Guest Post: Coming out of the shadows – do ecologists need to think more about light?

In the first of a series of Guest Posts, my PhD student Sue Bryceson (S.Bryceson@latrobe.edu.au) tackles the question: what do we really know about light? And how might it shape the understorey of eucalypt forests?
--------------------------------------------------------------------------------------------------------------------------

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?