Homoclimes (Similar climates)

 

Dr. David Symon
Senior Botanist Curator Waite Arboretum, Urrbrae SA (1956 – 1985)

Homoclime is not a widely used word. It is derived from homo meaning the same and cline a tract or region of the earth. It is used to describe regions of the globe that have similar climates. Many of the European and North American trees commonly grown in South – East Australia have their origins in very dissimilar climates. The originating tree material would have been imported without thought of matching Australian climates and most likely on visual characteristics. This has proven to narrow the use of such taxa (ie Aesculus hippocastanum) which originally would have been selected from a Northern European ecotype when more suitable ecotypes to suit Southern Australia may have been found in Southern Europe. Significant selection and investigative work in the field of tree selection exists in the Waite Arboretum collection, located in Urrbrae South Australia. This work is ongoing and uses specific scientific principles.

Various climatic indices may be used. J. A. Prescott & C. E. Lane Pool, in their study of the introduction of pines of the Mediterranean region to Australia, used rainfall, mean annual – temperature, temperature – amplitude and temperature – phase (the lag of temperature to day length). It should be noted that soils were not included and nor was altitude. The Authors compared these climatic values with those determined for the original sites of Pinus canariensis, P. pinaster and P. radiata. They were able to find close matches of the climate for these three tree species in the South West of Western Australia, Kangaroo Island, the South East of South Australia, parts of Victoria, the Islands of Bass strait and the North East corner of Tasmania. The major populations of P. radiata pine are now found in these areas with some extending North into New South Wales. In a later study of the climatology of the grape vine in Europe, Prescott used the temperature of the warmest month only, a far simpler index.

These general principles can be used to guide the introduction and suitability of new plants to Australia. Soil types may be important and watering regimes will allow many plants to survive that would normally have perished under normal rainfall conditions. The effect of water withdrawal can be seen in some trees that are slowly senescing after irrigation of the Waite Arboretum ceased in the 1960’s.

Day length, dependant on latitude, can be a strong factor in the control of bud burst timing within deciduous trees. With climates similar to South Australia, with hot – dry Summers, the earlier in spring the trees shoot, the better they can make use of available winter rainfall. Examples can be compared of the early growth (and flowering) of the Southern European Pyrus spp., Fraxinus spp., Aesculus californica and with the late shooting of Fraxinus excelsior ‘Aurea’. Within the Waite Arboretum, the A. californica often will have extension growth of as much as 15-20cm before plants originating from Northern Europe begin to grow.

A great many of our early plant introductions came from Northern Europe and North America, with most not thriving in the Mediterranean type climate (Geographic areas of Adelaide & Melbourne). For example many Ulmus spp., Betula spp., Fagus spp. and Tilia spp. will only survive and thrive with copious watering. I expect that all could be successfully re-introduced by locating new plant material of the same taxa from more southern eco-types.

The concept of homoclimes was used to guide the search of new trees for the Waite Arboretum in the late 1960’s. Some success from this period can be seen in the following list of specimens:

Southern Europe

Acer monspessulanum
Acer obtusifolium
Crataegus azarolus
Juniper phoenica
Phoenix theophrastii

Many Pyrus spp. (Including P. amygdaliformis, P. tadshikistanica)

South Africa

Celtis africana
Curtisia dentata

Cussonia spp.
Euclea pseudebunuus
Nuxia floribunda

Similar concepts can be applied to other parts of Australia, please use the concept of homoclime as a guide in the successful introduction of new and interesting or better – suited trees for the Australian urban landscape.