Download ornamental attributes of the natural variants of cordyline australis

ORNAMENTAL ATTRIBUTES
CORDYLINE AUSTRALIS
OF
THE
NATURAL
VARIANTS
OF
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. More trees need to flower before it can be
187
established that there are differences between populations in these characters and whether
this has a geographical pattern. As well, branching induced by flowering has had a
marked influence on the appearance of trees particularly at Christchurch where those that
flowered early branch on short trunks whereas trees that have not flowered have tall
single trunks (Fig. 2).
3.6. Pests and diseases
A variety of pests and diseases have affected the trees. Sudden decline has
occurred in the Auckland plantation and leaf damage caused by the cabbage tree moth
caterpillar (Epiphryne verriculata) has been widespread in all the plantations.
4. Discussion
New Zealand is of particular interest as a source of new ornamental plants suited
to cultivation outdoors in areas of southwest Europe of similar latitude and in other
temperate regions. Its usefulness as a source of plants is further enhanced by a latitudinal
range of 13º and high altitudes in the Southern Alps that provide habitats with a gradient
of temperature regimes to which its native plants have adapted. The country’s long
geographical isolation resulted in a high percentage of endemic plant species whose
novelty and exotic appearance attracted the interest of the first European visitors and they
quickly introduced them to their home countries. These early introductions were to a large
extent at random. More recently there has been more targeted collection and evaluation of
New Zealand plants for ornamental use particularly in regard to low temperature
tolerance (Harris and Decourtye, 1995; Harris et al., 2000).
Human influence on New Zealand’s flora (about 1000 years) has been short
compared with other regions of the world. Burning of vegetation by Mâori provided new
habitat for C. australis as the species requires vegetation disturbance for establishment.
This would have brought isolated and distinct populations of the species into closer
proximity and consequent hybridization would have acted to blur these distinctions. Even
more intensive clearing of vegetation in the two centuries of European settlement
accelerated this process. Then, as human population increased and land use intensified,
naturally established stands of C. australis were cleared so that now a large naturalized
flora occupies areas suiting C. australis fragmenting the natural occurrence of the species.
The genetic integrity of fragmented wild stands of C. australis is threatened by human
use, at first as a fibre and food plant by Mâori, and currently by its widespread use as an
ornamental plant. This has resulted in considerable movement of C. australis plants
around the country and insect-distributed pollen and bird-distributed seed from these are
able to contaminate its wild stands.
Concerns about genetic contamination of wild stands of C. australis have been
heightened by the marked increase of ecological restoration planting in recent decades.
The wisdom of using seed from local wild stands for these plantings was first emphasized
by Godley (1972) when there was very little information about geographic patterns of
genetic variation of New Zealand species. Just how finely tuned these patterns are is made
clear by this study of C. australis. The results emphasize the subtle adaptation that is
required to allow a perennial species to survive and grow well in a variety of wild
habitats. Molecular biotechnology may well be an effective tool in providing plants with
novel genetic combinations that can be grown under intensive cultivation, but the fitness
of these plants to perform effectively under more natural garden and landscape conditions
is less certain. It is important that there should be balanced investment in the provision of
genetic material for the development of new ornamentals by both the conservation of
already adapted genetic entities and the engineering of new transgenic entities.
“Freak” plants, often discovered when plants are propagated in nurseries, are
likely to remain the main source of new ornamental cultivars in New Zealand. The single
variegated tree found amongst the plants raised for this study is one such “freak” and is
188
being propagated for this purpose. It will not be difficult to establish that this plant is
distinct and, with propagation by cuttings of its stem or the toe of its rhizome, it should
provide a uniform and stable cultivar. This is a slow method of propagation, and it took
about 50 years to increase the stock of the variegated C. australis ‘Albertii’ to provide
sufficient plants for commercial release. Tissue culture is not an easy option to provide
uniform plants of variegated C. australis, and even C. australis ‘Karo Kiri’, which is not
variegated, provided plants with varying degrees of reversion to normal leaf types when
tissue cultured. However, it appears to be possible to avoid changes in coloured-leaf
clones, if not variegated clones, with appropriate tissue culture techniques (Anon, 2000).
Population differences in leaf arrangement point to the potential for more exact
selection of contrasting habits of cabbage tree. In contrast to Phormium where the
structural stiffness of leaves mostly results from leaf folding and curling (King et al.,
1996) thickness of the leaf blade is the main determinant of stiff leaves in C. australis.
The exceptionally stiff-leaved population 11 has this character because of the joint effects
of thick and curved leaves. Use of plants from this population would suit formal garden
landscapes whereas broad floppy-leaved plants of population 8 would fit a more relaxed
setting (Figs. 3B and 4). Similarly, tall, slim- and single-trunk trees have elegance that
give a different landscape impression than do trees with short trunks and early branching
(Fig. 3A). These shorter specimens bear their large inflorescences at a height where their
flowers, strong scent and fruits can be appreciated at close quarters. There is potential to
select for flower and fruit colour and more regular flowering should there be interest in
flowering trees. Cordyline australis is self incompatible (Beever and Parkes, 1996), but
the high frequency of stiff-leaved plants for population 11 (Fig. 4) indicates that a small
isolated group of trees of this population could provide seed-raised plants sufficiently
uniform for specified landscape and pot plant styles.
Cordyline australis trees that readily shed leaves can be a nuisance where the
tough fibrous leaves fall on lawns and get tangled in mower blades. By contrast, where
they are grown as tub plants or surrounded by paved areas, easy detachment of dead
leaves aids their grooming to leave bare trunks with tufts of green leaves.
The difference between the yellow-green and grey-green leaf blade colours is
perhaps not sufficiently marked by itself to provide a feature of ornamental interest.
However its association with a latitudinal gradient (Fig. 3C) suggests a possible adaptive
function in regard to light intensity and quality. There is interest in New Zealand in
ultraviolet B irradiance damage to plants (Hunt et al., 1996) and it was speculated that
red-brown pigmentation of seedlings might provide a screen to prevent this (Harris and
Beever, 2000). However a more plausible explanation is that the pigments may screen
frozen tissue from the damaging effect of high light levels after radiation frosts. Certainly
there are differences in the cold hardiness of C. australis populations that can extend the
area of usefulness of the species as an outdoor plant (Harris et al., 2001).
The yellow midribs of the leaves of populations 7 an 9 from the east of the North
Island (Fig. 3D) are sufficiently striking to make this feature of ornamental interest
especially as it is linked to the yellow-green blade colour (Fig. 3C) and these populations
have the tallest trees. Purple pigmentation is mostly concealed at the base of the leaves
but in a few plants it extends further up the midrib. Locations in New Zealand where it is
most likely to be encountered are defined by this study. Possibly even stronger expression
of this character may provide the purple-pink in the leaf colour variegation of C. australis
‘Albertii’ and the distinctiveness of some of the more recent coloured-leaved cultivars.
Pests and diseases that disfigure C. australis are probably the greatest deterrent to
its ornamental use in New Zealand. Although there has been some screening of their
occurrence and damaging effects on the trees in the experimental plantations, it is too
soon to conclude that there is scope for the selection and breeding of resistant plants.
Keeping regions where C. australis is introduced free of its pest and diseases is a
requirement to maintain its high usage as an ornamental.
189
Acknowledgements
Support was provided by Landcare Research through funding from the Foundation for
Research, Science and Technology, New Zealand and by my colleagues Ross Beever and
Sue Scheele.
References
Anon., 2000. Cordyline by the million – this UK nursery shows the way. Comm. Hort.
March 2000: 12-13.
Beever R.E., Andersen M.T., Winks C.J., Wood G.A., Sutherland P.W. and Forster
R.L.S., 2000. Phytoplasma diseases of native plants in New Zealand. In: Miller, H. ed.
Mundulla Yellows: a new threat to our native vegetation – meeting the challenge.
Conservation Council of South Australia, Adelaide, Australia, pp 7-12.
Beever R.E. and Parkes S.L., 1996. Self-incompatibility in Cordyline australis
(Asteliaceae). N.Z. J. Bot. 34: 135-137.
Godley E.J., 1972. Does planting achieve its purpose? Forest and Bird 185: 25-26.
Harris W. and Beever R.E., 2000. Genotypic variation of seedlings of wild populations of
Cordyline australis (Lomandraceae) in New Zealand. N.Z. J. Bot. 38: 595-606.
Harris W. and Decourtye L., 1995. Observations on cold damage to New Zealand plants
grown at Angers, France. Hort. in N.Z. 6(1): 9-19.
Harris W. and Heenan P.B., 1991. Cordyline ‘Ti Tawhiti’ and its relationship to
Cordyline ‘Thomas Kirk’. Hort. in N.Z. 2(2): 2-5.
Harris W., Beever, R.E. and Heenan, P.B., 1998. Phenotypic variation of leaves and stems
of wild stands of Cordyline australis (Lomandraceae). N.Z. J. Bot. 36: 593-604.
Harris W., Beever R.E. and Smallfield B., 2001. Variation in response to cold damage by
populations of Cordyline australis and of some other species of Cordyline. N.Z. J. Bot.
39: 147-159.
Harris W., Cadic, A. and Decourtye L., 2000. The acclimatization and selection of New
Zealand plants for ornamental use in Europe. Acta Hort. 508: 191-196.
Heenan P.B., 1991a. A cultivar checklist for the New Zealand species of Cordyline
(Asphodelaceae). Hort. in N.Z. 2(1): 8-12.
Heenan P. B., 1991b. Cordyline cultivar names – three new combinations. Hort. in N.Z.
2(1): 6-7.
Heenan P.B., Harris W. and Bayliss R., 1994. Cordyline australis ‘Karo Kiri’
(Asteliaceae): a new dwarf cabbage tree cultivar from New Zealand. Hort. in N.Z.
5(2): 2-7.
Hunt J.E., Kelliher F.M. and McNeil D.L., 1996. Response in chlorophyll a fluorescence
of six New Zealand tree species to a step-wise increase in ultraviolet-B irradiance.
N.Z. J. Bot. 34: 401-410.
King M.J., Vincent J.F.V. and Harris W., 1996. Curling and folding of leaves of
monocotyledons – a strategy for structural fitness. N.Z. J. Bot. 34: 411-416.
Metcalf L. J., 1987. The cultivation of New Zealand trees and shrubs. Reed Methuen,
Auckland.
Simpson P., 2000. Dancing leaves. The story of New Zealand’s cabbage tree, tî kôuka.
Canterbury University Press, Christchurch.
190
Fig. 1. Locations in New Zealand of the wild populations of Cordyline australis studied
for their genetic variation numbered latitudinally from north to south (see Harris et al.
1998 for details). The experimental garden sites are near to Auckland, Christchurch and
Dunedin.
191
Fig. 2. Cordyline australis trees in the Christchurch plantation showing variation in height
and stiff- and floppy-leaved forms. The measuring rod is 4m high.
Fig. 3. The relationship of latitude of population origin to variation of (A), mean tree
height-to-trunk circumference ratio; (B), mean leaf length-to-width ratio; (C), mean
percentage frequency of yellow-green leaf colour; and (D), mean percentage frequency of
leaves with distinctly yellow midribs of 28 populations of Cordyline australis.
Populations are numbered from north to south.
192
Fig. 4. The relationship between mean percentage frequency of floppy leaves in the upper
section of the tufts of 28 populations of Cordyline australis and (A), mean thickness of
the leaf blade; (B), mean percentage frequency of leaves with marked transverse
curvature of the leaf blade. Populations are numbered from north to south.
193