereas other populations there have not flowered. A pattern is emerging that the onset of first flowering and the time of flowering in a season are earlier the further south the origin of a population. Flowering has revealed variation in flower and fruit colour and the structure of the panicles.