The Causes of regular Pattern in Desert Perennials

Woodell, Mooney & Hill (1969) demonstrated a positive correlation between the density of 'individuals' of Larrea divaricata Cav. and rainfall, over twelve sites in the Sonoran and Mojave deserts, California. They also found 'a surprisingly strong association of aggregation with high and regularity with low rainfall', and suggested on the basis of these and other data that the regular spacing of L. divaricata in many sites of low rainfall is likely to be the result of root competition for available water. Anderson (1971) rejected this hypothesis. Barbour (1973) re-iterates Anderson's view. Some fallacies are pointed out here in the arguments put forward by Anderson (1971), some new evidence is considered, and some of the possible causes of regular patterns are discussed.