Cushion and shrub ecosystem engineers contribute differently to diversity and functions in alpine ecosystems

Prepared by Jian-Guo Chen, Xiao-Fang He, Song-Wei Wang, Yang Yang & Hang Sun

In severe environments, like dry deserts and high mountaintops, only a few plants can dominate the entire community. Thanks to their capacity to ameliorate micro-environmental conditions, such as modifying temperature, moisture, soil nutrients and wind speed, some of these plants are able to craft a suitable microhabitat in an otherwise hostile environment. Given their extraordinary capacity, these species have been also called ecosystem engineers. Without such ecosystem engineers, some less-tolerant species would show a low performance and survival rate. As a result, ecosystem engineers can also contribute positively to the local plant diversity. In the alpine region, cushion and shrub species are two common ecosystem engineers and their importance for alpine ecosystems have been independently confirmed by ecologists around the globe. However, very few studies have compared their contributions in the same ecosystem, along an environmental gradient. Insights from such studies could help us to understand what drives and maintains plant diversity in alpine ecosystems.

We selected one typical cushion (Arenaria polytrichoides) and shrub (Rhododendron rupicola) species that co-occur in the same communities along an altitudinal gradient in the Hengduan Mountains, in southwestern China, and compared their ability to ameliorate micro-environmental conditions and influence community attributes (plant species richness, abundance, aboveground production and livestock grazing).

In accordance to our expectations, the cushion and shrub species both modified the micro-environmental conditions (temperature, soil water content and nutrient availability) and thus acted as ecosystem engineers. Due to their modifications of the micro-environment, they offered suitable microhabitats for other less-tolerant species and thus increased community-level species richness, abundance and aboveground production. In addition, their special architectural morphologies has likely provided protection for other species from livestock grazing. These protective effects are especially important for palatable species and those that are sensitive to grazing disturbance. Those species (or individuals) within/beneath their canopies are more likely to reach a reproductive stage. In contrast, species (or individuals) that established away from cushions or shrubs might have been grazed by livestock more than once over the short growing season, which should prevent them from reaching reproductive stages. In the long term, the failure to recruit offspring could lead to species local extinctions.

Cushion and shrub species have different capacities for modifying the micro-environment and these capacities vary with environmental stresses. Hence, these two species groups contribute differently to community attributes in the alpine ecosystems. Our results show that under severe abiotic stress cushions can facilitate the performance of other species. However, in benign conditions, their positive effects are much weaker or even competitive. By comparison, the facilitative effect of shrubs was consistently high across the environmental stress gradient. In alpine regions, shrubs are mostly found at relatively lower elevations, while cushions occupy the top ridges. Together, the presence of both cushion and shrub species in the high mountains increases species diversity at the entire ecosystem level. In addition, by protecting other species from livestock grazing, shrubs and cushions can facilitate population recruitment and persistence of less common species. Hence, we argue that cushion and shrub plants are keystone species in alpine plant communities.

This is a plain language summary for the paper of Chen et al. published in the Journal of Vegetation science (https://doi.org/10.1111/jvs.12725).