Plant strategies in extremely stressful environments: are the effects of nurse plants positive on all understory species?

ABSTRACT

Facilitation by nurse plants likely increases from outside the canopy to the center of the canopy as stresses decrease towards the center of the canopy. These stress gradients may be important in controlling plant distributions, with stress-tolerant species specializing outside the canopy or at the canopy edge, and stress intolerant species specializing at the center of the canopy. We tested if interactions with nurse plants control the distribution of understory species, and if plants species specializing in the understory environment experience higher physiological stress when grown outside the canopy than species specializing in the open environments. We tested these predictions in field sites in the arid environment of Saudi Arabia. We measured the environmental conditions, understory species abundance, and functional and physiological traits of species found under nurse plant Acacia gerrardii. We found that Acacia trees have an overall facilitative impact on the understory species. Species found more commonly under nurse tree canopies experience significant physiological stress when growing outside canopies. In contrast, species found more commonly outside canopies do not experience significant physiological stress when growing either under canopies or outside canopies. Our results demonstrate that differences in species ability to tolerate environmental stresses are important in structuring herbaceous plant communities under nurse plants in these extremely stressful environments.

KEYWORDS:

• Arid environments
• functional traits
• plant–plant interactions
• species distributions
• stress tolerance

Acknowledgements

We thank Mr. Mohammad Basharat for the help in the field study. We thank also King Abdulaziz City for Science and Technology (KACST) who supports this work through a scholarship to A.A.N., and the National Wildlife Research Centre (NWRC) in Taif region who permitted and simplified our experiments in the reserve. J Facelli and S Solivares made helpful comments on a previous version of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Ali A. Al-Namazi

Ali Al-Namazi is an assistant professor at King Abdulaziz City for Science and Technology (KACST) with research interest in plant ecology particularly plant–plant interactions.

Stephen P. Bonser

Stephen Bonser is an associate professor at UNSW Sydney, Australia. He leads a research group in plant evolutionary ecology, and his research interests focus on the evolution of ecological strategies