About 40 per cent of the rainforest in the Amazon could be destroyed by 2050 unless Brazil adds measures to protect trees on private land, scientists say.

Cattle ranching and soy farming threaten to destroy rainforests unless laws are changed to preserve the land as parks, and farmers meet guidelines to manage their properties sustainably.

The researchers used a computer model to predict the effects of different practices, such as paving highways or preserving forests on private property.

Rainforest trees trunks flow down the Amazon (AP file photo)

"By 2050, current trends in agricultural expansion will eliminate a total of 40 per cent of Amazon forests, including six major watersheds and ecoregions," Britaldo Soares-Filho, of the Universidade Federal de Minas Gerais in Brazil, conclude in Thursday's issue of the journal Nature.

Eight of the Amazon River's 12 major watersheds could lose more than half their forest cover. The watersheds offer habit for more than half the region's native mammals.

On the other hand, if more areas are protected, then 73 per cent of the original trees would still be standing in 2050.

The trees store emissions of carbon dioxide, a greenhouse gas.