Dr Jennifer Gardner
As an experimental rain fed collection The University of Adelaide’s Waite Arboretum is becoming an increasingly valuable resource in view of global warming and widespread water restrictions. Nestled in the foothills of Adelaide, South Australia, 34o58’S 138o38’E at an altitude of 100–110m, the Arboretum was established in 1928 following a bequest to the University by Peter Waite. Under the terms of his gift, half his Urrbrae estate was to be held upon trust and in perpetuity as a park for the enjoyment of the public and the creation of an arboretum was in keeping with Waite’s interest in science. Meaningful evaluation of the performance of tree species and cultivars takes many years. The Deed of Trust ensures the security of tenure needed for such long-term testing.
The Arboretum occupies 30 ha and comprises over 2,200 labelled, documented and mapped trees from around the world growing under an average rainfall of 624 mm. Initial plantings sourced trees from the Eastern States featured many northern European species and were watered through the summer months. In 1960 the then Curator David Symon decided to cease summer watering and seek species from homoclimes such as the Mediterranean, Chile, South Africa and southern California. Since then many of the early plantings have died or declined, but the collection clearly demonstrates which species can survive without supplementary watering for more than 45 years. The information derived from the Arboretum is becoming increasingly relevant to inform species selection for our urban forests. Following is a selection of some of these successful species.
Dragon Tree Dracaena draco is the iconic species of the Arboretum. It is very slow growing but also very long-lived. Some of the Arboretum specimens are 80 years old and specimen in Brougham Place, North Adelaide is thought to be over 100. In its native Canary Islands specimens have lived for centuries. The longevity, striking flat-topped appearance and drought tolerance of this species suggest a possibility for Avenues of Honour. The dense canopy provides good shade in parks and gardens, provided the taller, more massive Canary Islands form is selected, rather than the much shorter Cape Verde Islands form.
Another impressive species from the Canary Islands is the Canary Island Pine Pinus canariensis with rich russet brown trunk and long pendulous needles. Its stature and long life also make it suitable for Avenues of Honour. Although it thrives in our climate, it does not seem to have the weedy potential of the Aleppo Pine Pinus halepensis, at least in Adelaide.
Waite Arboretum has a fine collection of over 80 species of Waterwise oaks. Cork Oak Quercus suber from the west and central Mediterranean merits much wider use. It is a broad handsome evergreen species with attractive corky bark and small leathery leaves. The very drought-tolerant, long-lived Algerian Oak Quercus canariensis from N. Africa and the Iberian peninsula would also make a fine species for Avenues of Honour. There are nine mature semi-deciduous or evergreen specimens in the Arboretum. Two more Mediterranean oaks flourishing in the Arboretum are the Kermes Oak Quercus coccifera a dense evergreen tree to 4.5m, and Tabor or Vallonea Oak Quercus ithaburensis an attractive deciduous species with bright green foliage in spring.
Seldom used in our urban forest, but of great potential for streets are the Californian oaks of which 22 species are represented in the Arboretum. Blue Oak Quercus douglasii is a large stately deciduous tree with upright branching and attractive blue-green foliage. The 50-year-old Arboretum specimens have heights and spreads of 12m. Other successful Californian species include: Californian Field Oak Quercus agrifolia with a large dense rounded canopy, an excellent shade tree; semi-evergreen Engelmann Oak Q. engelmannii; deciduous Valley Oak Q. lobata the largest North American oak and very drought resistant; and Interior Live Oak Q. wislizenii. Canyon Oak Quercus chrysolepsis is a very ornamental evergreen with bright yellow terminal twigs and petioles. The species occurs naturally over an astounding rainfall range, from <150 mm to >2.8m.
Turning to a few drought-tolerant Australian species, Wilga Geijera parvifolia though uncommon in cultivation in Australia is highly regarded in California as a street tree where it is called the Australian willow because of its narrow leaves and pendulous habit. A profusion of tiny cream flowers is produced in spring. Treenet, in collaboration with the City of West Torrens, established a trial in 2000 which is proving very successful.
Another tree being trialled in the City of West Torrens based on fine specimens in the Arboretum, is Crow’s Ash or Australian Teak Flindersia australis. It is an imposing tree and is recommended for wide verges and Avenues of Honour. Lacebark Brachychiton discolor is a striking species that is summer deciduous and when leafless is covered with large bright pink bell-shaped flowers. Whitewood Atalaya hemiglauca is a very hardy and drought-resistant tree 5 to10 m, with fragrant flowers. In the literature, it is reported as freely suckering, which would be disadvantageous in cultivation, but suckers have never been observed in the Arboretum where the specimens are 44 years old. The Tulipwood Harpullia pendula has a dense, shady canopy and decorative orange capsules. Native Olive Notelaea microcarpa which has a very wide distribution from Darwin and the Gulf District to the Southern Tablelands of NSW is a drought-resistant small tree. The mature specimens in the Arboretum have a dense rounded crown and are 6 m tall after 40 years without watering. It is anticipated that Treenet and Local Governments will collaborate to establish more trial sites and broaden the palette of waterwise species used for street planting.
Experimentation is ongoing in the Waite Arboretum with the planned establishment of a Dry Rainforest Demonstration Garden. This will incorporate not only species demonstrated to have performed well in the Arboretum such as Brachychiton spp., Wilga, and Rusty Fig Ficus rubiginosa, but also a range of trees and shrubs little known in cultivation. All the material has been propagated from wild collected seed by Daryl Kinnane of Native Rainforest Flora and the garden will also serve ex situ conservation of some of the rarer species.
BIODIVERSITY IN THE URBAN FOREST
While there are many introduced species that have merit for the urban forest there is a trend to look to indigenous species which are not only adapted to local soils and climate but also provide valuable habitat and food sources for native fauna. There is a need for improved cultivars of many Australian species selecting for good form and low fruiting. In Local Governments, interest is also growing to incorporate grasses and other understorey species in our streetscapes.
In the northwest section of Waite Arboretum are three fine Grey Box Eucalyptus microcarpa which are thought to predate white settlement. As the non-indigenous species die in that section they are not replaced. Grey Box seedlings are allowed to regenerate and the indigenous Black Forest understorey is being restored. Volunteers collect and propagate seeds of sheoak Allocasuarina verticillata, Golden Wattle Acacia pycnantha and Sticky Hopbush Dodonaea viscosa as well as a wide variety of herbs, bulbs and grasses from the Waite Conservation Reserve, 147 ha of Grey Box Grassy Woodland on the campus. Seedlings are hand watered and weeded for one to two years then reliant on rainfall. In the nine years since the project commenced the understorey species are self-seeding and wildlife is starting to return.
The Waite Arboretum is an important resource for landscape architects, planners and arboriculturists and the nursery industry. The Arboretum demonstrates the successful performance of species from around the world which survive our hot dry summers and rainfall of up to 624 mm without supplementary watering. This information is becoming increasingly valuable with global warming and water restrictions. There is great scope for Local Governments and organisations such as TREENET to collaborate and extend the Arboretum trials out into the harsher street environment and to make the results of the trials widely available through the TREENET website treenet.org. The nursery industry is urged to take up the challenge to make more widely available some of the uncommon species whose drought tolerance and good performance have been demonstrated in the Waite Arboretum.