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

Abstract

Questions:As ecosystem engineers, alpine cushion and shrub species have been widely documented, but their roles as co-existing nurse plants have not been explicitly compared. In this study, our questions were: (1) what are the differences in engineering effects between cushions and shrubs in alpine ecosystems in southwestern China? (2) what are the differences in their influences on the attributes of alpine plant communities? and (3) can we detect the effects of changes in environmental stress with elevation on the engineering effects of cushions and shrubs? Study Site: Three plant communities along an elevational gradient on Baima Snow Mountain, SW China.Methods: Field measurements were carried out to assess effects of cushion (Arenaria polytrichoides) and shrub (Rhododendron rupicola) species on soil temperature, soil nutrient and water availabilities, the species richness of co-existing vascular species and their abundance, above-ground production and the percentages of grazed individuals.Results: Shrub and cushion species modify micro-environmental conditions, especially at higher elevations, but in different ways. Grazing pressures are highest for plants in the surroundings, followed by within cushions, and finally under shrubs. This indicates that both cushion and shrub species can protect beneficiary species from grazing by livestock. The presence of cushion plants increased species richness, abundance and above-ground production in the higher community but not in the middle and lower communities. However, the presence of shrubs consistently increased those measurements across the elevational gradient. In addition, some species exclusively and some others preferentially occur within cushions or beneath shrubs. Conclusion:The results clearly suggest that alpine cushion and shrub species in the Hengduan Mountains both act as ecosystem engineers and contribute positively to community attributes. However, their engineering effects vary with environmental stresses. Cushions could facilitate other species under severe abiotic stress, but showed much weaker or even competitive effects in benign conditions; shrubs' facilitation may remain consistently strong under different stresses.

Alpine community recruitment potential is determined by habitat attributes in the alpine ecosystems of the Himalaya‐Hengduan Mountains, SW China

“…For all three mountains, regardless of habitat type, the most abundant taxa were members of the genera Saxifraga, Kobresia, Arenaria, Polygonum, Draba, and Viola, all with the highest occurrence values. In fact, these taxa were also frequently recorded at our study sites during previous research (Chen et al, 2019;Chen, Schöb, et al, 2015;Yang et al, 2010).…”

“…(c) Grassland or grass shrubland represents a habitat where some stress-tolerant species, such as cushion plants and shrubs (Chen et al, 2019;Chen, Schöb, et al, 2015;Yang et al, 2010), are able to modify the micro-environmental conditions, leading to an increase in species numbers and facilitating the formation of numerous vegetation patches of different sizes. Within such patches, the soils develop better and soil moisture and nutrients can be relatively well maintained compared with the surrounding bare ground.…”

Plant–plant facilitation increases with reduced phylogenetic relatedness along an elevation gradient

“…Elevation gradients have been widely used to test the SGH (Choler et al 2001, Cavieres et al 2002b, Zabinski et al 2002, le Roux and McGeoch 2010) because several environmental features such as temperature and growing season length decrease with elevation, while wind speed and light intensity increase, ultimately generating conspicuous gradients of environmental severity. Thus, communities having nurses that mitigate these environmental constraints, such as cushion plants (Cavieres et al 2002b, Schöb et al 2013) or shrubs (Pistón et al 2016, Chen et al 2019) tend to harbor a greater number of species as elevation increases (but see Cavieres et al 2016). However, these studies have been conducted above the treeline, so the study of facilitation requires further elevation gradients on other systems below the treeline (Ramírez et al 2006) to draw more general pictures.…”

Diversity patterns of cushion plants on the Qinghai-Tibet Plateau: A basic study for future conservation efforts on alpine ecosystems

“…Cushion-forming plants are considered ecosystem engineers in alpine and arctic ecosystems that create suitable micro-habitats for less stress-tolerant plants to inhabit ( Arroyo et al., 2003 ; Körner, 2003 ; Badano et al., 2006 ; Cavieres et al., 2005 , 2007 ; Chen et al., 2015b , 2019 ). As a result, cushion plants facilitate higher plant diversity and above-ground productivity at both local and regional scale ( Arroyo et al., 2003 ; Butterfield et al., 2013 ; Cavieres et al.…”