The Rings of Tantalus - Novel 1975
This is Book Two of Four in the series called 'The Expendables'. He wrote this series using the pseudonym 'Richard Avery'.
They were THE EXPENDABLES - some genuine volunteers, others not. They were the chosen ones - the people who had been 'selected' to travel far into space and find new planets for human colonisation.
The Expendables go to Tantalus and try to fathom the enigmatc rings that lay waiting to deal with intruders - as did the deadly vampire tree.