Spatial associations between surface rocks and succulents in the Agua Fria National Monument

Background/Question/Methods

In arid and semi-arid ecosystems, facilitative “nurse” associations often aid seedling establishment by ameliorating temperature fluctuations, increasing soil water availability and protecting against physical disturbance. Nurse plant associations have been well studied (e.g. leguminous trees and saguaro seedlings), but few studies have explored the potential abiotic facilitation of plants by rocks. Surface rocks are known to significantly affect soil moisture and temperature and were manipulated for these purposes and others by prehistoric dryland agriculturalists. In this study, we assessed the frequency and strength of spatial associations between succulents and surface rocks in semi-arid grasslands within the prehistorically active landscapes of the Agua Fria National Monument in central Arizona. In 15 hilltop plots (625m²), we compared the distribution of succulents relative to surface rocks to an expected frequency based on a survey of the proportion of microhabitat within the plots and analyzed the results using a goodness-of-fit test.

Results/Conclusions

We found a greater spatial association between rocks and succulents than expected by chance (G statistic =253, p <0.0001). This pattern strongly suggests that surface rocks facilitate succulent establishment or success in this semi-arid grassland. Succulents grow preferentially within a 2-cm buffer zone around surface rocks (64%), and relatively few succulents (18%) are found > 5 cm from a surface rock. In water-limited systems, rocks may increase soil moisture without significantly reducing light availability, as often occurs with nurse plants. Our surveys also showed that the strength of the rock-succulent association appears to vary between succulent species and by growth form. Smaller, low-growing species such as Agave parryi, Cylindropuntia leptocaulis and Echinocereus spp. show a stronger rock-succulent association than larger, shrub-like species such as Cylindropuntia acanthocarpa and Opuntia spp. Although the role of nurse plants in seedling success is well studied, our findings highlight the additional, possibly facilitative role played by abiotic landscape features. The strength of the spatial association between surface rocks and succulents will allow us to evaluate the lasting effects of prehistoric rock manipulation by agriculturalists on the modern day plant community composition and distribution in the Agua Fria National Monument.