East of the divide the precipitation rate falls sharply, due to the rain shadow created by the mountains. As well as being drier, the eastern valleys have also greater temperature extremes. Daily temperatures may vary from above 30 degrees C in summer to below freezing point in winter. Snow falls mostly between June and September, but falls may occur at any time of the year. See also The climatic influence in the New Zealand mountains .
Mountain beech is the dominant species, but red beech and silver beech are also common in places.
In the larger valleys the forest stops at the valley floors, which consists of gravel with a sparse vegetation of tussock and the prickly shrub matagouri. Generally the beech forests are fairly open in the drier south-eastern parts of the park, with little undergrowth. In the wetter areas close to the divide the beech forests are more complex, with a variety of shrubs, ferns, mosses, lichens and liverworts. Missing forest on eastern hillsides are usually the result of fires from both Maori and European times. As elsewhere in the Southern Alps the forest is very different on the wetter western side of the main divide.