(Original article written for the Newsletter of Hong Kong Gardening Society)
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| Albizia's pods with mistletoe's leaves behind |
Before
the sense of spring has fully come back, there are clumps of green scattered over
the bare crowns of some deciduous trees and if we have paid enough attention, these
leaves have remained over the whole winter.
It is quite easy to discover this on a few common tree species such as Celtis
sinensis (朴) and Bombax ceiba (木棉). In particular
these leaves look more prominent on Albizia lebbek (Mother-in-law tongue大葉合歡) when the dark green is under the contrast of
the light-coloured tree’s pod.
Such
simple opposite leaves are obviously not belonging to this exotic legume, which
bear compound leaves in a rather pale green tone. However, it would be vain trying to look for any
vine that creeps up the trees. Instead,
these “alien leaves” belong to a kind of mistletoes, oftenly, Taxillus chinensis (Mulberry Mistletoe廣寄生). This mistletoe is also
called Loranth due to its family Loranthaceae.
Mistletoe refers to parasitic plants that are growing
on the aerial part of other plants (usually trees or shrubs). They do produce their own food through
photosynthesis (so called hemiparasite) but rely on their hosts for water
mainly (some studies revealed that they can be even more greedy to take up sugar
and other nutrients as well). There are about
10 species of mistletoe locally but they have different specificity on hosts. Taxillus chinensis has a rather wide
range of hosts but is very popular on Albizia lebbek and other trees
with rough bark. The mistletoe seeds are
brought from birds’ dropping. There are
quite a number of birds which feed on the juicy and sticky fruits of this parasite. The dropping remains gluing and the birds try
to wipe them off their tummy on branches, where the seeds find their next host,
and if the bark is rough enough usually the seeds can settle firmly after
germination. A special organ called haustorium
then develops penetrating deep into the host xylem for absorption of water and
minerals. The infected trees usually suffer
from higher water stress and may gradually retrench the end of the affected
branches or may even decline ultimately.
It is sometime difficult to eradicate an established mistletoe on tree
unless you remove the whole infected parts including their “roots” inside the
host branch.
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| Taxillus's haustorium on host tissue |
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| Scarlet-backed Flowerpecker (AFCD) |
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| Red-base Jezebel (AFCD) |
The Albizia tree was introduced perhaps due
to their spreading shade (that may be too large for a local private garden to
house one) and attractive dry pods hanging above at fall (a bit noisy in a
windy day). As a tree relying on wind to
disperse their seeds together with the pods, Albizia lebbek is not very
attractive to local wildlife in general.
However, the presence of this native mistletoe colony may bring in
something unexpected, such as the colourful bird Scarlet-backed
Flowerpecker (朱背啄花鳥) and a beautiful butterfly Red-Base Jezebel (報喜斑粉蝶), which highly relies on mistletoes for their living. More mistletoes bring in even more different bird
species, but more mistletoe seeds may get into your garden too! Mulberry Mistletoe also colonizes shrubs such
as Sweet Osmanthus and Crepe Mytle!
Before
the sense of spring has fully come back, there are clumps of green scattered over
the bare crowns of some deciduous trees and if we have paid enough attention, these
leaves have remained over the whole winter.
It is quite easy to discover this on a few common tree species such as Celtis
sinensis (朴) and Bombax ceiba (木棉). In particular
these leaves look more prominent on Albizia lebbek (Mother-in-law tongue大葉合歡) when the dark green is under the contrast of
the light-coloured tree’s pod.
