Patch-level facilitation fosters high-Andean plant diversity at regional scales

By Sabrina S. Gavini, Cecilia Ezcurra & Marcelo A. Aizen

The alpine ecosystem is one of the most stressful environments in the world due to the extreme abiotic conditions to which organisms are subject to. Surprisingly, under this harsh abiotic scenario, positive or facilitative rather than negative or competitive interactions prevail. Facilitation is generally seen as the favorable effects of the presence of neighbors, for example, when one organism improves the microhabitat favoring another organism establishment and survival, a benefit that, in the face of greater environmental stress, exceeds the cost of living near other individuals that would result in competing for resources.

One of the most extraordinary and best-adapted organisms in the high mountain are cushion plants. Their short stature and compact shape allow them to reduce wind abrasion and buffer extreme temperatures during the day and night. Moreover, the soils below cushions retain more water and contain more nutrients than the surrounding bare soil. Because cushion plants create microhabitats, that are more favorable than the surrounding environment, they are considered nurse plants by facilitating several alpine plant species inside them.

Studies of the importance of cushion plants on plant diversity in the Patagonian Andes are still very scarce. We assessed if facilitation by cushion plants influences patterns of plant diversity at different spatial scales, and wondered about the consequences in species richness in the hypothetical absence of the cushion plants. We recorded the number of plant species growing within and outside cushion plants patches at three high altitudes across seven mountains in northwestern Patagonia. With this data, numbers of species growing in either cushion plants or in the surrounding open areas were compared with the species numbers growing exclusively in open areas (i.e., the scenario that completely lacks the nurse effect exerted by cushions); approach performed at the patch, community, mountain and regional scale.

We identified fifteen species forming cushions and acting as nurse plants in the high-Andean studied communities. We found a higher number of species associated with cushion plants than in open area patches. Moreover, the number of plant species decreased with increasing elevation. This decrease, however, was only observed in the surrounding open area, since the number of species within cushions remained constant throughout the elevation gradient. On the other hand, the “hypothetical” communities lacking cushion plants showed lower species richness than observed communities, which encompasses both the open area and the nursing cushion plants. Moreover, facilitation by cushion plants became considerably stronger with increasing altitude, where environmental conditions become more severe. Remarkably, the increase in species richness attributed to the presence of cushion plants persisted at the mountain and regional scales. Our findings also suggest that approximately one-third of the region’s high-Andean flora owes its presence to the existence of cushion plants. This study provides evidence that ecological processes taking place at local scales are crucial in promoting and maintaining alpine biodiversity at increasing spatial scales. Therefore, the loss of these cushion plants would have major repercussions for plant diversity as they provide refuges that prevent biodiversity loss.

This is a plain language summary for the paper of Gavini et al. 2020 published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12922). The post was prepared by Sabrina S. Gavini.