Complex sentence: The Sierra Madre del Sur pine-oak forests occupy the higher reaches of the range; they are part of a chain of pine-oak forests that stretch from the southwestern United States to Costa Rica along the American Cordillera.
Split into simpler sentences: The Sierra Madre del Sur pine-oak forests occupy the higher reaches of the range. These forests are part of a chain of pine-oak forests that stretch from the southwestern United States to Costa Rica along the American Cordillera.