The next higher forest zone is the mountain hemlock zone, the highest forest zone in the Olympic mountains. In this zone most precipitation falls as snow and the snow-free growing season is very short. In the drier rainshadow side of the mountains, mountain hemlock is largely replaced by subalpine fir. At the higher elevations of the mountain hemlock/subalpine fir zone, tree cover is reduced to isolated stands of trees called tree islands, surrounded by subalpine meadows, in the subalpine parkland zone. Variations in substrate, topography, moisture, and snowpack depth and duration determine the vegetation community. Tree islands form on convex topography, which tends to gather less snow and shed it sooner than surrounding level or concave topography.