A savanna or savannah is a grasslan ecosystem with scattered trees or shrubs. In savannas trees are small or widely spaced so that the canopy does not close. It is often believed that savannas are characterized by widely spaced, scattered trees, however in many savanna communities tree densities are higher and trees are more regularly spaced than in forest communities. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses. Savannas are also characterised by seasonal water availability, with the majority of rainfall being confined to one season of the year. Savannas can be associated with several types of biomes. Savannas are frequently seen as a transitional zone, occurring between forest and desert or prairie.