Heidelberg Mistletoes Revisited: Decadal Changes in the Distribution of Creeping Mistletoe 'Muellerina Eucalyptoides' on Introduced Trees in Suburban Melbourne

2007·1.6 MB·English

by  Gregg Muller

Contributions
Heidelberg mistletoes revisited: decadal changes in the
distribution ofCreeping Mistletoe Muellerina eucalyptoides
on introduced trees in suburban Melbourne
Gregg Muller
SchoolofOutdoorEducationandEnvironment,
LaTrobeUniversity,Bendigo
Email:[email protected]
Abstract
Introducedtreehostsofcreepingmistletoe in Heidelberg,Victoria,wereresurveyedafteran interval
often years. There was substantial turnover ofhosts in the decade, and increasing disparity in the
density ofboth infected trees and mistletoes between the elevated western block compared to the
adjacent valley slopes to the east, with more than five times the density ofinfected trees and ten
timesmoremistletoesinthewest. Differentpotentialhostdensitiesbetweenthesitesdonotexplain
thedifferencesin infection rates.(TheVictorianNaturalist124(1),2007,27-32).
Introduction
Mistletoes have an intriguing biology - Plum Primus sp.. Plane Tree Platanus sp.,
they are hemiparasites (they photosynthe- Oak Quercus sp.. Elm Ulmus sp. and Birch
sise, but obtain their moisture and nutrient Betula sp. in a 300 hectare area ofsubur-
requirements from their host), that rely, at ban Heidelberg, north ofMelbourne. There
least in southern Victoria, on the have been few studies into changes in the
Mistletoebird Dicaeum hirundinaceum to spatial distribution of Australian mistle-
spread their seed. While mistletoe has toes, so the inclusion ofa detailed distribu-
oftenbeen seen as a pest, recentwork indi- tion map in that paper (Fig. 1) suggested a
cates that mistletoes are important compo- follow-up study to investigate changes in
nents ofwoodland and forest ecosystems host distribution and infection patterns
(Watson 2001). They provide reliable nec- overthe interveningdecade.
tar and fruit resources, often when little
Study area and methods
eflosrebiisrdasv.aiPlaobslse,umasndprsehfeelrteerntaianldlynebstroswitsees The study site spans the uneven rectangle
on mistletoe, and a number of species of bounded by Waterdale Road to the west
and Rosanna Road to the east, and
butterfly rely on mistletoes as a host for
Southern Road and St James Road to the
theircaterpillars.
Some mistletoes are host specific, but north and Banksia Street to the south (Fig.
most species can parasitise a number of 1). The area is divided into two approxi-
mately equal blocks by Upper Heidelberg
genera(Downey 1998). Creeping Mistletoe
Road/Waiora Road, which runs north-
Muellerina eucalyptoides has successfully
adopted a number ofintroduced deciduous south through the site, and which also
tree genera as hosts in suburban forms a topographic boundary between the
Melbourne. The lack ofleaves on the hosts relatively flat elevated area to the west,
in winter, and the closely spaced suburban and the slopes descending to the Yarra
street network means that surveying for Rivertotheeast.
mistletoes in the suburbs can be consider- The area is a mix of older residential
housing surrounding Burgundy and Bell
ablymoreefficientthan innativeforests.
liIsnhed19a97s,pecTihael eVdiicttioorniaonnNmaitsutrlaetloiesst(pVuobl-. bStarneethso,uasnidngpotsottSheeconnodrtWh,orwlidthWasrcastutbeurre-d
p1a1p4e(r3)().SeIenbcelucdked19in97t)herecpololretcitnigonownasthea psahrokpspianngd sstormipeslaiglhotngindtuhsetrmiaaljoarrearsoaadns.d
distribution of Creeping Mistletoe Little native vegetation grows within the
Muellerina eucalyptoides growing on area, apart from a small number of old
introduced host trees, primarily Cherry eucalypts between Brown Street and St
James Road.
Vol. 124(1)2007 27
Contributions
Following the methodology of Seebeck effortof 16.16 km perkm2 a slightly high-
(1997), each street was surveyed from a er figure than in the ea,st due to the
slowly moving vehicle in August 2005, subdivision geometry.
when the lack ofleaves on deciduous trees In April 2006 afurthersurveywas under-
facilitated the detection of mistletoes. taken to establish the density ofpotential
Roadside trees and private gardens were host trees in the area. Approximately 20
surveyed, but the two campuses of the percent ofthe roads (4.148 km in the east,
Austin Hospital (Austin and Repatriation) and 5.444 km ofroad in the west) in each
were not. Mistletoes in evergreen trees block were surveyed from a slow-moving
were not surveyed. Each distinct clump of vehicle, and the genus and location ofeach
mistletoewasrecorded, alongwith the host potential host tree was recorded and
tree genus, and the position was logged loggedwith aGPS unit.
using a Global Positioning System (GPS)
unit. Since Creeping Mistletoe may have a Results
creeping habit along the branches of its Mistletoes
host, these clumps may not represent dis- The location ofinfected trees is shown in
tinct individuals, but for the purposes of Fig. 2. Infected street trees are plotted at
this paper they are considered as such. the actual location (typically accurate to
Where trees were heavily infested or +/- 10 ni using the GPS), but those occur-
observations were doubtful, closer inspec- ring on private property are plotted at the
tion on foot and/or with binoculars was nearest point on the street, and may be up
carried out. Host trees could generally be to30 metresfrom theiractual location.
identified by morphology and bark. Mistletoe and infected tree densities are
Doubtful identifications were rechecked shown inTable 1. Thesefiguresmayunder-
when theplantswere in leafin April 2006. represent the true densities, since buildings
Within the east block (area =1.428 km2 and foliage, particularly in back and side
18.69 km ofroad was surveyed, represent-) yards, may have obscured mistletoes and
ing a survey effort of 13.09 km per km2 In host trees occurring on private property.
trhoeadwewsats(saurrevaey=e1d.,33r7epkrems2e)n,t2i1n.g61a skumrv.eoyf bSlinoccekst,hethseurmveoysteffaocrctudriaftfeeremdeainsutrheetfwoor
comparison ofmistletoeand hostdensity is
Southernft Heidelberg mistletoe hosts: 1995
©go G
• ••• °
Ljr: -h1
UmT If* StJameaf
Bq
G
• G • Prunus N
fI 0 8fownstQ + « Plane
' G Oak
+ Birch
G Elm
G •L.
Bur9unOyS( #
G
0.4 0.8
kilometres
Fig. 1.Mapshowingdistributionofmistletoehostsin 1995.
28 The Victorian Naturalist
Contributions
Table 1. Densityofinfectedhostsandmistletoes
Block Numberof Hostsper Numberof Mistletoesper
hosts surveykilometre mistletoes surveykilometre
East 30 1.61 61 3.26
West 153 7.08 715 44.24
Table2. Densityofpotentialhosttrees
Block Numberof Surveydistance Potentialhostsper
potentialhosttrees (km) surveykilometre
East 130 4.148 31.34
West 141 5.444 25.90
Fig.2.Mapshowingdistributionofmistletoehostsin2005.
mistletoes per survey kilometre and hosts susceptibility to infection. Ifone area had
persurveykilometre. more hosts of a particularly susceptible
In the west block there were more than genus, then that might give rise to greater
five times as many infected trees and more infected host densities in that area. A com-
than ten times the density of mistletoes parison ofpotential host density with actu-
compared with the east black, both in al hostdensity by hosttree generabetween
absolute terms and relative to survey the east and west blocks indicates that dif-
effort. ferences in potential hostdensitiesbetween
the blocksdoesnotexplainthe marked dif-
Potentialhosttreedensities ference in infected host density between
The number of potential host trees per
theblocks(Fig. 3).
kilometre ofsurvey in each block is shown While relative densities of cherry plum
in Table2. are comparable across the blocks (black
Incidental observation indicated that dif- dots, left hand axis), actual infection rates
ferent host genera might have differing (open squares, right hand axis) are consid-
Vol. 124 (1) 2007 29
Contributions
erably lower in the east block. Birch and tohavebeen used, andthenumberofpoints
oak densities in the east are more than shown on the map is less than the number
twice those in the west, but infection rates of infected trees referred to in the text.
are approximatelythree times lower. These Where the infected trees are moredispersed
results should be treated with caution for it is probably safe to assume that all ofthe
some genera. Plane trees, and to a lesser infectedtreeswereplottedonthemap.
extent oaks, occur as discrete patches of In spite of uncertainty in re-identifying
street plantings, leading to a very ‘lumpy’ some ofthe hosts in the older study, most
distribution across the study site, and the individual hosts - particularly where only
potential host survey, which only sampled a singlemature specimen ofthe host genus
a portion ofeach block, may have mis-rep- occurs in a location- can still be identified
resented the actual density ofthese genera (Tables 3 and 4). Ifthe Seebeck map is
in the blocks. Since cherryplum, birch and generally reliable, then overall mistletoe
elm are more evenly dispersed across the infection rates in the east block (where no
area, they are considered unlikely to suffer dense clusters of infected trees occur)
from thislimitation. appear to be relatively stable, but with a
2C0h0a5nges in infection patterns, 1995 - coNnosidcelreaabrlesptautrinaolvepratitnerhnostisn.persistence,
A direct comparison ofchanges in the spa- ahobsatnsdionntmheenetastorblroecckruwiatsmeenvtidoefnt.mistletoe
tial distribution ofmistletoes is not possible The picture is less clear in the west
since Seebeck recorded infected host trees, block. Since a high proportion of infec-
rather than mistletoe plants. While
tions in the west block occur in tight clus-
Seebeck'stext isnotexplicit, itappearsthat ters, estimates ofpersistence, recruitment
where multiple mistletoes occurred in an and abandonment within these patches are
individual tree, only the host tree was
unlikely to yield reliable data. The incom-
troeecsorwdietdhirnatthheer thohsatn.tThheenummabperinocflumdiesdtlei-n tphleetrecdoamtpaliinccaltuedsedthien tihsesuSe.eeTbhecekdmataappfruer--
that paper is also incomplete, since not all sented in Table 4 are only indicative of
host trees bearing mistletoe referred to in changes in the dispersed host areas outside
thetextappearon themap, forwhatappears
oftheclustered infection areas.
to be reasons ofcartographic simplicity. However, within these dense clusters
Where dense clusters of infected trees changes can be inferred from Seebeck’s
occurred, some cartographic licence seems text. Mistletoes in the row ofoaks along
Fig.3.Comparisonofpotentialhostsand infectionsbygenus,eastandwestblock.
30 The Victorian Naturalist
Contributions
Table3.Changesinhosttrees. EastBlock, 1995-2005.
Genus Persistenthost Abandonedhost Newhost
Cherryplum 4 6 9
OEalmk 20 3l 21
Birch 1 1 2
Plane 1 0 0
Other 0 0 2
Total 8 11 16
Tabic4.Changesindispersedhosttrees, West Block, 1995-2005.
Genus Persistenthost Abandonedhost Newhost
Cherryplum 11 23 41
Elm
Oak 01 21 41
Birch 2 3 8
Plane 0 0 0
Other 0 0 3
Total 13 29 57
Lloyd St have expanded west from the persisted over ten years, while there has
original 15 hosts (out of 49) clustered at been considerable turnover in the mistletoe
the east end ofthe street, to 39 infected population, suggests this is not the case.
trees (although Seebeck’s map only shows The increase in infected trees in the west,
12hosts). while infection levels in the east have
Similarly, inthe line ofplanetrees in Saint remained relatively stable, lends weight to
Hellier St, the mistletoe population has thatview.
expanded from nine trees clustered toward Like any organism, the population of
the east end ofthe street, to 21 hosts, with mistletoes is a function of the balance
the infection spreading west along the plan- between recruitment and mortality. In the
tation. The group of mistletoes in the case ofmistletoes, however, this is compli-
Dresden Stplanetreeplantation hasexpand- cated by reliance on a specific vector
ed south and increased from the original six (Mistletoebirds, Diaceum hirundinaceum)
trees to nine (out of 17). However, in the for spread, and host specificity for estab-
nearby group ofnine similarly aged planes lishment.
in Edwin St infected trees have increased From the data presented here, potential
fromonlyoneinfectedtreetotwo. host densities are not the cause ofdiffer-
Apartfrom atrend in hostcycling similar ences between the blocks, since more
to that noted in the east block, there potential hosts occurin theeastwherethere
appears to be a spread of infection from is less mistletoe. Underlying geology and
the high density patches at the east ofthe tree cover density (Muller, in prep.) appear
block toward the less densely infected nottobethecausative factorseither.
areasto thewest. The differences may lie in the biology
and behaviour ofthe vector or population
Discussion
tWhheildeistthriisbruetsieoanrcphathtaesrnssheadndsocmheanliggehst oinn econnctersolaraigseinntgs,froormmithcerotcolpimoagtriacphdyiftfheart-
affect mistletoe establishment or vigour.
maibsletlneutomebehrosotfdqeunessittiyo,nsitrreagiasredsinagctohnesicdaeurs-- Perhaps Mistletoebirds prefer the elevated
area to the west ofUpper Heidelberg Road
esofthedifferencesbetweentheblocks.
The difference in density between the to the valley slopestothe east. Department
east and the west may be just a chance of Sustainability and Environment data-
occurrence, butthe factthatthe pattern has baserecords shed little lightonthe issue of
Mistletoebird visitation, with only three
Vol. 124(1)2007
31
Contributions
records for the study area, all in the east urban ecosystems. If this is true, then
block. mistletoes on introduced trees may be a
Differential distribution of possums, critical element in establishing and main-
implicated as mistletoe control agents in taining diverse ecosystems in our cities
other studies (Reid and Yan 2000) may be and towns, particularly since the densities
the cause. Again, records in the reported here are considerably higher than
Department of Sustainability and have observed for Box Mistletoe
I
Environment database are sparse. Only Amyema miquelii in forests in central
four records for Common Brushtail Victoria(unpubl. data).
Possum Trichosurus vulpeculaand threeof The high visibility ofmistletoes in decid-
Common Ringtail Possum Pseudocheirus uous trees during winter makes mistletoe
peregrinus occur in the study area. study in urban areas considerably easier
Anecdotal reports from residents and the than in native forestsettings. Therelatively
local municipality suggest that possums good historical records that exist for urban
are fairlywidespread although no quantita- areas, and the ease of access and large
tivedata areavailable. number of potential observers in these
The apparent spread of mistletoes into locations suggestthatthe suburbs maybea
previously unoccupied hosts in the west prime location foruntangling the complex-
block indicates an increase in recruitment itiesofmistletoeecology.
occurring in the west block but not in the
east. This may be due to chance, to DRoewfneeryenPcOes(1998). An inventory ofhost species of
changed circumstances occurring over the each aerial mistletoe species (Loranthaceae &
paalstterdneatciavdeelyi,n tbheecawuesstebluotngno-ttetrhme eeaqsuti,lio-r ReViidscaNceaane)diYnaAunstrZal(ia2.00C0u)nniMnigsthlaemtioae,s5,a6n8d5-7o2t0h.er
phanerogamsparasiticoneucalypts. InDiseasesand
brium in the mistletoe population has not PathogensofEucalyptspp353-384 Eds Keane, PJ,
yetoccurred in thewestblock. Kile, GA, Podger, FD and Brown, BN (CSIRO
Anthropogenic factors may be another Publishing:Melbourne).
SeebeckJ (1997)CreepingmistletoeMuellerinaeuca-
causative factor. Differences in gardening Ivptoides in suburban Melbourne. The Victorian
hcahboiitcsesan-d wchhoiicches,mabyehaivnifoluure,ncaendbopteht WaNtastounr,aliDsMt1(1240,0113)0-M1i3st4l.etoe-akeystoneresourcein
forestsand woodlandsworldwide.AnnualReviewof
Mistletoebirds and possums-mayall have Ecology>andSystematics32,219-249.
some influenceon mistletoedistribution.
Mistletoes are considered to be keystone
resources in forests and woodlands
(Watson 2001 and the same may hold for
) Received5June2006;accepted16November2006
AValentine’s Day poem
Goodeniaovataisyellow in flower
Asbrightasmy love, foryou everyhour
WhiletheCommon Hovea ispurple in hue
(Well it’sactually mauve, between meandyou)
The Caladeniarosellahaspetalsofred
’Tisthecolourofpassion, it’soften been said
Butthe bestplantofall forthejoboftype-casting
Is theBracteanthabracteate-theGolden Everlasting
Itsname says itall, in colourandstyle
Likemy love, it is pureandgoes on all the while
written byoneofthe Editors
for his wife
32 The Victorian Naturalist