Nurse cushion plants show independent trait adaptation to differing habitats

In some alpine regions, plants in close proximity are exposed to two contrasting types of abiotic stress – in the lower elevations, they are met with drought and seasonal high temperatures, while in the upper elevations, they are met with the threat of frost damage. One such species living in dry Mediterranean mountains is the nurse cushion plant Arenaria tetraquetra subsp. amabilis. These plants host beneficiary species under their canopies, improving the microclimate and the beneficiaries’ survival, though the protected plants may in turn have positive or negative effects upon the host.

In a new article published in Annals of Botany, lead author Ana I. García-Cervigón and colleagues studied intraspecific variation in the cushion plant’s xylem anatomy, architecture, and leaf functional traits, relating them to elevation and habitat. The goal was to understand how and to what extent the plant adapts its hydraulic system according to its environment, and how tightly linked these traits are to one another.

Xylem anatomy and plant architecture showed the greatest responsiveness to environmental challenges, as well as the greatest coordination between them. In the hotter areas, cushion plants were smaller, with more compact canopies and more isolated vasculature to minimize embolism expansion risk in the case of severe drought.

In the colder areas, xylem anatomy presented as larger vessels with a higher proportional conductive area. Fewer traits appeared to show adaptation to the northern climate, which the authors speculate may be because some adaptations to increased cold stress are compensated by lower drought stress, producing a more neutral response to higher elevations.

Overall, traits were less tightly constrained by one another than expected, allowing a more fine-tuned optimization to the plant’s environment. In the face of ongoing climate change, this is encouraging, because it suggests the species has greater physiological leeway to adapt to changing abiotic conditions. “In [this] case, trait independence under harsher conditions might also imply that each subset of traits is controlled by different environmental drivers, which might allow for higher adjustment potential against future changes in environmental conditions,” wrote the authors.