Some gardeners like to keep one or a few maple
trees for their attractive seasonal colour change. But unlike template region, it is not always
easy to achieve this goal in Hong Kong, particularly in the central urban area
where warm air prevails for most of time.
Seasonal foliage colour changes depend on a
basket of factors such as species properties and weather. While there are quite a lot of tree species
which change colour before fall, not all turn red as maple trees do before leaves
drop. Some species like Ginkgo biloba
(Maiden Leaves 銀杏) are well-known and popular for their golden colour in
late autumn. Ginkgo biloba is not
common in the subtropical Hong Kong, but we have Celtis sinensis and Melia
azedarach, both of which dress yellow before mid-winter.
Gingo biloba which turns yellow in autumn (taken at Japan)
The yellow leaves come from the pigment
xanthophyll. Sometimes carotenoids also
play their role too. These two pigments present
in leaves for most of the time but are always masked by another green pigment namely,
chlorophyll when the leaves are still actively producing food through photosynthesis. Chlorophyll is essential for fixing carbon in
the air into carbohydrate with the aid of sunlight, but it is also broken down
gradually in sunlight. In growing season
healthy trees replenish chlorophyll through continued supply of minerals and
water from other parts of the plant.
At some point after the growing season, this
supply is gradually blocked by the formation of an abscission zone at where a
leaf is attached to the branch. The
level of chlorophyll then drops and the yellowish and orangish hue then becomes
visible after fading of the green shade.
Also due to this blockage, in some species – maple trees for example -
the leaves gradually turn red / purple due to the accumulation of anthocyanins from
the transformation of the trapped sugar in leaves.
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| progression of colour change in Liquidambar formosana |
While variation in daylength triggers the formation
of abscission zone, weather condition plays an important role to the duration
and completion of the whole process. After
the abscission zone has formed, abundant light and low temperatures will destroy
chlorophyll more rapidly. Cool air particularly
at night promotes the formation of red/purple pigments. This explains why people found that maple
trees in Hong Kong (at Tai Tong) showed their brightly red only in some years,
when there had been a sharp and prolonged cooling.
There are other conditions which are
detrimental to the colour show. If the
trees have suffered a prolonged drought, the abscission zone will form earlier
and the leaves will drop before colour change.
Occasional heavy wind or rain during late autumn will also cause leaf
drop before the colour change.
True maple trees are Acer spp. (槭樹),
from which maple syrup is extracted (from some species). Locally Acer spp. are not as common as
Liquidambar formosana (Sweet Gum 楓香), which is usually mistakenly referred
as maple by many people due to their similarity in appearance and they turn bright
red. In fact these two genera belong to
different taxonomic groups. They can be
distinguished simply by the leaf arrangement – with Acer spp. being opposite and
Liquidambar spp. being alternate.
Their fruits are completely different too, with Acer spp. baring winged
seeds in pairs, while Liquidambar spp. have their fruits arranged in
spherical shape.
 |
| Liquidambar formosana |
Though
being a minority in local flora, occasionally Acer buergerianum (Trident
Maple三角槭) is seen in countryside or even planted in some parks. It is referred as an exotic species to Hong
Kong although its natural distribution covers Guangdong to the southmost. Other than this large tree species, there are
also other smaller amenity species of Acer spp. planted in local gardens. Some of them look reddish even all year round
- which is another mystery perhaps.
 |
| Acer buergerianum also turns orangish red before fall |
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