ECOLOGY OF ASCLEPIAD LIANES 
95 
strongly related to latitude, reaching a maxi¬ 
mum of 109 days in the south of the area in 
northwestern Victoria (Australian Division of 
National Mapping 1986, Badawy 1982). Given 
the recent emphasis on absolute minimum 
temperature in defining distribution limits 
(Woodward 1987, Booth 1990), such values 
range from 4.5°C at Tennant Creek, -7.5°C at 
Alice Springs, -2.8°C at Broken Hill, -4.0°C at 
Mildura, -5.8°C at Ouyen and -2.0°C at Wal- 
peup (Australian Bureau of Meteorology unpub¬ 
lished screen temperatures). 
Within the area studied in detail, north¬ 
western Victoria, mean annual rainfall increases 
from 250 mm in the north (Neds Corner) to 343 
mm in the south (Walpeup). Temperatures de¬ 
crease and frost season increases at the same 
time; the areas south of about 35°S lacking Mars- 
denia and Rhyncharrhena are both wetter and 
colder than those areas supporting them 
(Badawy 1982). As growth of both species occurs 
especially in summer, decreased summer rain¬ 
fall could possibly limit their distribution. 
However, there is no such decrease going from 
the north to the south of their Victorian range; 
some factor related to declining temperatures 
seems more likely to set their southern limits. 
There is no clear evidence that the two species 
differ in their climatic tolerances. 
A survey in 1985-6 of 1,300 20 m x 20 m 
quadrats in predominantly native vegetation in 
northwestern Victoria produced four records of 
Marsdenia and eight of Rhyncharrhena (Cheal & 
Parkes 1989 and personal communication), so 
these are not common species. 
The Victorian distribution maps show 23 
Marsdenia minor grid records and 15 of 
Rhyncharrhena (Fig. 3), partly reflecting the 
view that Rhyncharrhena is much the rarer of 
the two, e.g. in Victoria (J. N. Macfarlane per¬ 
sonal communication) and around Broken Hill 
(Morris 1975). 
In central Australia, most Rhyncharrhena 
plants are found in Acacia aneura communities, 
while Marsdenia occurs in most habitats (P. K. 
Latz 1982 and personal communication). How¬ 
ever, in New South Wales both species occur in a 
wide range of communities, including those 
dominated by Acacia aneura, Casuarina pauper, 
Eucalyptus intertexta and various mallee species 
of eucalypt (“mallee”) (Cunningham et al. 
1981). In Victoria, the two asclepiads have very 
similar habitat ranges, both occupying relatively 
fertile sandy loams to clay loams carrying (1) 
□ 
■ V 
Y r— 
B 
% 3. Victorian distribution of (A) Rhyncharrhena linearis and (B) Marsdenia australis based on presence or 
absence within 10‘ latitude x 10‘ longitude grid squares for post-1 950 records. All records from the Flora Survey 
and Management Group, Department of Conservation and Environment, with the addition of A31 and F29 for 
M. australis. 
