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DREZNER, T. D. (Department of Geography, N430 Ross, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada). Nurse tree canopy shape, the subcanopy distribution of cacti, and facilitation in the Sonoran Desert, J. Torrey Bot. Soc. 137: 277-286. 2010.-Facilitation between different plant species has been documented in terrestrial and marine ecosystems worldwide. The classic example of such positive relationships, the large keystone saguaro cactus (Carnegiea gigantea), always establishes under nurse plants that provide protection to young seedlings through subcanopy climate and environmental modification. Plants (e.g., nurse plants) are not necessarily symmetrical and variations in shape and zone of influence may impact the distribution of other species associated with them. The saguaro is distributed disproportionately to the south of its nurse tree Cercidium microphyllum (palo verde) where subcanopy temperatures are warmer on cold winter nights. I determine whether the tree’s canopy shape contributes to the non-random pattern of its protege cacti in its subcanopy. Data for tree canopy width for each of the four cardinal directions were collected. ANOVA and circular statistics (Rayleigh and V tests) found a significant canopy bias to the southwest. A second dataset shows that Carnegiea are found significantly more often to the south of Cercidium trunks than to the north. When the center of canopy is used rather than the trunk, the statistical bias no longer holds. The observed biases in distribution may be a function of the nurse’s canopy shape and resultant microclimate effects. The findings of this study provide an explanation to several already documented observations: 1) winter temperatures are more ameliorated to the south of the trunk than to the north, 2) more of the shade-dependent cacti are found on the south side of their Cercidium nurse’s trunk than to the north, and 3) saguaro cacti show a distributional preference to dense-canopied nurses. Variations in nurse canopies become crucial elements of the microenvironment and will alter patterns of establishment and survival of the prot g species that rely on them as even small changes can dramatically alter distributional patterns and expected survivorship. Nurse protection may lead to patterns quite different from the seed rain, and ultimately has a dominant role in the population dynamics of the species.