Complex sentence: This type of pine is called "heart" because when it reaches maturity the tree is mostly heartwood, and contains almost twice the resin content of other types of pine, such as loblolly and slash and had much much higher structural strength.
Split into simpler sentences: This type of pine is called "heart" because when it reaches maturity the tree is mostly heartwood. It contains almost twice the resin content of other types of pine and had much much higher structural strength.