Hierarchical effects of rainfall, nurse plants, granivory and seed banks on cactus recruitment

Aims Plant facilitation is recognized as an important process, contributing not only to the ecology of individual species, but the diversity, organization and stability of communities as a whole. While decades of studies have enhanced our knowledge of the mechanisms contributing to plant facilitation, greater attention is now being given to the conditionality of the mechanisms, whereby multiple biotic and abiotic factors may act independently and jointly with one another in shaping plant–plant facilitation. Our objective was to evaluate the conditionality and hierarchy of mechanisms contributing to cactus germination and early seedling recruitment arising from plant–plant facilitation, including the effects of seed banks, rainfall, granivory and nurse plants. Location Bahia de Kina, Sonora, Mexico; Sonoran Desert of North America. Methods We employed descriptive analyses and a factorial experiment to examine the consequences of rainfall, nurse plants, granivory and seed banks for senita cactus recruitment (Pachycereus schottii) in two plant communities of the Sonoran Desert: one degraded of its key nurse plants and the other with its nurse plants intact. Results Seed banks did not occur in either of the plant communities, indicating that seed banks and seed dormancy do not affect senita recruitment and thereby do not modulate other mechanisms that contribute to nurse plant facilitation and cactus recruitment. Juvenile senita cacti were consistently associated with nurse plants in both plant communities. Yet, the density and species composition of nurse plants differed between the communities, resulting in reduced senita recruitment and shifts to alternative nurse plants in the community degraded of its key nurse plants compared to the intact community. In a factorial experiment manipulating rainfall, granivory and nurse plants, we found that rainfall and granivory had significant main factor effects, leading to increases and decreases in seedling recruitment, respectively. There was no main factor effect of nurse plants, but there was a significant nurse plant by granivory interaction, indicating that nurse plants increase seedling recruitment through escape from granivory conditional upon rainfall. Conclusions Taken together, these results reveal a conditional hierarchy of mechanisms of plant–plant facilitation shaping senita cactus recruitment, namely the predominant effects of rainfall increasing recruitment and of granivory decreasing recruitment, for which nurse plants played a key role in increasing seedling recruitment with rainfall and by reducing granivory.