Testing the Stress-Gradient Hypothesis at the Roof of the World: Effects of the Cushion Plant Thylacospermum caespitosum on Species Assemblages

Miroslav Dvorský,Martin Kopecký,Jan Altman,Jiří Doležal,Francesco De Bello,Zuzana Chlumská,Kateřina Janatková,Klára Řeháková,Christian Rixen

doi:10.1371/journal.pone.0053514

Abstract

Many cushion plants ameliorate the harsh environment they inhabit in alpine ecosystems and act as nurse plants, with significantly more species growing within their canopy than outside. These facilitative interactions seem to increase with the abiotic stress, thus supporting the stress-gradient hypothesis. We tested this prediction by exploring the association pattern of vascular plants with the dominant cushion plant Thylacospermum caespitosum (Caryophyllaceae) in the arid Trans-Himalaya, where vascular plants occur at one of the highest worldwide elevational limits. We compared plant composition between 1112 pair-plots placed both inside cushions and in surrounding open areas, in communities from cold steppes to subnival zones along two elevational gradients (East Karakoram: 4850–5250 m and Little Tibet: 5350–5850 m). We used PERMANOVA to assess differences in species composition, Friedman-based permutation tests to determine individual species habitat preferences, species-area curves to assess whether interactions are size-dependent and competitive intensity and importance indices to evaluate plant-plant interactions. No indications for net facilitation were found along the elevation gradients. The open areas were not only richer in species, but not a single species preferred to grow exclusively inside cushions, while 39–60% of 56 species detected had a significant preference for the habitat outside cushions. Across the entire elevation range of T. caespitosum, the number and abundance of species were greater outside cushions, suggesting that competitive rather than facilitative interactions prevail. This was supported by lower soil nutrient contents inside cushions, indicating a resource preemption, and little thermal amelioration at the extreme end of the elevational gradient. We attribute the negative associations to competition for limited resources, a strong environmental filter in arid high-mountain environment selecting the stress-tolerant species that do not rely on help from other plants during their life cycle and to the fact the cushions do not provide a better microhabitat to grow in.

Highlights

The response of alpine and subnival plants to recent climate warming and their potential distributional shifts along elevational gradients may, to a large extent, hinge on interactions between species [1]
We investigated which of the two contrasting predictions would apply: would the nurse effect take place, as in most of the key studies on cushions worldwide, or would we find no facilitation as implied by the refined stress-gradient hypothesis? we asked the following questions: 1) Are other vascular plants positively or negatively associated with cushions of T. caespitosum? 2) Does the effect of T. caespitosum on the surrounding vegetation depend on its size? 3) Does T. caespitosum influence the surrounding abiotic environment? 4) Do these effects and patterns change with elevation and water availability?
To test the generality of plant interactions, we analysed the associations between T. caespitosum and other vascular plants from the cold steppes up to the subnival zones in two mountain ranges (Eastern Karakoram, Little Tibet)

Summary

Introduction

The response of alpine and subnival plants to recent climate warming and their potential distributional shifts along elevational gradients may, to a large extent, hinge on interactions between species [1]. Many species from the lower alpine zone have been recently shown to extend their range limits toward the subnival zone [2,3,4] This process can be facilitated or hindered by resident species. Plants must cope with multiple stress factors including low temperature, strong wind and abrasion, unstable substrate and solifluction, and low nutrient and water availability [6] Under such conditions, the main mechanisms behind the facilitative processes could be thermal amelioration [7,8], water and nutrient provision [8,9,10] and protection from strong desiccant winds by cushion plants [8,11]

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Conclusion