The subalpine zone mostly consists of old-growth birch forests. These forests are exceptional in terms of density and richness, making it possible for significant quantities of sediment material to be carried from the mountainside by the runoff and deposited in the watercourses. This kind of transfer is particularly noted in the Rapa Valley. In general, the transition between the conifer forests and the ones consisting of birch is more or less gradual, with the number of birches present in the coniferous forests increasing along with the altitude until the conifers have completely disappeared. The size of the trees also diminishes with increasing altitude. The upper altitude limit for the forests — which is also the tree line varies greatly throughout the park, from in the Tjoulta valley to over in the Rapa Valley.