The Influence of Shade, Soil, and Water on Saguaro Seedling Establishment

Experiments utilizing 3299 transplanted saguaro (Carnegiea gigantea [Engelm.] Britt. & Rose) seedlings were conducted at the Saguaro National Monument, Tucson, Arizona. The multiple-factor tests were designed to determine the effect upon saguaro establishment of shade, irrigation, and soil source. Shade was essential to seedling survival; mortality among one group of 1200 unshaded plants reached 100% in 1 year compared with 65% mortality among an equal number of shaded plants. The most critical time for the seedlings was the dry, hot period just before the beginning of the summer rains in July. Irrigation did not enhance survival of seedlings growing under the most favorable combinations of shade and soil; rainfall alone provided adequate moisture. The effect of soil source on survival was possibly a function of soil-surface albedo; more seedlings died on the darker and, by inference, hotter soils. Insects killed as many as 20% of the seedlings. These animals were considered as only one of many minor agents affecting saguaro seedling establishment. Saguaro populations, it was concluded, will be strongly influenced by forces which alter the number of shade-producing perennial plants with which the saguaro grows-forces like grazing and climatic change.