The new findings highlight the critical importance of early life-cycle stages of plants in shaping tropical forests.
They also stress the need for greater and more widespread inclusion of the very smallest plant classes into plot-based tropical forest research worldwide, which often tends to focus just on the larger plants.
La Trobe University ecologist Dr Peter Green says: 'We have demonstrated scientifically for the first time that what happens in rainforests now – the lungs of our planet – can produce changes in these forests 300 years down the track.'
For the past 20 years the joint US-Australia funded research project, which began in the 1960s, has been led by Dr Green and Professor Kyle Harms from Louisiana State University in Baton Rouge.
From 1971 to 2013 they tracked the fate of nearly 8,000 seedlings, saplings and big trees from a total of 186 species.
Dr Green says the work confirms that the rarity or commonness of large trees is influenced most strongly by events that occur when plants are small and especially vulnerable.
Using computer simulations, he and his colleagues compared the diversity of plants that survived over four decades with what would have been expected over the same period if all had an even probability of dying.
'The difference is a measure of how strongly processes such as whether they are eaten by animals, attacked by disease or affected by drought can influence patterns of diversity.
'Because we divided plants into different size classes, we were able to gauge how influential these processes were for tiny plants compared with much larger plants,' he says.
'Our simulations showed that what happens to the very smallest plants in the forest has the greatest influence in determining patterns of composition and diversity once trees eventually reach the canopy.'
Understanding the balance of nature
Dr Green says although this model has been implicit in most research on tropical seedling dynamics, it has never formally been tested until now in any forest.
'The early stages are in fact more important than tropical ecologists have traditionally assigned to them. It's only because we have been following the fate of individual plants here in Australia for decades that we have finally been able to test these ideas.
'This is critical to our understanding of the balance of nature,' says Dr Green. 'Knowing the strongest influence of the ecological process is extremely important.'
The research began in Australia more than 50 years ago. That's when American scientist Dr Joseph Connell – now aged 91 and an Emeritus Professor at the University of California – was seeking an English speaking, politically stable post-war base for his studies and began tagging vegetation on two plots in northern and southern Queensland.
One of these, the major site, is on 1.7 hectares at Davies Creek National Park near Cairns and the other on two hectares south of Brisbane, in Lamington National Park.
Dr Green and ten hardy field workers last year braved stinging trees, ticks, leeches and biting flies to record the fate of thousands of tagged plants to bring the recent PNAS study to conclusion.
All up, repeated censuses over the five decades on both plots have resulted in a database of about 100,000 plants from 300 species.
New species – and slowest growing trees
'The work involves meticulously tagging, labelling and recording almost everything that shoots out of the ground,' says Dr Green.
'Not all plants we found were well-known to science. The most common species at Davies Creek is in the genus Niemeyera, but it still hasn't been formally described by taxonomists.'
He says another finding is that the large Australian rainforest trees surveyed are among the slowest growing anywhere in the tropics.
'Many plants that were one or two feet tall in 1965 are only a few feet taller now. Age estimates using our growth data indicate that seven-foot saplings of many species could be eighty to one hundred years old.'
The research has been financed by the US National Science Foundation and more recently by Australia's Long Term Ecological Research Network.
Photo: Give me the saplings and I'll show you the trees: Dr Green, top right, with members of his research team in North Queensland.
Media Contact: Dr Peter Green, Department of Ecology, Environment, and Evolution, T 03 9479 3675, E firstname.lastname@example.org, or Ernest Raetz, Media and Communications, M 0412 261 919.