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By Thea Williams 29 June 2015 7 min read

One of the study's authors, CSIRO's Matt Bradford, with a Gossia shepherdii (Lignum) estimated at close to 1000 years old on the Robson Creek rainforest plot, south west of Cairns.

The Amazon gets great press. And, yes, it is the world’s largest single rainforest catchment. But a recent research paper[Link will open in a new window] turns the tables on the assumption that the Amazon is the be all and end all of rainforests.

The first comprehensive estimate of tropical trees across the globe has found that in our neck of the woods, the Indo-Pacific tropical region, is every bit as species-rich as the American tropics.

Undertaking this research was no mean feat. The paper, published in June in the Proceedings of the National Academy of Sciences, listed 170 authors from all over the world, including two researchers from CSIRO. Matt Bradford was number 18 on the list and Dr Dan Metcalfe, a research group leader with Biodiversity and Ecosystems Knowledge and Services, was number 92.

Spare a thought for lead researcher Ferry Slik, from the Universiti Darusallam in Brunei, each time he sent a draft of the paper for comment. He was the one who, in October 2012, first flagged the idea of combining research data on rainforests from across the globe in one survey .

What made this survey work possible is the increasing number of inventory plots across the tropics where every species in a defined area is identified.

Forget Joseph Banks’-style specimen collecting. This is detailed mapping, measuring and studying communities of trees. This survey is based on 207 such tree inventory plots scattered across tropical regions to which the researchers had access. As Dan puts it, over the past three years they “plumbed the depths” of all significant survey field work in tropical regions.

That scale of detailed research enabled the team to produce standardised species lists for regions. Using that data and probability-based statistical modelling, they could estimate the richness of species across continents, using the number of stems sampled at each plot, the number of species identified at each plot, and the density of plots across the regional landscapes.

The tropic of trees

To really appreciate the diversity of rainforests, consider this: Europe has 124 species of tree.

“Pretty boring,” according to Dan, who first came to South East Asia from the UK as a PhD student. “Contextually, Europe would have been covered by glaciers. After the ice retreated, a very small number of species managed to reinvade over the Alps and the Pyrenees. You’re looking at a weedy flora.”

But in the tropics, this research shows, there are at least 40,000 and possibly more than 53,000 tree species. What also characterises the tropics, they’ve found, is that large numbers of tree species with exceptionally small population sizes flourished to develop very complex, interdependent “communities”.

In the Indo-Pacific region, explains Dan, rainforests benefited from mountains and islands that created refuge for this abundance of species. So too, the Americas. The research estimates that rainforests in both regions have a minimum of between 19,000 and 25,000 tree species each.

Africa wasn’t so lucky with its dry, flat plains, and has a reduced diversity.

This survey also shows that very few species are shared across the regions. Each tropical region is unique, highlighting the need for localised responses to the threat of deforestation.

“There’s an enormous investment in research in the Americas, particularly North America, and that drives the perception that the Amazon basin is where everything happens,” Dan says. “There is a lot of European and Asian research in the Indo-Pacific region but we don’t have the National Geographic channel running after people doing field work.”

The cassowary lives in the dense tropical rainforest of north east Queensland. Image: final gather/Flickr

Australia pulls its weight

Australian field work has, in fact, been critical to this work. That’s because Australian data was so reliable and exhaustive that researchers could use it to test their statistical modelling and gauge the number of species in sites where counting hadn't taken place, and for which there was far less information.

Of the 207 inventory plots used to provide data for this survey, eight are in Australia - in the Wet Tropics, the Whitsundays and forest straddling the Queensland-New South Wales border.

What really helped is that most of the Australian plots were set up by the Commonwealth Forest and Timber Bureau in the 1970s.

“Rather than being single-use survey plots, we have got 45 years of data - an understanding of the number of species (490) and an understanding of rare and common species in the Australian bio-region,” says Dan.

For a picture of the sort of data they’re collecting, let’s look at the work done at a newer plot site established in 2009 at Robson Creek on the Atherton Tablelands near Cairns, also used in the survey.

Over a three-year period, the Terrestrial Ecosystem Research Network (TERN) and CSIRO measured, mapped and identified every tree on the 25 ha site that had a stem greater than 10 cm in diameter - 23,416 of them, some more than 1000 years old.

Every tree growing in the Robson Creek rainforest plot has now been identified to species and measured. Image: TERN.

And on that one 500 m x 500 m plot, they recorded 208 different species of tree.

More recently, an army of researchers and volunteers from CSIRO, James Cook University, University of Queensland, RMIT, Flinders University, University of Technology Sydney and the Queensland Government braved the leeches and descended on the Robson Creek site to advance the plot survey.

Scores of people scoured the forest canopy for leaf samples, others took to the air in planes using advanced radar equipment and hyperspectral scanning technology to measure the plot. Together they helped build the most complete picture of the forest ecosystem possible.

The plot is now part of Australia’s first rainforest 'SuperSite' - the Far North Queensland Rainforest SuperSite[Link will open in a new window].

A laser scan of the rainforest at Robson Creek, showing the three dimensional forest structure. Image: P. Scarth, University of Queensland.

The role of unique and rare rainforest species

What Australian researchers know, as Dan explains, is that only 1 per cent of the Australian landscape is now rainforest “but with a very high degree of endemism”. That’s to say species that are unique to one particular geographical area.

Plant with reddish-purple berries
Halfordia kendack (kerosenewood) which like other related species produces fruit through the winter, providing a resource to fruit-eating birds and mammals after the main summer fruiting period has quietened down. Image: CSIRO.

Take Australia’s Wet Tropics, for example: a third of the species are unique to that location and another third only found in Australia.

These rare species provide “critical ecosystem services” to the rainforest community. Only by better understanding their role can researchers work out how best to preserve remnant rainforests.

One example of a client for these ecosystem services is the cassowary – the somewhat alarming mega-bird that lives in Australia’s dense northern tropical rainforest. It’s nationally endangered, with a population of less than 3000. Most of the time, it lives on a varied diet of fruit. But during the winter months, it survives on a very monotonous diet – the fruit of the kerosenewood tree (Halfordia kendack), which fruits through winter. No kerosenewood, no cassowary.

And that would not only be a great loss in itself, but would have knock-on effects. The cassowary is what’s known as a keystone species [Link will open in a new window]because of its major role in maintaining the ecological balance of the rainforest.

Without cassowaries, some species of rainforest plants would have no means of distribution. If cassowaries were to disappear from an area, some of these species may eventually become locally extinct, threatening other species that depend on them and changing the forest dynamic.

This cassowary poo containing a single species of fruit (not kerosenewood in this case), shows that at some times animals are dependent on single species for food. Image: CSIRO.

“We have, as a first world country, the ability to generate a very high level of understanding of the forests that we’re custodians of and confidence in our knowledge of our regional species pool", says Dan.

“Estimating the total number of global tree species may seem a bit nerdy but the significance is in using well-known floras such as Australia to extrapolate sensibly into places we don’t know well, such as Myanmar or the eastern Congo, and thus get a reasonable estimate of diversity in areas where threats are greatest and forests are disappearing.”

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