Regional environmental conditions affect microsite response in a keystone desert species

• Keystone species variation in growth in the desert.
• Faster growth when shaded at hyper-arid site.
• Slower growth when shaded at cooler, wetter site.
• Access to surface water hastened growth in hyper-arid site, not in moderate site.
• Nurse association and proximity to conspecifics did not significantly impact growth.

Little is known about the environmental conditions that influence the growth rate of the keystone species Carnegiea gigantea. This study assesses variations in growth based on proximity to other conspecific individuals (intraspecific clumping), proximity to surface water, nurse species, and the impact of shading in an extreme desert environment. I observed that (1) at the cooler and wetter site, shading slowed growth relative to Carnegiea growth in the open, while at the hotter and more arid site, shaded Carnegiea individuals grew faster, likely related to the extremities of this site and thus more pronounced necessity of shade and a nurse. (2) Presence near runnels (concentrated drainage) did not significantly impact growth in the less extreme cooler and wetter site, but proximity to surface water drainage did increase growth rates at the more arid site. (3) Growth was faster under nurse trees than the smaller shrubs, however, not significantly so. (4) Proximity to other conspecifics (clumping) of this shallow-rooted species did not significantly change growth rates. Increased stress may be particularly crucial at the more extreme site in defining ecological responses to varying microsite conditions.