Complex sentence: The upper flanks of the volcanoes San Martin and Santa Marta are covered with a Neotropical rainforest, while the lower portions are covered with stunted pastures, and much of the Sierra de Los Tuxtlas are in the Los Tuxtlas Biosphere Reserve.
Split into simpler sentences: The upper flanks of the San Martin and Santa Marta volcanoes are covered with a neotropical rainforest, while the lower portions are covered with stunted pastures. Much of the Sierra de los Tuxtlas are in the Los Tuxtlas Biosphere Reserve.