Ultraviolet and Visible Light in the Daintree Rainforest of Northern Queensland, Australia

Frances Baines

Daintree National Park is a stretch of ancient tropical lowland rain forest in Australia north of the Daintree River 110 km north of Cairns (-16°10' S 145°25' E). It is one of the oldest rain forests on the planet, more than 135 million years old, and covers approximately 1,200 sq. km.

This article reports measurements of sunlight in the Cooper Creek area at the foot of Thornton Peak (Fig.1) on a fine day in early October at the very end of the dry season.

Figure 1. Thornton Peak as seen from from Cooper Creek, Daintree.

The rain forest hardwoods and fan palms cover the slopes of the mountains to the sea and to the riverbanks, which are lined with mangroves. In March 2006, cyclone Larry damaged the forest, and there are new clearings in some areas where trees have fallen, although the canopy has grown back well. Sunbeams reach the forest floor in many places creating a mosaic of light and shade.

The measurements were made using the following equipment:

• Solarmeter 6.2 UVB meter (µW/cm²)
• Solarmeter 6.5 meter (UV Index)
• SkyTronic LX101 model 600.620 digital lux meter
• Electronic Temperature Instruments Ltd. model TN1 infra-red non-contact thermometer
• Unbranded battery-operated digital "patio" thermometer/hygrometer with probe

I am a member of the UV Guide UK team (www.uvguide.co.uk), a small group of amateur herpetologists researching UV lighting for reptiles. This project formed part of my holiday study of the native habitat of reptiles.

We were lucky enough to be escorted around the Cooper Creek Wilderness Reserve by the land manager herself, Mrs. Prue Hewett, who shared her deep knowledge and love of this amazing place with us and showed us many of its secrets, including the fan palms, tree ferns, a pair of Southern Cassowary birds (Casuarius casuarius) and four beautiful Boyd's Forest Dragons (Hypsilurus boydii) (Fig. 2). We were able to quietly approach these arboreal lizards quite closely. They were perched in areas of dappled light and shade and did not seem to be basking. Prue said that they do appear to thermoregulate to some extent by moving up and down the tree, in and out of deeper shade.

Figure 2. One of four enchanting Boyd's Forest Dragons (Hypsilurus boydii) we encountered at Cooper Creek.

As can be seen in Figs. 3 and 4, the amount of UVB available to the dragon depends upon its exact position on the tree trunk. 25µW/cm² was the highest reading I could obtain close to him. There were shafts of direct sunlight on adjacent trees that gave readings up to 238µW/cm² (at 3.00pm). The UV Index graduated from 0.1 in the shade to 4.8 in the sunlight.

Figure 3. The UVB in the vicinity of a Boyd's Forest Dragon at mid-afternoon.		Figure 4. Another UVB measurement in the vicinity of a Boyd's Forest Dragon at mid-afternoon.

Overall light levels were surprisingly high. At 3.00 pm, just before taking the accompanying photographs, I measured 2,000 lux in the shade and 90,000 lux in the sunlit area (global readings). The direct solar reading was 130,900 lux.

The full set of measurements is given in Table 1. Direct measurements refer to those made with the meter sensor aimed at the sun. Global readings are taken with the sensor aimed directly up at the zenith sky. "Maximum basking temperatures" refer to the highest surface temperature located at the site (e.g. on a rock, log or bare ground) exposed to the sun, measured with the infrared non-contact thermometer.

The measurements in Table 1 are a simple "snapshot" of the conditions found on one fine day at the end of the dry season. In no way do they give a full picture of the microhabitat of this remarkable rain forest.

It is interesting to note the wide range of temperatures, light levels and UVB light levels available on the forest floor--all within feet of each other, and constantly changing as the sunlight moves through the canopy above.

Below are more photographs of the rain forest and some of the UVB meter readings taken on a walk through the rain forest at about half past eight in the morning on 8 October 2006.

It was a great privilege and a wonderful experience to visit this beautiful and special place.

Figure 5. Cooper Creek.

Figure 6. The fan palm canopy over Cooper Creek.

Figure 7. Sunlight may even reach the buttress roots in the rain forest.		Figure 8. Light through the forest canopy.

Figure 9. There is wide variation in sunlight on the forest floor. At this site the solar irradiance was 112 µW/cm² between 8:32 - 8:36 am on 8 October 2006.

Figure 10. At this site the solar irradiance was 151 µW/cm² between 8:32 - 8:36 am on 8 October 2006.


Frances Baines is an amateur scientist who studies UV-B lighting for reptiles. She has developed a PowerPoint on her specialty for the British Herpetological Society and writes that "I am always so excited to think there is still such a big role amateurs can play in 'real' science, but it's a big challenge for me to be standing up in a room full of knowledgeable academics."