Tarzan may have to find a new way to commute given dramatic changes in the growth of hanging vines in the rainforest. Dick Ahlstrom reports.
THE vine transport network favoured by Tarzan on his journeys through the jungle may be on the way out. Vine growth has surged over the past two decades, possibly as a result of higher atmospheric carbon dioxide levels.
More vines might seem like an advantage to Tarzan, presenting more commuting options, but in fact the rapid growth of the tree-climbing vines, lianas, will make things worse not better. They are tree parasites that break tree limbs and block sunlight, slowing tree growth. The vines can actually prevent the growth of some tree species.
The rapid growth of lianas in the Amazon River basin (rather than Tarzan's Africa) is detailed today in a letter published in the journal, Nature. "Liana populations have become more dense, liana basal area size of individual lianas has increased," according to lead author, Dr Oliver Phillips of Leeds University.
It had long been assumed that the old-growth rainforests were close to equilibrium "but this view has been challenged by recent findings that neotropical forests are accumulating carbon and biomass, possibly in response to the increasing atmospheric concentrations of carbon dioxide," the authors write.
Old-growth forests are capable of absorbing between one and two billion tonnes of carbon a year, but it had not been considered how changes in the availability of this carbon might affect different types of plants.
"Lianas in particular are ignored in forest inventories and models alike, in spite of their key functional roles," the authors say. As structural parasites, lianas exert an unexpectedly powerful ecological effect. They represent only five per cent of total forest biomass, but a colossal 40 per cent of leaf productivity.
"They also suppress tree growth and encourage tree mortality, and affect the competitive balance among trees," the authors add.
The team looked at 47 sites in four Amazonian regions, north Peru, south Peru, Bolivia and Ecuador.
"We find that the density and the basal area of large lianas have increased substantially over the last two decades of the 20th century."
This finding confounds assumptions made about the forest's response to rising carbon dioxide levels. The lianas seem to be out- competing the trees, but this in turn may undermine the forests themselves. Lianas increase the risk of tree mortality by about 30 per cent, and so could stop the forests from assisting in the mopping up of excess atmospheric carbon dioxide.