THE ASSOCIATION OF DESERT ANNUALS WITH SHRUBS

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IT HAS LONG been noted by both professional and lay observers that many species of annual plants characteristic of desert habitats are more or less strictly confined to sites beneath shrubs. Not all species of desert annuals are thus associated with shrubs. In fact, the herbs of a given area can be classified rather clearly into two groups, one nearly always growing beneath some species of shrubs, the other preponderantly occupying the broad expanses of otherwise bare soil between the widely-spaced shrubs. A few herbaceous species occur more or less indiscriminantly in both kinds of habitats. Furthermore, there are some species of shrubs that do not harbor dependent annual herbs or do so only rarely. The pattern of distribution is thus a very clear one comprising widely-spaced shrubs of several species, annual herbs growing between the shrubs, other annual species more or less confined to the bases of certain shrubs, and other shrub species which seldom or never harbor dependent herbs. This pattern of distribution and association of shrub and herb species was studied by Went (1942) in the Mojave Desert and in the Borego Valley of the Colorado Desert in Southern California. He showed a strong dependence of Malacothrix californica DC., Rafinesquia neomexicana Gray, Phacelia distans Benth., and Emmenanthe penduliflora Benth. upon sites provided by Franseria dumosa Gray, Larrea tridentata CoV., Opuntia echinocarpa Engelm. and Bigel., Hymenocloa salsola Torr. and Gray, and similar woody species. The shrub Encelia farinosa Gray, on the other hand, seldom harbored these annual herbs. He noted also that dead shrubs of any species harbored these herbs in abundance. Where the herbs were found apparently independent of shrubs, investigation invariably revealed a rotting stump of some dead shrub at the site of each. The failure of Encelic fctrinosa to harbor shrubdependent herbs was investigated by Gray and Bonner (1948a). They demonstrated in the leaves of Encelia a growth-inhibiting substance which, in either aqueous or ether extracts, killed tomato seedlings in water culture in 1-3 days. They showed also a marked depression of growth of tomato seedlings in sand culture by the application of Encelia leaves to the top of the sand and irrigation through the Encelia leaf mulch. It is suggested in their paper