Monday, 3 June 2013

Follow up on Mistletoes

Following my previous post on the subject of mistletoes our friend, and fellow ANPS member, Bill sent me a very interesting email which I reproduce below with his permission.  He also asked that I acknowledge David M Watson, Mistletoes of Southern Australia as the principal reference.

"There are no mistletoes in the Brindabellas?
There are no mistletoes in Tasmania. Mistletoes became locally extinct about 100 000 years ago during the last ice age.  After that time they never established a sufficiently large population to persist.  The Mistletoe bird has been observed in Tasmania, however the journey across the Bass Strait is far longer than the time for seeds to pass through the bird. Mistletoes would also have to contend with their No1 enemy, the Possum.
"Is this is also the case for the absence of Mistletoes in the Brindabellas? Are there Mistletoe birds in the Brindabellas? How far east of the Brindabellas is the closest colony of, say the most common mistletoe, the Drooping Mistletoe, Ameyema miqelii?  Perhaps just far enough away for seeds to pass through the bird before the bird can reach the mountains?  
"Research has shown that the host plants, principally Eucalpyts, have a variety of defences against mistletoe parasitism:·      Eucalpyts can restrict vascular flow of water and nutrients to ‘infected’ branches.·      Trees can actively shed branches at the collar. The cells of a tree’s collar are constructed of specialised cells that allow for the excision of a branch during trauma events.·      Trees can shut down heavily shaded branches.·      Acacias exude sap at the site of penetration physically forcing the seed away with a resin plug,·      Bark thickness is known to affect ‘infection’. Muellerina spp (Brachychiton mistletoe) being an exception.·      Flaky or curling bark can potentially remove seed.·      Some trees such as Euc albens are only mildly affected and generally not throughout the range of the plants. Few recorded Corymbia spp infections implying some form of natural or chemical immunity as found on some trees attacked by Viscum sp. in Europe."
Thinking about some of these issues led me down a track I should have followed in the first place, in seeing what the existing records in the Atlas of Living Australia (ALA) show about Mistletoe distribution.  I will begin with two continent scale maps.

This first small scale map is for Amyema miquelli showing a wide distribution, most everywhere with (I suspect) a high correlation to the presence of observers.
 For Amyema pendula (or A pendulum for traditionalists) the picture is very different.
These images obviously support Bill's comment that there are no Mistletoes in Tasmania.   The yellow circles relate to sites from which the preserved specimens were collected.

Moving in to the area of interest (ie around the ACT) we find these situations (A. miquelii followed by A. pendula).

There are a few records (mainly of preserved specimens) in the Brindabellas but to a large extent the High Country is free from mistletoe.

I will comment (briefly) on the issue of Mistletoebirds and the spread of mistletoes.  While David Watson seems to be The Man on mistletoes themselves, Nick Reid is the equivalent person for links between birds and mistletoe.  While I have not been able to access his list of species that are suspected of spreading mistletoes in another paper (1995) he comments:
"About 16 bird species have been recorded feeding on mistletoe fruit and are potentially legitimate dispersers of mistletoe in Australia (Reid 1986). A further 17 species recorded feeding on mistletoe fruit are almost certainly seed predators. Considerable work is required to determine if a bird species is a legitimate seed disperser. The evidence to date suggests that only three Australian bird species are undeniably involved in mistletoe dispersal: mistletoebird (Dicaeum hirundinaceum), painted honeyeater (Grantiella picta) and spiny-cheeked honeyeater (Acanthagenys rufogularis) (Liddy 1983, Reid 1986, 1989, Yan 1993), although several other honeyeater species are probably dispersers as well.
Of the three 'certainties' only the Mistletoebird is commonly found in the Canberra area.

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