Nurse rocks are more important than nurse plants in determining the distribution and establishment of globose cacti (Mammillaria) in the Tehuacan Valley, Mexico

While it is widely acknowledged that nurse-plants facilitate the establishment of seedlings in drylands, there are several anecdotal reports on associations between cacti and rocks. We assessed the preference for rocks or shrubs from the spatial distribution of eight cactus species (Mammillaria spp.). We experimentally assessed if distribution results from facilitation evaluating the survival of Mammillaria pectinifera seedlings growing next to rocks or under nylon sunblockers that simulated shrub shading. We found that more than half of the species avoided shrubs, while 50% of them were positively associated with rocks. Longevity and growth were highest for seedlings placed next to stones, so it may be concluded that the observed association is due to differential survival and not to other processes such as non-random seed dispersal. It is broadly accepted that the association between cacti and shrubs is mainly due to shading, however it may also represent a cost for the seedling in terms of photosynthetically active radiation. Rocks may provide a fresh and moist environment without reducing sunlight. Our results are relevant for conservation and reintroduction programs. It is important to continue the research on stress amelioration in drylands and to consider other kinds of nurse objects besides plant canopies