The trees are carefully trimmed to provide attractive scenes, and to prevent them from blocking other views of the garden. Their growth is also controlled, in a technique called niwaki, to give them more picturesque shapes, and to make them look more ancient. It has been suggested that the characteristic shape of pruned Japanese garden trees resemble trees found naturally in savannah landscapes. This resemblance has been used to motivate the so called Savannah hypothesis. Trees are sometimes constrained to bend, in order to provide shadows or better reflections in the water. Very old pine trees are often supported by wooden crutches, or their branches are held by cords, to keep them from breaking under the weight of snow.