Complex sentence: The Arboretum was created by Colonel Sir Leonard Ropner (1895-1977), he began the Arboretum in 1931 and in July 2006 the gardens celebrated their 75th anniversary by planting the 1,750th tree.
Split into simpler sentences: The Arboretum was created by Colonel Sir Leonard Ropner (1895-1977). He began the Arboretum in 1931 and in July 2006 the gardens celebrated their 75th anniversary by planting the 1,750th tree.