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Every seed catalogue and every nursery pot label indicates
the optimum sun exposure conditions for the plants they sell: ‘sun’, ‘shade’ or
‘partial shade’ (which usually means ‘full sun for one part of the day’).
Australian plant growers have inherited this standard practice from Northern Hemisphere
thinking, and it permeates horticulture, botany and by default, ecology. I’m
starting to think that this needs to be refined.
As many a tourist to Australia will bemoan, it can be hard
to find shade from sun or shelter from rain under eucalypts compared to
the protection offered by a plane tree, elm, or oak. A well-known characteristic
of our iconic tree is that the leaves angle downwards, a trait that enables
them to avoid the intensity of the midday sun. This characteristic also means
that when the sun is high in the sky, the leaves cast dappled shade.
I started trying to characterise eucalypt shade compared to
the shade of exotic species. What is the shade like, and how does it affect
what can grow under it? Does this affect the dynamics of eucalypt-dominated
ecosystems? So I photographed a 1 x 1 m white quadrat under eucalypt woodland
shade (top image), then posterised the images (centre) to quantify the shade levels.
I did the same thing under exotic canopies (bottom).
I wanted to know more, so I took a photo every 15 minutes
for 4 hours under a dozen different woodland canopies in Melbourne, native and
exotic. The images below show one of this series.
The shade patches shift randomly and rapidly. There’s no
temporal pattern, just blasts of light amid splotches of shade. Anything growing
under this dappled shade would need to be able to cope with full sun conditions
‑ although only for minutes at a time ‑ rather than the muted light of steady shade.
Any understorey shrub or groundcover plant that can take advantage of these
minutes of strong light could have a competitive edge.
An agricultural study in the tropics found that full sun
plants like melon and capsicum can grow equally well under the dappled shade of
a passionfruit (but not deep shade). Studies of light flecks in tropical rainforests
typically find that plants can fire up their
photosynthesis mechanisms as soon as the sun-specks hit them. It’s not a long
stretch to think that this phenomenon is probably happening right through the
dappled shade of Australia’s eucalypt woodlands.
How many of Australia’s common eucalypt-woodland plants
could grow under the shade of a North American elm or savanna oak? I suspect
that the canopy light conditions mean there are clear halos under northern
hemisphere trees where the plant life is clearly different to the surroundings,
whereas in Australia any such vegetation shift is less apparent.
So as well as looking at soil patterns to explain species
distribution in woodlands, perhaps we should also be looking upwards to see
what’s going on with the light?
