Tropical Topics A n i n t e r p r e t i v e n e w s l e t t e r f o r t h e t o u r i s m i n d u s t r y

Light in the rainforest Vol 1 No. 5 September 1992 The solar panel canopy Notes from the When we look down from an aeroplane on to the rainforest canopy we see a green roof – an almost solid mass of vegetation obscuring the ground below. Editor What we are seeing are billions of leaves feeding. They are guzzling sunlight. Sunlight is food. Without it no can live. The abundance of sunlight Animals eat other organisms, living or not fully exposed to the sun. This in the tropics produces more living dead - a pre-prepared meal of probably prevents them from being plant material per hectare than nutrients. , on the other hand, damaged by the intensity of the anywhere else on the planet. make their own food. They are the tropical sun. (Leaves held up to only living things that can capture receive the full force of the sun were o Ideal growing conditions have also energy from the sun and use it recorded to be 10 C hotter than those (through the process of at their natural, inclined, angle.) This produced one of the most diverse arrangement also allows sunlight to be ecosystems on Earth (it is possible photosynthesis) to produce sugars to find 120 tree species per hectare and other materials from which they shared by leaves lower in the canopy. build their own structures. Each leaf is There the leaves, unlike those at the in ’s tropical forests top, grow horizontally and can capture compared with only 30 per hectare a solar cell. The canopy is a vast solar in temperate forests). Much panel. all the rays reaching them. scientific research has been done in an attempt to understand what The architecture of the forest is Researchers also discovered that, like produces such diversity. One determined by a hunger for sunlight. the best solar panels, leaves move into theory is that different gaps - There is fierce competition for this the most efficient positions; leaf openings in the forest canopy - vital energy. Stretching high on tall, angles in these mangrove trees alter producing different amounts of light straight trunks, trees don’t waste with changes in light between wet (see page 2) is responsible. energy on producing branches until (cloudy) and dry (sunny) seasons. they reach the canopy and are able to The forest may need some compete successfully with their disturbance but one thing is certain; neighbours for available light. Trunks human impact on tropical forests can be branchless to heights of 30m does nothing to maintain diversity. or more. On the other hand, the Clearing affects the nutrient balance same trees, grown in full light (by removing trees, increasing soil - for example in a park or erosion, fires, etc.) and the ability of garden without competition the forest to regrow (by crushing from neighbours - will branch saplings and compacting soil). Not early in life and grow into least, these gaps encourage growth shorter, bushy trees with of a limited number of hardy, light- completely different shapes. loving species such as lawyer cane and stinging trees. The canopy is an interlocking network of sun-hungry leaves. Research has shown that full It is so efficient that only regeneration of a forest can take 800 between three and 15 percent of years - even longer if the sunlight penetrates. Not all of disturbance is more severe. With the light is caught by the this in mind we can be thankful that topmost leaves. Studies of the forests of the Australian Wet mangrove (Rhizophoraceae Tropics are now protected. The long family) forests have shown that process of regeneration can begin. leaves at the top of the canopy tend to be inclined so that they are A story of Oskars Although it looks stable, rainforest is continually changing. Old trees fall and others take their place. Menu Look around the forest floor. In the If the gap is large, different types of gloom, where only one to five percent trees join the race — pioneer species. at the of available light falls, vegetation is Unlike the Oskars (otherwise known as sparse. Apart from ferns, palms and climax species) pioneers cannot Photosynthesis Cafe other plants which have adapted to germinate or grow in low light. Instead low light levels, there are some spindly their seeds, which remain viable (able Chlorophyll saplings with few leaves. to germinate) for much longer than Essential for digestion of your Astonishingly, these unimpressive those of climax species, wait for a plants may be 20-year-old trees. burst of sunlight. Then they sprout meal, this tadpole-shaped and take off. These pioneer saplings molecule is tastefully arranged Botanists have nicknamed these little grow much faster in bright light. They around a single atom of saplings ‘Oskars’. Oskar, a character in are sprinters whereas climax Oskars are magnesium. the novel The Tin Drum by German long distance runners. author Gunter Grass, was a little boy Sunlight who didn’t grow up. That is what has This is not all bad news for the climax happened to these saplings. Deprived species. Pioneers are able to grow in Strong, hot and spicy. A real of sunlight they are unable to reach dry hot sunny conditions which energy booster after a long dark their potential as magnificent Oskars dislike. Some have deep tap night. rainforest trees. But there is hope. All roots and can reach nutrient and water they need is a gap in the canopy. stores beyond the reach of shallow- Hydrogen Perhaps an old tree or even a branch rooted climax species. They prevent will fall and give them a chance. soil erosion and create shady damp Tastefully combined with conditions where more little Oskars oxygen and presented in a That increase in light is enough. can germinate. Usually fast-growing crystal clear beverage. Suddenly energy is available for pioneers live for just 20-50 years. growth — and the race is on. Generally Eventually a small gap opens and a Carbon and oxygen the largest Oskar wins. Once it reaches patient Oskar on the forest floor has a the canopy its shade will kill chance at last. Although slower- Irresistibly combined in a heady competitors. If, however, there is no growing, these trees live much longer concoction of carbon dioxide gap, Oskars eventually die after a — for hundreds of years. It is these and wrapped in an invisible (and couple of pointless decades of waiting climax species, the long distance disposable) mix of nitrogen, while more germinate, taking their runners, which comprise a mature, oxygen, argon and water place. well-established rainforest. vapour. Recipe More light notes Sugar glucose Greedy leaves The photosynthetic method For a young sapling the bigger its leaves, the better are its chances of gathering light. Some forest trees produce young leaves which are very different from the mature leaves. First ensure that you have a good supply of chlorophyll Darlingia darlingiana produces large, lobed leaves (left) (green matter) in your leaves. at first. Gradually they become less lobed and the mature This is an essential ingredient. leaves (right) are a conventional ‘leafy’ shape.

Trap some sunlight and, using Angling for light your chlorophyll, convert it When competition for light is as intense as it is in the rainforest, the last thing to energy. you want to do is shade your own leaves. Next time you see a sapling, look at the way its leaves are arranged to gain maximum exposure. Many spiral out from the Take up some water through trunk — you can count five or more leaves before you find two growing at the same angle. Others produce their leaves at right angles, each new layer growing your roots. Use some of your out further to avoid shading previous layers. energy to split it into hydrogen and oxygen. Drip tips Breathe in some air. Select the Many rainforest leaves have a glossy upper surface carbon dioxide and split it and pointed leaf tips — known as ‘drip tips’ — so into carbon and oxygen. water runs off quickly. This helps to prevent the growth of algae and lichens which are more likely (Don’t forget to breathe out to take hold on a damp surface. By covering the excess oxygen. It keeps the surface of the leaf, they would cut out light and, animal kingdom alive.) therefore, decrease its ability to photosynthesise. Quick rainwater runoff may also prevent the leaf from becoming too cool, which would slow down Finally combine two parts of the activities of the living cells. hydrogen with one each of carbon and oxygen. The result Red leaves in the rainforest is sugar glucose, your basic New growth on many rainforest plants, notably Syzygium species, is often a very building block. attractive red colour, but the reason for this is a long-standing puzzle. The red or pink is caused by the pigment anthocyanin, (which also gives beetroot its colour). Its actual colour varies from red to brown and purple depending on Spice of diversity the pH of the fluid in the leaf cells. Research by Dr Sharon Robinson of the As we have seen, growth of the forest Australian National University suggests that the red colour may act to protect varies according to the types of gaps the photosynthetic mechanism inside the new leaves. New red leaves are not yet created. It is not only the size of the photosynthetically active — they are still developing the internal organs gap which determines life-and-death (chloroplasts) responsible for capturing the sun's energy and converting it to issues for hopeful saplings and seeds. stored energy. Anthocyanin pigment reflects red light like a mirror and absorbs The amount of light penetrating to the light from the blue end of the spectrum. Anthocyanin can therefore act as a forest floor can depend on its sunscreen, reducing the amount and type of light penetrating the leaf until the orientation; an east-west gap receives chloroplasts mature and can use the light photosynthetically. We see the red much more sunlight than a north- colouration because anthocyanin reflects red light away from the leaf and south gap. Angles of light change absorbs blues and greens. with the seasons and can vary according to slopes. Gaps may appear more frequently in certain places. Gathering sunflecks Few plants can survive in the very dim light of the forest floor. Those that do The variation is infinite — but the depend on sunflecks — patches of sunlight which dodge the leaves above and different effects may, to some degree, reach parts of the floor for as little as a few seconds a day. be responsible for the immense diversity in tropical forests. Research Dark green forest leaves are very efficient at capturing sunlight. In addition, these suggests that light conditions in plants seem to respond very quickly when they are lit up. With most plants there different gaps favour regeneration of is a time lag after exposure to light, before photosynthesis begins. Then, as soon different species. Thus variety in gap as the light is ‘switched off’, the rate of photosynthesis begins to decline. size — which in natural conditions However, research indicates that forest floor plants switch on may be caused by anything from quicker — as soon as the sunfleck hits them — and cyclones and landslips to falling continue to photosynthesise longer after the branches — is considered, by some light has gone. scientists, to be the driving force behind biological diversity in the rainforest. Staghorn fern This epiphytic fern has two distinct leaf types. ‘Shield’ leaves, which are green at first but become brown, are Whether they are purely structural. They hold the plant in place and contain the roots. As new stretchers, climbers, ‘shields’ grow on the outside, older jumpers, hitchhikers or ones decompose and provide food for the roots inside. The longer green sunfleck gatherers, all leaves photosynthesise and produce spore— ‘seeds’ for the next plants are aiming to generation. capture the sun’s rays. Here are some of the different strategies they use to reach for the sky. Basket Orchid fern Many orchids are epiphytes. Their Surrounding a tree roots have a spongy sheath of special trunk. The brown cells up to 18 layers thick which can bracket, or ‘nest’ leaves absorb water and nutrients rapidly, hold the plant together and taking advantage of a shower of trap leaf litter for food. rain or cloud or mist. Some orchids are leafless but have green roots which photosynthesise. Woody liane Often all we can see of the numerous lianes are woody stems heading up towards the canopy - they seem to have climbed up without visible support. Bird’s-nest fern Actually, as young Another epiphyte commonly plants they seen on the branches of forest have wound trees. The leaves are arranged themselves in a funnel to collect falling around saplings leaves, which provide it with which have since nutrients, and moisture. died and disappeared. The evidence is the Pothos longipes empty woody To make the best of low light levels, coils. this plant produces what look like double leaves. In fact, each ‘leaf’ consists of a flattened stem, containing chlorophyll for the process of photosynthesis, and a proper leaf. It climbs up trunks using little claw-shaped rootlets. It is possible that some climbers using rootlets produce a chemical that adheres to tree bark.

Saplings, ferns and other understorey plants These shade-loving plants feed on sunflecks. Stretchers Forest trees, growing tall on Strangler fig branchless Epiphytes are not parasites — trunks to reach they don’t extract food from the sky, are the the host tree on which they climbing frame grow but use them to get for other closer to the source of light. plants. Rhaphidophora Mistletoe The strangler fig is the Seeds of this climber Unlike epiphytes, biggest. Its seed is planted (in germinate only in parasitic mistletoe a bird dropping) in the fork of deep shade, in other grows roots into the a tree where it germinates and words, the base of a branch on which the plant puts down roots to the tree which will has germinated and feeds ground. More roots follow and support it. on its host. the fig grows up to the light, overshadowing its unfortunate The tiny black and red host tree. Roots coalesce mistletoe bird is one of the around the trunk of the host few animals which can eat until it is finally encased and the berries. The highly ‘strangled’. Eventually the toxic seeds bypass its dead host tree rots away, digestive crop — then it leaving the strangler standing carefully ‘plants’ them on a high in the forest. branch, ensuring a future crop of food. Look at the way the roots merge together — you can see the scars where they have joined. This commonly happens to roots — but out of sight under the ground.

Lawyer cane Actually a palm, this climber thrives when more light is Flagellaria indica available, often dominating Look for the curling disturbed forests where there tendrils at the end of has been logging or the sides Flagellaria’s leaves of roads. It puts out lines of (left). These twine ‘grappling hooks’ which circle around other around until they catch on to vegetation as the something. Then it uses that plant hoists itself support to pull itself up. The upwards. process is repeated as the lawyer cane claws its way up. It often becomes too heavy for its support and falls back to the ground, but soon starts hooking its way up again. Questions & Answers Facts and stats on light and leaves Q Why do marlin come in to the from its body. During a day, a hectare of Lizard Island area at this time of forest can cycle 500 000 year? The fluoro spots have also been litres of carbon dioxide. called eyes, but experiments showed With all plants on the earth A From September to December, them to be the least sensitive to light doing this our atmosphere is marlin from the north-west Coral Sea of all areas tested. The spots seem completely recycled about congregate just outside the reef, to be patches of tiny light-scattering every 250 years. sometimes within a few hundred structures known as iridophores. metres of the reef edge, to spawn. (Another type of iridophore Chlorophyll, the green Main concentrations are in reefs off produces the bright colours in the matter in leaves, has the Cairns - Lizard Island region. exposed fleshy mantle of the reef existed for at least two clam Tridacna.) The function is billion years. Its unknown. One suggestion is that the From May to October, there are also decomposition products have blue glow produced is diffused to concentrations of marlin around been found in rocks light-sensitive areas of the body, Cape Bowling Green, south of containing algae of that age. Townsville. In this case, they are increasing the sea urchin’s ability to juveniles in the 10-40kg, six months perceive differences in light and shadow. It certainly seems able to do Almost all the oxygen in the to two-and-a-half years old, bracket earth’s atmosphere (20 and are probably feeding on this. Try putting your hand over the percent) has been derived pilchards and herrings. Another top of an urchin and watch it point from photosynthesis over the billfish, the sailfish, also swims with its spines at the shadow, ready to past two billion years. the marlin. defend itself against attack. The long red and short More than 95 percent of the fish Q My bird book tells me that caught are tagged and released. metallic starlings are migratory. blue wavelengths of visible However, after their long struggle, it This year, I have been aware of them light are absorbed more is possible that many are exhausted all year. Just now I notice they are easily by chlorophyll than and fall prey to sharks or simply starting to nest again. Are they medium wavelengths. Most of drown. becoming sedentary? these are reflected and enter our eye producing the effect we Q The long-spined black sea A There are two possibilities. recognise as green. urchin (Diadema) appears to have Metallic starlings which we are an eye on the top and four to five seeing around in the winter may Although 75-80 percent of fluorescent blue spots. What are have migrated from further south the sunlight falling on a leaf they? while those which nested in the is absorbed, only about 10 Cairns area have migrated to the percent is actually captured A The ‘eye’ is, in fact, the anus. It or further north by the chlorophyll and turned is a flexible cone, or tube, which in to New Guinea. It is also possible into energy. one species, Diadema setosum, has that juvenile starlings may hang an orange ring, like the iris of an around while adults do the migrating. Almost all twiners twist eye. Watch the urchin and you will An extensive banding programme themselves in an anti- see it bend the tube over and would be necessary to determine clockwise direction — distribute balls of faecal matter - the exactly what happens. whether in the northern or tube enables it to deposit it away southern hemispheres. Scientists are still trying to Tourist talk understand why. ENGLISH GERMAN JAPANESE Epiphytic ferns can become very heavy and break sun Sonne taiyo branches off trees. Some light Licht hikari trees have defence leaves Blätter ha mechanisms against photosynthesis Photosynthese kougousei epiphytes. Very smooth bark gap Lücke aida discourages them from gaining a canopy Baum-kronen zaikan roothold. Others have flaky bark which shade Schatten ka-ge pulls away from the tree with the epiphyte Epiphyte chakusei epiphyte’s weight. It is thought that shokubutsu some trees may produce a toxin to germinate keimen hatsuga suru inhibit epiphyte growth. Nature notes A diary of natural events creates a pleasing journal which grows richer with the passage of time. Watching for the recurrence of an event after noting it in a previous year, and trying to understand what could have caused changes in timing, is intriguing.

These notes are from the author's own notebook, or were offered by researchers and fellow naturalists. Readers will, inevitably, note variations between their observations and those appearing here. If you do not keep a nature diary perhaps this will inspire you to begin one.

A small, glossy-leaved beach tree known as the sea hearse (Hernandia nymphaeifolia) should carry ripe fruit this month. The gloomy name is an allusion to its unusual fruit, a dark, ridged seed case, partially concealed within ‘a loose smooth cream envelope with a circular opening at the top through which the black fruit can be seen ... the fruit’s structure apparently suggesting a carved coffin surrounded by a pale shroud.’ (Cribb, A.B. and J.W., 1985: Plant Life of the Great Barrier Reef and Adjacent Shores.) September marks the start of the period when Spring in Wet Tropics rainforests will our skies are enlivened by see the blossoming of a plant known Pied imperial-pigeons. as ‘native guava’ ( laurina). These striking white and These flowers are thought to show black birds return from New some primitive characteristics. For Guinea to nest offshore, on islands example, what look like numerous such as Hinchinbrook and Low Isles, white petals are, in fact, sterile as well as in trees on the mainland, stamens, revealed after the united including some lucky urban petals are pushed off as a cap, similar backyards. Rainforest fruits which to the process in eucalypt flowers. The feature prominently in their diet sterile stamens are food for a small include many of the laurel family weevil which lives in close association (Cryptocarya spp), lawyer palms with this flower, assisting in its (Calamus spp), figs and woodland pollination. trees of the Canarium family (Canarium vitiense and others).

Pied imperial-pigeons raise one young Numbers of migratory waders will also at a time, the parents taking turns to be reaching a peak during September, feed the nestling on ‘pigeon milk’ returning to our hemisphere after a secreted from glands in the crop, a busy breeding season in far-off procedure common to all pigeons. nations to the north, during their Birds may raise a second offspring summer. Most numerous of about 30 after the first fledges, but this has not species of waders recorded on the been confirmed. (Acknowledgments to Cairns mudflats are sharp-tailed and Francis Crome, CSIRO Wildlife, curlew sandpipers and red-necked Atherton.) stint. Relatively easy to identify are Female green turtles may begin the large eastern curlew, its smaller mating during September, accepting relative, the whimbrel, and two and storing sperm from several males. species of godwit - the black- During mating, which takes place at tailed and bar-tailed. Of the the surface, the shell of the male can smaller species, two of the be seen as an almost immobile lump on most recognisable may be the the water. Fertilised green turtles may Terek sandpiper with its squat begin egg laying at traditional body on orange legs and the beach sites about greenshank, with dark November, producing green legs. several clutches of (Acknowledgements up to 150 eggs at to John Crowhurst, intervals until about Cairns City Council.) March next year. (Acknowledgments to Mark Simmons of Whimbrel GBRMPA.) Bar-tailed godwit Bookshelf There are no books, as such, on has a wealth of general light in the rainforest but a information on the forest and its number of chapters and articles dynamics. Sections include the tackle the subject. forest and environment interacting, environmental New Scientist 14 March 1992 factors, the forest community, Logging rainforests the tree growth physiology, dynamic natural way? forest ecosystems and Nick Brown and Malcolm Press management of tropical forest land. Although dealing with forests in Borneo, this article has much Tropical Rainforest Research relevant information on the in effect of sunlight in various gaps N. Goudberg et al, Ed. on forest regeneration, plus an Institute for Tropical Rainforest interesting section on sunflecks. Studies (1991) Chapter: Aspects of the Australian Tropical Micrometeorology of Rainforests Rainforests in the Wet Tropics L.J. Webb and J. Kikkawa of Northeast Queensland CSIRO (1990) S.M. Turton Chapter: Disturbance: The Forest Transformer The Ecology of Australia’s M.S. Hopkins Wet Tropics R. Kitching Ed. This chapter looks at regrowth Ecological Society of Australia after both natural disturbances, (1988) including cyclones, and ‘man- Chapter: Solar Radiation sized gaps’. Regimes in a North Queensland Rainforest Tropical Forest and its S.M. Turton Environment K.A. Longman and J. Jenik Longman Scientific and Technical (1974/1987)

While only a few pages deal with light and shade, this book

This newsletter was produced by the Queensland Department of Environment and Heritage (now The Environmental Protection Agency) with funding from the Wet Tropics Management Authority.

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